LETTER IX.

_BENEFITS DERIVED FROM INSECTS._

INDIRECT BENEFITS.

My last letters contained, I must own, a most melancholy though not an overcharged picture of the injuries and devastation which man, in various ways, experiences through the instrumentality of the insect world. In this and the following I hope to place before you a more agreeable scene, since in them I shall endeavour to point out in what respects these minute animals are made to benefit us, and what advantages we reap from their extensive agency.

God, in all the evil which he permits to take place, whether spiritual, moral, or natural, has the ultimate good of his creatures in view. The evil that we suffer is often a countercheck which restrains us from greater evil, or a spur to stimulate us to good: we should therefore consider every thing, not according to the present sensations of pain, or the present loss or injury that it occasions, but according to its more general, remote, and permanent effects and bearings;--whether by it we are not impelled to the practice of many virtues which otherwise might lie dormant in us--whether our moral habits are not improved--whether we are not rendered by it more prudent, cautious, and wary, more watchful to prevent evil, more ingenious and skilful to remedy it--and whether our higher faculties are not brought more into play, and our mental powers more invigorated, by the meditation and experiments necessary to secure ourselves. Viewed in these lights, what was at first regarded as wholly made up of evil, may be discovered to contain a considerable proportion of good.

This reasoning is here particularly applicable: and if the ultimate benefit to man seems in any case problematical, it is merely because to discover it requires more extended and remote views than we are enabled by our limited faculties to take, and a knowledge of distant or concealed results which we are incompetent to calculate or discover. The common good of this terraqueous globe requires that all things endowed with vegetable or animal life should bear certain proportions to each other; and if any individual species exceeds that proportion, from beneficial it becomes noxious, and interferes with the general welfare. It was requisite therefore for the benefit of the whole system that certain means should be provided, by which this hurtful luxuriance might be checked, and all things taught to keep within their proper limits: hence it became necessary that some should prey upon others, and a part be sacrificed for the good of the whole.

Of the counterchecks thus provided, none act a more important part than insects, particularly in the vegetable kingdom, every plant having its insect enemies. Man, when he takes any plant from its natural state and makes it an object of cultivation, must expect that these agents will follow it into the artificial state in which he has placed it, and still prey upon it; and it is his business to exert his faculties in inventing means to guard against their attacks. It is a wise provision that there should exist a race of beings empowered to remove all her superfluous productions from the face of nature; and in effecting this, whatever individual injury may arise, insects must be deemed general benefactors. Even the locusts which lay waste whole countries clear the way for the renovation of their vegetable productions, which were in danger of being destroyed by the exuberance of some individual species, and thus are fulfilling the great law of the Creator, that of all which he has made nothing should be lost. A region, Sparrman tells us, which had been choked up by shrubs, perennial plants, and hard half-withered and unpalatable grasses, after being made bare by these scourges, soon appears in a far more beautiful dress, clothed with new herbs, superb lilies, and fresh annual grasses, and young and juicy shoots of the perennial kinds, affording delicious herbage for the wild cattle and game[457]. And though the interest of individual man is often sacrificed to the general good, in many cases the insect pests which he most execrates will be found to be positively beneficial to him, unless when suffered to increase beyond their due bounds. Thus the insects that attack the roots of the grasses, and, as has been before observed, so materially injure our herbage, the wire-worm, the larvæ of _Melolontha vulgaris_, _Tipula oleracea_, &c., in ordinary seasons only devour so much as is necessary to make room for fresh shoots, and the production of new herbage; in this manner maintaining a constant succession of young plants, and causing an annual though partial renovation of our meadows and pastures. In the rich fields near Rye in Sussex I particularly observed this effect; and I have since at home remarked, that at certain times of the year dead plants may be every where observed, pulled up by the cattle as they feed, whose place is supplied by new offsets. So that, when in moderate numbers, these insects do no more harm to the grass than would the sharp-toothed harrows which it has been sometimes advised to apply to hide-bound pastures, and the beneficial operation of which in loosening the sub-soil these insect-borers closely imitate.

Nor would it be difficult to show that the ordinary good effects of some of those insects, which torment ourselves and our cattle, preponderate over their evil ones. Mr. Clark is inclined to think that the gentle irritation of _Gasterophilus Equi_ is advantageous to the stomach of the horse rather than the contrary. On the same principle it is not improbable that the Tabani often act as useful phlebotomists to our full-fed animals; and that the constant motion in which they are kept in summer by the attacks of the Stomoxys and other flies, may prevent diseases that would be brought on by indolence and repletion. And in the case of man himself, if I do not go so far as with Linné to give the louse the credit of preserving full-fed boys from coughs, epilepsy, &c., we may safely regard as no small good, the stimulus which these, and others of the insect assailants of the persons of the dirty and the vicious, afford to personal cleanliness and purity.

I might enlarge greatly upon the foregoing view of the subject: but this is unnecessary, as numerous facts will occur in subsequent letters which you will readily perceive have an intimate bearing upon it; and I shall therefore proceed to point out the more evident benefits which we derive from insects, arranging them under the two great heads of _direct_ benefits, and those which are _indirect_; beginning with the latter.

The insects which are _indirectly_ beneficial to us, may be considered under three points of view: First, as removing various nuisances and deformities from the face of nature: Secondly, as destroying other insects, that but for their agency would multiply so as greatly to injure and annoy us: and Thirdly, as supplying food to useful animals, particularly to fish and birds.

To advert in the _first_ place to the former. All substances must be regarded as nuisances and deformities, when considered with relation to the whole, which are deprived of the principle of animation. In this relation stand a dead carcase, a dead tree, or a mass of excrement, which are clearly incumbrances that it is desirable to have removed; and the office of effecting this removal is chiefly assigned to insects, which have been justly called the great scavengers of nature. Let us consider their little but effective operations in each of their vocations.

How disgusting to the eye, how offensive to the smell, would be the whole face of nature, were the vast quantity of _excrement_ daily falling to the earth from the various animals which inhabit it, suffered to remain until gradually dissolved by the rain or decomposed by the elements! That it does not thus offend us, we are indebted to an inconceivable host of insects which attack it the moment it falls; some immediately beginning to devour it, others depositing in it eggs from which are soon hatched larvæ that concur in the same office with tenfold voracity: and thus every particle of dung, at least of the most offensive kinds, speedily swarms with inhabitants which consume all the liquid and noisome particles, leaving nothing but the undigested remains, that soon dry and are scattered by the winds, while the grass upon which it rested, no longer smothered by an impenetrable mass, springs up with increased vigour.

Numerous are the tribes of insects to which this office is assigned, though chiefly if not entirely selected from the two orders _Coleoptera_ and _Diptera_. A large proportion of the genera formed, by different authors, from Scarabæus of Linné, viz. _Scarabæus_, _Copris_, _Ateuchus_, _Sisyphus_, _Onitis_, _Onthophagus_, _Aphodius_, and _Psammodius_; also _Hister_, _Sphæridium_; and amongst the _Brachyptera_, the majority of the _Staphylinidæ_, many _Aleocharæ_, especially of Gravenhorst's third family, many _Oxyteli_ and some _Omalia_, _Tachini_ and _Tachypori_, of that author, including in the whole many hundred species of beetles--unite their labours to effect this useful purpose: and what is remarkable, though they all work their way in these filthy masses, and at first can have no paths, yet their bodies are never soiled by the ordure they inhabit. Many of these insects content themselves with burrowing in the dung alone; but _Ateuchus pilularius_[458], a species called in America the _Tumble-dung_, whose singular manœuvres I shall subsequently have to advert to, _Copris lunaris_, _Geotrupes stercorarius_ and many other lamellicorn beetles, make large cylindrical holes, often of great depth, under the heap, and there deposit their eggs surrounded by a mass of dung in which they have previously enveloped them; thus not only dispersing the dung, but actually burying it at the roots of the adjoining plants, and by these means contributing considerably to the fertility of our pastures, supplying the constant waste by an annual conveyance of fresh dung laid at the very root; by these canals, also, affording a convenient passage for a portion of it when dissolved to be carried thither by the rain.

The coleopterous insects found in dung inhabit it in their perfect as well as imperfect states: but this is not the case with those of the order _Diptera_, whose larvæ alone find their nutriment in it; the imago, which would be suffocated did it attempt to burrow into a material so soft, only laying its eggs in the mass. These also are more select in their choice than the _Coleoptera_--not indeed as to delicacy,--but they do not indiscriminately oviposit in all kinds, some preferring horse-dung, others swine's-dung, others cow-dung, which seems the most favourite pabulum of all the dung-loving insects, and others that of birds. The most disgusting of all is the rat-tailed larva that inhabits our privies, which changes to a fly (_Eristalis tenax_) somewhat resembling a bee.

Still more would our olfactory nerves be offended, and our health liable to fatal injuries, if the wisdom and goodness of Providence had not provided for the removal of another nuisance from our globe--the _dead carcases_ of animals. When these begin to grow putrid, every one knows what dreadful miasmata exhale from them, and taint the air we breathe. But no sooner does life depart from the body of any creature, at least of any which from its size is likely to become a nuisance, than myriads of different sorts of insects attack it, and in various ways. First come the Histers and pierce the skin. Next follow the flesh-flies, some, that no time may be lost, (as _Sarcophaga carnaria_, &c.) depositing upon it their young already hatched[459]; others (_Musca Cæsar_, &c.) covering it with millions of eggs, whence in a day or two proceed innumerable devourers. An idea of the dispatch made by these gourmands may be gained from the combined consideration of their numbers, voracity and rapid development. One female of _S. carnaria_ will give birth to 20,000 young; and the larvæ of many flesh-flies, as Redi ascertained, will in twenty-four hours devour so much food, and grow so quickly, as to increase their weight two hundred fold! In five days after being hatched they arrive at their full growth and size, which is a remarkable instance of the care of Providence in fitting them for the part they are destined to act: for if a longer time was required for their growth, their food would not be a fit aliment for them, or they would be too long in removing the nuisance it is given in charge to them to dissipate. Thus we see there was some ground for Linné's assertion under _M. vomitoria_, that three of these flies will devour a dead horse as quickly as would a lion.

As soon as the various tribes of _Muscidæ_ have opened the way, and devoured the softer parts, a whole host of beetles, _Necrophori_, _Silphæ_, _Dermestes_, _Cholevæ_, and _Staphylinidæ_, actively second their labours. Wasps and hornets also come in for their portion of the spoil; and even ants, which prowl every where, rival their giant competitors in the quantity consumed by them; so that in no very long time, especially in warm climates, the muscular covering is removed from the skeleton, which is then cleansed from all remains of it by the little _Corynetes cæruleus_ and _ruficollis_, (which last is so interesting, as having been the means of saving the life of Latreille[460],) and several _Nitidulæ_[461]. Even the horns of animals have an appropriate genus (_Trox_) which inhabits them, and feeds upon their contents. And not only are large animals thus disposed of, even the smallest are not suffered long to annoy us. The burying beetle (_Necrophorus Vespillo_) inters the bodies of small animals, such as mice, several assisting each other in the work; and those to which they commit their eggs afford an ample supply of food to their larvæ[462]. Ants also in some degree emulate these burying insects, at least they will carry off the carcases of insects into their nests; and I once saw some of the horse-ants dragging away a half-dead snake of about the size of a goose-quill[463]. Some insects will even attack living animals and make them their prey, thus contributing to keep them within due limits. The common earth-worm is attacked and devoured by a centipede (_Geophilus electricus_). Mr. Sheppard saw one attack a worm ten times its own size, round which it twisted itself like a serpent, and which it finally mastered and devoured.

But insects are not only useful in removing and dissipating dead animal matter; they are also intrusted with a similar office with respect to the _vegetable_ kingdom. The interior of rotten trees is inhabited by the larvæ of a particular kind of crane-fly with pectinated antennæ (_Ctenophora_[464]), and other insects, which there find an appropriate nutriment; and a similar diet is furnished to the grubs of the rose-beetle (_Cetonia aurata_) by the dead leaves and stalks usually to be found in an ant's nest. _Staphylinidæ_, _Sphæridia_, and other _Coleoptera_, are always found under heaps of putrescent vegetables; and an infinite number are to be met with in decomposing fungi, which seem to be a kind of substance intermediate between animal and vegetable. The Boleti in particular have a genus of coleopterous insects appropriated to them[465], and the Lycoperdons another.--Stagnant waters, which would otherwise exhale putrid miasmata and be often the cause of fatal disorders, are purified by the innumerable larvæ of gnats, Ephemeræ, and other insects which live in them and abstract from them all the unwholesome part of their contents. This, Linné says, will easily appear if any one will make the experiment by filling two vessels with putrid water, leaving the larvæ in one and taking them out of the other. For then he will soon find the water that is full of larvæ pure and without any stench, while that which is deprived of them will continue stinking[466].

Benefits equally great are rendered by the wood-destroying insects. We indeed, in this country, who find use for ten times more timber than we produce, could dispense with their services; but to estimate them at their proper value, as affecting the great system of nature, we should transport ourselves to tropical climes, or to those under the temperate zones, where millions of acres are covered by one interminable forest. How is it that these untrodden regions, where thousands of their giant inhabitants fall victims to the slow ravages of time, or the more sudden operations of lightning and hurricanes, should yet exhibit none of those scenes of ruin and desolation that might have been expected, but are always found with the verdant characters of youth and beauty? It is to the insect world that this great charge of keeping the habitations of the Dryads in perpetual freshness has been committed. A century almost would elapse before the removal from the face of nature of the mighty ruins of one of the hard-wooded tropical trees, by the mere influence of the elements. But how speedy its decomposition when their operations are assisted by insects! As soon as a tree is fallen, one tribe attack its bark[467], which is often the most indestructible part of it; and thousands of orifices into the solid trunk are bored by others. The rain thus insinuates itself into every part, and the action of heat promotes the decomposition. Various fungi now take possession and assist in the process, which is followed up by the incessant attacks of other insects, that feed only upon wood in an incipient state of decay. And thus in a few months a mighty mass, which seemed inferior in hardness only to iron, is mouldered into dust, and its place occupied by younger trees full of life and vigour. The insects to which this duty is intrusted have been already mentioned in a former letter (p. 235--); but none of them do their business so expeditiously or effectually as the Termites, which ply themselves in such numbers and so unremittingly, that Mr. Smeathman assures us they will in a few weeks destroy and carry away the trunks of large trees, without leaving a particle behind; and in places where, two or three years before, there has been a populous town, if the inhabitants, as is frequently the case, have chosen to abandon it, there shall be a very thick wood, and not the vestige of a post to be seen.

* * * * *

I observed in a former letter, that the devastations of insects are not the same in every season, their power of mischief being evident only at certain times, when Providence, by permitting an unusual increase of their numbers, gives them a commission to lay waste any particular country or district. The great agents in preventing this increase, and keeping the noxious species within proper limits, are other insects; and to these I shall now call your attention.

Numerous are the tribes upon which this important task devolves, and incalculable are the benefits which they are the means of bestowing upon us; for to them we are indebted, or rather to Providence who created them for this purpose, that our crops and grain, our cattle, our fruit- and forest-trees, our pulse and flowers, and even the verdant covering of the earth, are not totally destroyed. Of these insects, so friendly to man, some exercise their destructive agency solely while in the larva state; others in the perfect state only; others in both these states; and lastly, others again in all the three states of larva, pupa, and imago. For order's sake, and to give you a more distinct view of the subject, I shall say something on each separately.

The first, those which are insectivorous only in their _larva_ state, maybe further subdivided into _parasites_ and _imparasites_, meaning by the former term those that feed upon a _living_ insect, and only destroy it when they have attained their full growth; and by the latter, those that prey upon insects already dead, or that kill them in the act of devouring them.

The _imparasitic_ insect devourers chiefly belong to the _Hymenoptera_ order; and though it is in the larva state that their prowess is exhibited, the task of providing the prey is usually left to the female, of which each species for the most part selects a particular kind of insect. Thus many species of _Cerceris_ and the splendid _Chrysidæ_ or golden wasps feed upon insects of their own order. One of the latter (_Parnopes incarnata_) commits her eggs to the progeny of _Bembex rostrata_: another (_Chrysis bidentata_) attacks the young of _Epipone spinipes_.

_Bembex_ and _Mellinus_ confine themselves to _Diptera_, the former preying upon _Eristalis tenax_, _Bombylii_, and the like[468]; the latter amongst others ridding us of the troublesome _Stomoxys calcitrans_. One of these last I have observed stationed on dung watching for flies, which when seized, she carried to her burrow.

_Epipone spinipes_, belonging to the family of Wasps, feeds upon certain green apod larvæ, of which the female deposits ten or twelve with each egg. _Ammophila vulgaris_ destroys caterpillars of a larger size, and it is probable that most of the other Vespoid and Sphecoid _Hymenoptera_, viz. _Trypoxylon_, _Philanthus_, _Larra_, _Crabro_, &c. assist in this great work.

_Pompilus_, to which genus probably several species mentioned by Reaumur as preying on these insects should be referred, has it in charge to keep the number of spiders within due bounds: and some Sand-wasps lend their aid. One of these last, mentioned by Catesby (_Pronæus cæruleus_), has been known to seize a spider eight times its own weight[469]. Another species of this genus, which is common in the Isle of France, attacks an insect still more difficult, one would think, to turn to its purpose, the all-devouring _Blatta_ or cock-roach, and is therefore one of the great benefactors to mankind. When this insect perceives a Blatta (called there Kakerlac and Cancrelas) it stops immediately: both animals eye each other; but in an instant the sand-wasp darts upon its prey, seizes it by the muzzle with its strong jaws, and bending its abdomen underneath it, pierces it with its fatal sting. Sure of its victim, it now walks or flies away, leaving the poison to work its effect; but in a short time returns, and, finding it deprived of power to make resistance, seizes it again by the head, and drags it away, walking backwards to deposit it in a hole or chink of a wall[470].

Grasshoppers are the prey of another sand-wasp, supposed to be the _Sphex pensylvanica_ of Linné, a native of North America, each of which in its larva state devours three of a large green species with which its mother has provided it[471].

From none of the imparasitic insectivorous larvæ do we derive more advantage than from those which devour the destructive Aphides, whose ravages, as we have seen above, are more detrimental to us in this island than those of any other insect. A great variety of species, of different orders and genera, are employed to keep them within due limits. There is a beautiful genus of four-winged flies, whose wings resemble the finest lace, and whose eyes are often as brilliant as burnished metals (_Hemerobius_), the larvæ of which, Reaumur, from their being insatiable devourers of them, has named the lions of the Aphides. The singular pedunculated eggs from which these larvæ proceed I shall describe when we come to treat upon the eggs of insects; the larvæ themselves are furnished with a pair of long crooked mandibles resembling horns, which terminate in a sharp point, and like those of the ant-lion are perforated, serving the insect instead of a mouth; for through this orifice the nutriment passes down into the stomach. When amongst the Aphides, like wolves in a sheep-fold, they make dreadful havoc: half a minute suffices them to suck the largest; and the individuals of one species clothe themselves, like Hercules, with the spoils of their hapless victims.

Next in importance to these come the aphidivorous flies (many species of _Syrphidæ_), whose grubs are armed with a singular mandible, furnished like a trident with three points, with which they transfix their prey. They may often be seen laid at their ease under a leaf or upon a twig, environed by such hosts of Aphides, that they can devour hundreds without changing their station; and their silly helpless prey, who are provided with no means of defence, so far from thinking of escaping, frequently walk over the back of their enemy, and put themselves in his way. When disposed to feed, he fixes himself by his tail, and, being blind, gropes about on every side, as the Cyclops did for Ulysses and his companions, till he touches one, which he immediately transfixes with his trident, elevates into the air, that he may not be disturbed by its struggles, and soon devours. The havoc which these grubs make amongst the Aphides is astonishing. It was but last week that I observed the top of every young shoot of the currant-trees in my garden curled up by myriads of these insects. On examining them this day, not an individual remained; but beneath each leaf are three or four full-fed larvæ of aphidivorous flies, surrounded with heaps of the skins of the slain, the trophies of their successful warfare; and the young shoots, whose progress had been entirely checked by the abstraction of sap, are again expanding vigorously.

But even these serviceable insects must yield the palm to the lady-bird or lady-cow (_Coccinella_), the favourite of our childhood, which, as well as most of its congeners, in the larva state feeds entirely on Aphides[472]; and the havoc made amongst them may be conceived from the myriads upon myriads of these little interesting animals, which are often to be seen in years when the plant-louse abounds. In 1807 the shore at Brighton and all the watering-places on the south coast was literally covered with them, to the great surprise and even alarm of the inhabitants, who were ignorant that their little visitors were emigrants from the neighbouring hop-grounds, where in their larva state each had slain his thousands and tens of thousands of the Aphis, which under the name of the _Fly_ so frequently blasts the hopes of the hop-grower. It is fortunate that in most countries the children have taken these friendly Coccinellæ under their protection. In France they regard them as sacred to the Virgin, and call them _Vaches à Dieu_, _Bêtes de la Vierge_, &c.; and with us, commiseration for the hard fate of a mother, whose "house is on fire and children at home," ensures them kind treatment and liberty. Even the hop-growers are becoming sensible of their services, and, as I am informed, hire boys to prevent birds from destroying them.--If we could but discover a mode of increasing these insects at will, we might not only, as Dr. Darwin has suggested, clear our hot-houses of Aphides by their means, but render our crops of hops much more certain than they now are. Even without this knowledge, nothing is more easy, as I have experienced, than to clear a plant or small tree by placing upon it several larvæ of Coccinellæ or of aphidivorous flies collected from less valuable vegetables.

Lastly, to close this list of imparasitic insectivorous larvæ, I may mention those of Geoffroy's genus _Volucella_ so remarkable for their radiated anus, which live in the nests of humble-bees, braving the fury of their stings and devouring their young; and the ant-lion (_Myrmeleon_) and Reaumur's improperly named worm-lion (_Leptis_), whose singular stratagems will be detailed in a subsequent letter, both of which destroy great numbers of insects that are so unfortunate as to fall into their toils.

The _parasitic_ larvæ, an extremely numerous tribe, must next be considered. These, with the exception of a very few individuals, belong to the order _Hymenoptera_, and were included by Linné under his vast genus _Ichneumon_, so named from the analogy between their services and those of the Egyptian Ichneumons (_Viverra Ichneumon_), the former as destroyers of insects, being equally important with the latter as devourers of serpents, the eggs of crocodiles, &c.

The habits of the whole of this tribe[473], which properly includes a great number of distinct genera, are similar. They all oviposit in living insects, chiefly while in the larva state, sometimes while pupæ (_Misocampus Puparum_); and even while in the egg state (_Ich. Ovulorum_, L[474].); but not, as far as is known, in perfect insects. The eggs thus deposited soon hatch into grubs, which immediately attack their victim, and in the end ensure its destruction. The number of eggs committed to each individual varies according to its size, and that of the grubs which are to spring from them; being in most cases one only, but in others amounting to some hundreds.

From the observations hitherto made by entomologists, the great body of the Ichneumon tribe is principally employed in keeping within their proper limits the infinite host of _lepidopterous_ larvæ, destroying, however, many insects of other orders; and perhaps if the larvæ of these last fell equally under our observation with those of the former, we might discover that few exist uninfested by their appropriate parasite. Such is the activity and address of the Ichneumonidans, and their _minute_ allies (_Pupivora_, Latr.), that scarcely any concealment, except perhaps the waters, can secure their prey from them; and neither bulk, courage, nor ferocity avail to terrify them from effecting their purpose. They attack the ruthless spider in his toils; they discover the retreat of the little bee, that for safety bores deep into timber; and though its enemy Ichneumon cannot enter its cell, by means of her long ovipositor[475] she reaches the helpless grub, which its parent vainly thought secured from every foe, and deposits in it an egg, which produces a larva that destroys it[476]. In vain does the destructive _Cecidomyia_ of the wheat conceal its larvæ within the glumes that so closely cover the grain; three species of these minute benefactors of our race, sent in mercy by Heaven, know how to introduce their eggs into them, thus preventing the mischief they would otherwise occasion, and saving mankind from the horrors of famine[477]. In vain also the _Cynips_ by its magic touch produces the curious excrescences on various trees and plants, called galls, for the nutriment and defence of its progeny: the parasite species attached to it discovers its secret chamber, pierces its wall however thick, and commits the destroying egg to its offspring. Even the clover-weevil is not secure within the legumen of that plant; nor the wire-worm in the earth, from their ichneumonidan foes. I have received from the late Mr. Markwick that of the former, and Mr. Paul has shown me the destroyer of the latter, which belongs to Latreille's genus _Proctotrupes_. Others are not more secured by the repulsive nature of the substance they inhabit; for two species at least of Ichneumon[478] know how to oviposit it in stercorarious larvæ without soiling their wings or bodies.

The ichneumonidan parasites are either external or internal. Thus the species above alluded to, which attacks spiders, does not live within their bodies, but remains on the outside[479]; and the larva of _Ophion luteum_, which adheres by one end to the shell of the bulbiferous egg that produced it, does not enter the caterpillar of _Euprepia villica_, the moth upon which it feeds[480]. But the great majority of these animals oviposit within the body of the insect to which they are assigned, from whence, after having consumed the interior and become pupæ, they emerge in their perfect state. An idea of the services rendered to us by those Ichneumons which prey upon noxious larvæ may be formed from the fact, that out of thirty individuals of the common cabbage caterpillar (the larvæ of _Pontia Brassicæ_) which Reaumur put into a glass to feed, twenty-five were fatally pierced by an Ichneumon (_Microgaster globatus_[481]). And if we compare the myriads of caterpillars that often attack our cabbages and broccoli with the small number of butterflies of this species which usually appear, we may conjecture that they are commonly destroyed in some such proportion--a circumstance that will lead us thankfully to acknowledge the goodness of Providence, which by providing such a check has prevented the utter destruction of the Brassica genus, including some of our most esteemed and useful vegetables.

The parasites are not wholly confined to the order _Hymenoptera_: some insects of other orders, though comparatively very few, destroy our little enemies in the same way. _Tachina Larvarum_, and another like it described by De Geer, lay their eggs in caterpillars and other larvæ[482]; and Reaumur describes several other flies of similar habits[483]. The order also of _Strepsiptera_, lately established[484], appears to be altogether parasitic; but with this difference from the _Pupivora_, that these extraordinary animals are found only upon _Hymenoptera_ in their perfect state, and do not appear to destroy the insects upon which they prey, but probably prevent their breeding. The species at present known are formed into two genera, _Xenos_ and _Stylops_, which are confined to _Melitta_[485] and _Vespa_[486].

The next description of insect destroyers are those which devour them in their _first_ and _last_ states.--No beetles are more common after the summer is confirmed, than the species of the genus _Telephorus_. Preysler informs us that the grub of _T. fuscus_ destroys a great many other larvæ[487], and I have observed the imago devour these and also _Diptera_.--Linné has with justice denominated the _Cicindelæ_ the tigers of insects. Though decorated with brilliant colours, they prey upon the whole insect race; their formidable jaws which cross each other are armed with fearful fangs, showing to what use they are applicable; and the extreme velocity with which they can either run or fly, renders hopeless any attempt to elude their pursuit. Their larvæ are also equally tremendous with the imago, having eight eyes, four on each side, seated on a lateral elevation of the head, two above and two very minute below, which look like those of spiders, and besides their threatening jaws armed with a strong internal tooth, being furnished with a pair of spines resembling somewhat the sting of a scorpion, which stand erect upon the back of the abdomen, and give them a most ferocious aspect[488]. This last apparatus, according to Clairville, serves the purpose of an anchor for retaining them at any height in their deep cells[489]. Most of the aquatic beetles, at least the Gyrini and Dytisci, prey upon other insects both in their first and final state. The larvæ of the latter have long been observed and described under the name of _Squillæ_, and are remarkable for having their mandibles adapted for suction like those of Hemerobius and Myrmeleon; but they are not like them deprived of a mouth, being able to devour by mastication as well as by suction.--Another tribe of this order which abounds in species, those predaceous beetles which form Linné's great genus _Carabus_ (_Eutrechina_[490]), is universally insectivorous. One of the most destructive is the grub of a very beautiful species, an English specimen of which would be a great acquisition to your cabinet, it being one of our rarest insects[491], I mean _Calosoma Sycophanta_. This animal takes up its station in the nests of _Lasiocampa processionea_ and other moths, and sometimes fills itself so full with these caterpillars, which we cannot handle or even approach without injury, as to be rendered incapable of motion and appear ready to burst. Another beautiful insect of this tribe, _Carabus auratus_, known in France by the name of _Vinaigrier_, is supposed to destroy more cockchafers than all their other enemies, attacking and killing the females at the moment of oviposition, and thus preventing the birth of thousands of young grubs[492]. Lastly come the _Brachyptera_, many of which prey upon insects as well as on putrescent substances. Mr. Lehmann tells us that some of them are very useful in destroying a weevil, _Apion flavifemoratum_[493], the great enemy of our crops of clover seed.

Amongst the devourers of insects in their _perfect_ state only, must be ranked a few of the social tribes, ants, wasps, and hornets. The first-mentioned indefatigable and industrious creatures kill and carry off great numbers of insects of every description to their nests, and prodigious are their efforts in this work. I have seen an ant dragging a wild bee many times bigger than itself; and there was brought to me this very morning while writing this letter, an _Elater_ quite alive and active, which three or four ants in spite of its struggles were carrying off. An observing friend of mine[494], who was some time in Antigua, informed me that in that island, a kind of ant which construct their nests in the roofs of houses, when they meet with any animal larger than they can carry off alive, such as a cock-roach, &c., will hold it by the legs so that it cannot move, till some of them get upon it and dispatch it, and then with incredible labour carry it up to their nest. Madam Merian, in her account of the periodical ants mentioned to you before[495], and which is confirmed by Azara[496], notices their clearing the houses of cock-roaches and similar animals; and _Myrmica omnivora_ is very useful in Ceylon in destroying the former insect, the larger ant, and the white ant[497].

You are not perhaps accustomed to regard wasps and hornets as of any use to us; but they certainly destroy an infinite number of flies and other annoying insects. The year 1811 was remarkable for the small number of wasps, though many females appeared in the spring, scarcely any neuters being to be seen in the autumn[498]; and probably in consequence of this circumstance, flies in many places were so extremely numerous as to be quite a nuisance. Reaumur has observed that in France the butchers are very glad to have wasps attend their stalls, for the sake of their services in driving away the flesh-fly; and if we may believe the author of Hector St. John's _American Letters_, the farmers in some parts of the United States are so well aware of their utility in this respect, as to suspend in their sitting-rooms a hornet's nest, the occupants of which prey upon the flies without molesting the family.

There are other devourers of insects in their perfect state, the manners and food of whose larvæ we are unacquainted with. St. Pierre speaks of a lady-bird, but it probably belonged to some other genus, of a fine violet colour, with a head like a ruby, which he saw carry off a butterfly[499]. Linné informs us that _Clerus formicarius_ devours _Anobium pertinax_. A fly related to the _Panorpa communis_ appears created to instill terror into the pitiless hearts of the tyrants of our lakes and pools,--the all-devouring _Libellulina_[500]. The _Asili_ also, which are always upon the chase, seize insects with their anterior legs and suck them with their haustellum. The cognate genus _Dioctria_, particularly _D. œlandica_, prey upon _Hymenoptera_, by some unknown means instantaneously killing the insect they seize. Many species also of _Empis_, whose haustellum resembles the beak of a bird, carry off in it _Tipulariæ_ and other small _Diptera_; and what is remarkable, you can seldom take these insects in coitu, but the female has a gnat, some fly, or sometimes beetle, in her mouth. Can this be to deposit her eggs in, as soon as they are impregnated by the male? or is it designed for the nuptial feast? Even _Scatophaga stercoraria_ and _scybalaria_, and probably many others of the same tribe, feed upon small flies, though their proboscis does not seem so well adapted for animal as for vegetable food.

The most unrelenting devourers of insects appear to be those belonging to my fourth division, which attack them under _every_ form. These begin the work of destruction when they are larvæ, and continue it during the whole of their existence.--The earwig that haunts every close place in our gardens, and defiles whatever it enters, probably in some degree makes up for its ravages by diminishing the number of other insects. The cowardly and cruel _Mantis_, which runs away from an ant, will destroy in abundance helpless flies, using its anterior tibiæ, which with the thigh form a kind of forceps, to seize its prey. The water-scorpions (_Nepa_, _Ranatra_, and _Naucoris_), whose fore legs are made like those of the _Mantis_, the water-boatman (_Notonecta_), which always swims upon its back, and _Sigara_, all live by rapine, and prey upon aquatic insects. Some of this tribe are so savage that they seem to love destruction for its own sake. One (_Nepa cinerea_) which was put into a basin of water with several young tadpoles, killed them all without attempting to eat one.

Those remarkable genera of the tribe of water-bugs (_Hydrocorisæ_ Latr.), which glide over the surface of every pool with such rapidity, being gifted with the faculty of walking upon the water, _Hydrometra_, _Velia_, and _Gerris_, subsist also upon aquatic insects. A large number of the land-bugs (_Geocorisæ_ Latr.) plunge their rostrum into the larvæ of _Lepidoptera_, and suck the contents of their bodies; and _Reduvius personatus_, which ought on that account to be encouraged, is particularly fond of the bed-bug.

But of all the insects that are locomotive and pursue their prey in every state, none are greater enemies of their fellow tribes than the _Libellulina_, and none are provided with more powerful and singular instruments of assault. In the larva and pupa states, during which they live in the water and prey upon aquatic insects, they are furnished with two pair of strong jaws, covered by a kind of mask armed with a pair of forceps or claws, which the animal has the power of pushing from it to catch any thing at a distance[501]. When an aquatic insect passes within its reach, it suddenly darts forth the mask, opens the forceps, seizes the unfortunate victim, and brings it within the action of its jaws.

When they assume the imago state, their habits do not, like those of the white ants, become more mild and gentle, but on the contrary are more sanguinary and rapacious than ever; so that the name given to them in England, "Dragon-flies," seems much more applicable than "Demoiselles," by which the French distinguish them. Their motions it is true are light and airy; their dress is silky, brilliant and variegated, and trimmed with the finest lace:--so far the resemblance holds; but their purpose, except at the time of love, is always destruction, in which surely they have no resemblance to the ladies. I have been much amused by observing the proceedings of a species not uncommon here, _Anax Imperator_ of Dr. Leach. It keeps wheeling round and round, and backwards and forwards, over a considerable portion of the pool it frequents. If one of the same species comes in its way, a battle ensues; if other species of _Libellulina_ presume to approach, it drives them away, and it is continually engaged in catching case-worm flies and other insects (for the species of this tribe all catch their prey when on the wing, and their large eyes seem given them to enable them the more readily to do this,) that fly over the water, pulling off their wings with great adroitness and devouring in an instant the contents of the body. From the number of insects of this tribe which are every where to be observed, we may conjecture how useful they must be in preventing too great a multiplication of the other species of the class to which they belong.

Lastly, under this head, not to dwell upon some other apterous genera, devourers of insects, as the scorpion and centipede, _Phalangium_ and _Galeodes_, must be enumerated the whole world of Spiders, extremely numerous both in species and individuals, which subsist entirely upon insects, spreading with infinite art and skill their nets and webs to arrest the flight of the heedless and unwary summer tribes that fill the air, which are hourly caught by thousands in their toils; one of them (_Theridium_ 13-_guttatum_ Rossi), we are told, even attacking the redoubted Scorpion[502].

So much for the insect benefactors to whom it is given in charge to keep the animals of their own class within their proper limits; and I cannot doubt that you will recognise the goodness of the Great Parent in providing such an army of counterchecks to the natural tendency of almost all insects to incalculable increase. But before I quit this subject I must call your attention to what may be denominated _cannibal insects_, since in spite of those declaimers who would persuade us that man is the only animal that preys upon his own species[503], a large number of insects are guilty of the same offence. Reaumur tells us, that having put into a glass vessel twenty caterpillars of the same species, which he was careful to supply with their appropriate food, they nevertheless devoured each other until one only survived[504]; and De Geer relates several similar instances[505]. The younger larvæ of _Calosoma Sycophanta_ often take advantage of the helpless inactivity into which the gluttony of their maturer comrades has thrown them, and from mere wantonness it should seem, when in no need of other food, pierce and devour them. A ferocity not less savage exists amongst the _Mantes_. These insects have their fore legs of a construction not unlike that of a sabre; and they can as dexterously cleave their antagonist in two, or cut off his head at a stroke, as the most expert hussar. In this way they often treat each other, even the sexes fighting with the most savage animosity. Rösel endeavoured to rear several specimens of _M. religiosa_, but always failed, the stronger constantly devouring the weaker[506]. This ferocious propensity the Chinese children have, according to Mr. Barrow, employed as a source of barbarous amusement, selling to their comrades bamboo cages containing each a _Mantis_, which are put together to fight. You will think it singular that both in Europe and Africa these cruel insects have obtained a character for gentleness of disposition, and even sanctity. This has arisen from the upright or sitting position, with the fore legs bent, assumed in watching for their prey, which the vulgar have supposed to be a praying posture, and hence adopted the belief that a child or traveller that had lost his road would be guided by taking one of these pious insects in his hands and observing what way it pointed. _Mantis fausta_, though not as some suppose worshiped by the Hottentots, is yet greatly esteemed by them, and they regard the person upon whom it alights as highly fortunate[507]. A similar unnatural ferocity is exhibited by _Gryllus campestris_, of which having put the sexes into a box, I found on examining them that the female had begun to make her meal off her companion.--The malign aspect of the scorpion leads us to expect from it unnatural cruelty, and its manners fulfill this expectation. Maupertuis put a hundred scorpions together, and a general and murderous battle immediately began. Almost all were massacred in the space of a few days without distinction of age or sex, and devoured by the survivors. He informs us also that they often devour their own offspring as soon as they are born[508]. Spiders are equally ferocious in their habits, fighting sanguinary battles, which sometimes end in the death of both combatants; and the females do not yield to the Mantes in their unnatural cruelty to their mates. Woe be to the male spider that after an union does not with all speed make his escape from the fangs of his partner! Nay, De Geer saw one that, in the midst of his preparatory caresses, was seized by the object of his attentions, enveloped by her in a web, and then devoured--a sight which, he observes, filled him with horror and indignation[509].

* * * * *

Such are the benefits which we derive from the insects that keep each other in check. Here they are the _destroyers_ to which we are chiefly indebted: but we are in another point of view under nearly equal obligations to the _destroyed_; for they are insects, either wholly or in part, that form the food of some of our most esteemed fishes, and of birds that are not more valuable to us as articles for the table, than as the songsters that enliven our groves. But before proceeding to the details which this view of the subject involves, I ought not to omit pointing out to you that many quadrupeds, which though not all of direct utility to us are doubtless of importance in the scale of being, derive a considerable part of their subsistence from insects.

The harmless hedgehog and the mole, to begin at the lower end of the series, are both said to be insectivorous[510]; the latter devouring large quantities of the wire-worms. The greedy swine will root up whole acres in search of the grubs of cockchafers, of which they are very fond; and perhaps the good they do is greater than the harm, if their attack be confined to grass that having been undermined by these grubs would soon die: they also dig up the larvæ of the destructive _Cicada septendecim_, called the American locust[511], on which, when in their perfect state, the squirrels are said to grow fat[512]. The badger, Lesser informs us, will eat beetles: and its kinsman the bear has the character of being very fond of ants and of honey; which last is also said to be a favourite article with the fox, who has sometimes the audacity to overturn bee-hives, and even to attack wasps' nests in search of it. He will also eat beetles.

Sparrman has given an amusing account of the honey-ratel, (_Viverra mellivora_,) which has a particular instinct enabling it to discover bees, and attack them in their entrenchments. Near sun-set the ratel will sit and hold one of his paws before his eyes, in order to get a distinct view of the object of his pursuit; and when, in consequence of his peering about in this manner, he sees any bees flying, he knows that at this time of the day they are making for their habitations, whither he follows them, and so attains his end[513]. Another species of Viverra (_V. prehensilis_) is also reputed to be an eager insect-hunter. The young armadillos feed on a species of locust; but no quadruped can with more propriety be called insectivorous than the ant-eaters (_Myrmecophaga_), which, as their name imports, live upon ants. The great ant-eater, when he comes to an ant-hill, scratches it up with his long claws, and then unfolds his slender worm-like tongue, (which is more than two feet long, and wet with saliva,) and when covered with ants draws it back into his mouth and swallows thousands of them alive, renewing the operation till no more are to be found. He also climbs trees in search of wood-lice and wild-honey. Bats, as every one knows, are always flitting about in summer evenings, hawking for insects: and the Lemur and monkeys will also eat them.

Insects likewise afford a favourite kind of food to many reptiles: the tortoise; frogs and toads; and lizards too of different kinds. St. Pierre mentions a small and very handsome species in the island of Mauritius, that pursues them into the houses, climbs up the walls, and even walks over glass, watching with great patience for an opportunity of catching them[514]. The common snake also is said to receive part of its nutriment from them.

But to revert to insects as indirectly advantageous to us, by furnishing food to fishes and birds, beginning with the former.

Our rivers abound with _fish_ of various kinds, which at particular seasons derive a principal part of their food from insects, as the numerous species of the salmon and carp genus. These chiefly prey upon the various kinds of _Trichoptera_, in their larva state called case- or caddis-worms, and in their imago may-flies (though this last denomination properly belongs only to the _Sialis lutaria_, which generally appears in that month,) and _Ephemeræ_. Besides these, the waters swarm with insects of every order, as numerous in proportion to the space they inhabit, as those that fill the air, which form the sole nutriment of multitudes of our fish, and the partial support of almost all.

Reaumur has given us a very entertaining account of the infinite hosts of Ephemeræ that by myriads of millions emerge at a certain season of the year from some of the rivers in France, which, as it is well worth your attention, I shall abridge for you.

These insects in their first and intermediate state are aquatic: they either live in holes in the banks of rivers or brooks below the water, so that it enters into their habitations, which they seldom quit; or they swim about and walk upon the bed of the stream, or conceal themselves under stones or upon pieces of stick. Though their life, when they assume the perfect state, is usually extremely short, some being disclosed after sun-set, laying their eggs and dying before sun-rise; and many not living more than three hours; yet in their preparatory state their existence is much longer, in some one, in others two, in others even three years.

The different species assume the imago at different times of the year; but the same species appear regularly at nearly the same period annually, and for a certain number of days fill the air in the neighbourhood of the rivers, emerging also from the water at a certain hour of the day. Those which Swammerdam observed, began to fly about six o'clock in the evening, or about two hours before sun-set; but the great body of those noticed by Reaumur did not appear till after that time; so that the season of different harvests is not better known to the farmer, than that in which the Ephemeræ of a particular river are to emerge, is to the fishermen. Yet a greater degree of heat or cold, the rise or fall of the water, and other circumstances we are not aware of, may accelerate or retard their appearance. Between the 10th and 15th of August is the time when those of the Seine and Marne, which Reaumur described, are expected by the fishermen, who call them _manna_: and when their season is come, they say "the manna begins to appear, the manna fell abundantly such a night;"--alluding, by this expression, either to the astonishing quantity of food which the Ephemeræ afford the fish, or to the large quantity of fish which they then take.

Reaumur first observed these insects in the year 1738, when they did not begin to show themselves in numbers till the 18th of August. On the 19th, having received notice from his fisherman that the flies had appeared, he got into his boat about three hours before sun-set, and detached from the banks of the river several masses of earth filled with pupæ, which he put into a large tub full of water. This tub, after staying in the boat till about eight o'clock, without seeing any remarkable number of the flies, and being threatened with a storm, he caused to be landed and placed in his garden, at the foot of which ran the Marne. Before the people had landed it, an astonishing number of Ephemeræ emerged from it. Every piece of earth that was above the surface of the water was covered by them, some beginning to quit their slough, others prepared to fly, and others already on the wing; and every where under the water they were to be seen in a greater or less degree of forwardness. The storm coming on, he was obliged to quit the amusing scene; but when the rain ceased to fall he returned to it. As soon as the cloth with which he had ordered the tub to be covered was removed, the number of flies appeared to be greatly augmented, and kept continually increasing: many flew away, but more were drowned. Those already transformed, and continually transforming, would have been sufficient of themselves to have made the tub seem full; but their number was soon very much enlarged by others attracted by the light. To prevent their being drowned, he caused the tub to be again covered with the cloth, and over it he held the light, which was soon concealed by a layer of these flies, that might have been taken by handfulls from the candlestick.

But the scene round the tub was nothing to be compared with the wonderful spectacle exhibited on the banks of the river. The exclamations of his gardener drew the illustrious naturalist thither: and such a sight he had never witnessed, and could scarcely find words to describe. "The myriads of Ephemeræ," says he, "which filled the air over the current of the river, and over the bank on which I stood, are neither to be expressed nor conceived. When the snow falls with the largest flakes, and with the least interval between them, the air is not so full of them as that which surrounded us was of Ephemeræ. Scarcely had I remained in one place a few minutes, when the step on which I stood was quite concealed with a layer of them from two to four inches in depth. Near the lowest step a surface of water of five or six feet dimensions every way was entirely and thickly covered by them: and what the current carried off was continually replaced. Many times I was obliged to abandon my station, not being able to bear the shower of Ephemeræ, which, falling with an obliquity less constant than that of an ordinary shower, struck continually, and in a manner extremely uncomfortable, every part of my face:--eyes, mouth and nostrils were filled with them." To hold the flambeau on this occasion was no pleasant office. The person who filled it had his clothes covered in a few moments with these flies, which came from all parts to overwhelm him.--Before ten o'clock this interesting spectacle had vanished. It was renewed for some nights afterwards, but the flies were never in such prodigious numbers. The fishermen allow only three successive days for the great fall of the manna: but a few flies appear both before and after, their number increasing in one case, in the other diminishing. Whatever be the temperature of the atmosphere, whether it be cold or hot, these flies invariably appear at the same hour in the evening, that is, between a quarter and half-past eight: towards nine they begin to fill the air; in the following half-hour they are in the greatest numbers; and at ten there are scarcely any to be seen. So that in less than two hours this infinite host of flies emerge from their parent stream, fill the air, perform their appointed work, and vanish. A very large proportion of them falls into the river, when the fish have their grand festival and the fishermen a good harvest[515].

Under this head I may observe how much the patient angler is indebted to insects for some of his choicest baits, for the best opportunities of showing his skill, and for the most gratifying part of his diversion. The case-worm and several other larvæ are the best standing bait for many fish. The larva of the Ephemera, there called bait and bank-bait[516], is much used in some parts of Holland. The case-worms, and grubs (I suppose of flies) from the tallow-chandlers are in request with us for roach and dace; and I am told by an acute observer of these things, the Rev. R. Sheppard, that the _Geotrupes_ and _Melolonthæ_ are good baits for chub[517]. But to be an adept in fly-fishing, which requires the most skill and furnishes the best diversion, the angler ought to be conversant in Entomology, at least sufficiently so to distinguish the different species of _Phryganea_ and other _Trichoptera_, and to know the time of their appearance.--The angler is not only indebted to insects for some of his best baits, but also for the best material to fasten his hooks to, and even for making his lines for smaller fish--the Indian grass or gut as it is called, (termed in France _Cheveux de Florence_,) which is said to be prepared in China from the matter contained in the silk reservoirs of the silk-worm, but according to Latreille is the silk vessel itself when dried[518].

* * * * *

One of the most important ends for which insects were gifted with such powers of multiplication, giving birth to myriads of myriads of individuals, was to furnish the feathered part of the creation with a sufficient supply of food. The number of birds that derive the whole or a principal part of their subsistence from insects is, as is universally known, very great, and includes species of almost every order.

Amongst the _Accipitres_ the kestril (_Falco Tinnunculus_, L.) devours abundance of insects. A friend of mine, upon opening one found its stomach full of the remains of grasshoppers and beetles, particularly the former, which he suspects constitute great part of the food of this species. One of the shrikes, also, or butcher-birds (_Lanius Collurio_)--and it is probable that other species of this numerous genus may have the same habits--is known to feed upon insects, which it first impales alive on the thorns of the sloe and other spinous plants, and then devours. If meat be given it, when kept in a cage, it will fix it upon the wires before it eats it. _Lanius Excubitor_ also impales insects, but Heckewelder denies that it feeds upon them. If he be correct, the object of this singular procedure with that species, may be to allure the birds, which it preys upon, to a particular spot[519].

Amongst the _Picæ_ or Pies the _Crotophaga_, called the Ani, which is a native of Africa and America, lives upon the locust and _Ixodes Ricinus_, which it picks in great numbers from the backs of cattle; but none are greater devourers of insects in this order than rooks. It is for the grubs of _Melolontha_, _Tipula_, &c., that they follow the plough; and they always frequent the meadows in which these larvæ abound, destroying them in vast numbers. Kalm tells us, that when the little crow was extirpated from Virginia at an enormous expense, the inhabitants would willingly have brought them back again at double the price[520]. The icteric oriole is kept by the Americans in their houses for the sake of clearing them of insects; and the purple grackle is so useful in this respect, that when, on account of their consuming grain, the American farmers in New England offered a reward of threepence a head for them, and they were in consequence nearly extirpated, insects increased to such a degree as to cause a total loss of the herbage, and the inhabitants were obliged to obtain hay for their cattle not only from Pennsylvania but even from Great Britain[521]. Of this order also is the bee-cuckoo (_Cuculus Indicator_) so celebrated for its instinct, by which it serves as a guide to the wild bees' nests in Africa. Sparrman describes this bird, which is somewhat larger than a common sparrow, as giving this information in a singular manner. In the evening and morning, which are its meal-times, it excites the attention of the Hottentots, colonists, and honey-ratel, by the cry of _cherr, cherr, cherr_, and conducts them to the tree or spot in which the bees' nest is concealed, continually repeating this cry. When arrived at the spot, it hovers over it, and then alighting on some neighbouring tree or bush, sits in silence, expecting to come in for its share of the spoil, which is that part of the comb containing the brood[522].--The wryneck and the woodpeckers, the nut-hatch and tree-creeper, live entirely upon insects and their eggs[523], which they pick out of decayed trees and out of the bark of living ones. The former also frequents grass-plats and ant-hills, into which it darts its long flexible tongue and so draws out its prey. The woodpecker likewise draws insects out of their holes by means of the same organ, which for this purpose is bony at the end and barbed, and furnished with a curious apparatus of muscles to enable them to throw it forwards with great force. Some species spit the insects on their tongue, and thus bring them into their mouth. In America, the tree-creeper is furnished with a box at the end of a long pole to entice it to build in gardens, which it is found to be particularly useful in clearing from noxious insects.

Amongst the _Grallæ_ or Waders, many of the long-billed birds eat the larvæ of insects as well as worms: and they form also no inconsiderable part of the food of our domestic poultry, especially turkeys, which may be daily seen busily engaged in hunting for them, and, as well as ducks, will greedily devour the larger insects, as cockchafers, and in North America _Cicadæ_. Mr. Sheppard was much amused one day in July last year with observing a cow which had taken refuge in a pond, probably from the gad-fly, and was standing nearly up to its belly in water. A fleet of ducks surrounded it, which kept continually jumping at the flies that alighted upon it. The cow, as if sensible of the service they were rendering her, stood perfectly still though assailed and pecked on all sides by them. The partridge takes her young brood to an ant-hill, where they feast upon the larvæ and pupæ, which Swammerdam informs us were sold at market in his time to feed various kinds of birds[524]. Dr. Clarke also mentions having seen them, as well as the ants themselves, exposed to sale in the market at Moscow as a food for nightingales[525]. Latreille tells us that singing birds are fed in France with the larvæ of the horse-ant (_Formica rufa_).

But the Linnean order of _Passeres_ affords the greatest number of insectivorous birds; indeed almost all the species of this order, except perhaps the pigeon-tribe, and the cross-bill and other Loxiæ, more or less eat insects. Amongst the thrush tribe, the blackbird, though he will have his share of our gooseberries and currants, assists greatly in clearing our gardens of caterpillars; and the locust-eating thrush is still more useful in the countries subject to that dreadful pest: these birds never appear but with the locusts, and then accompany them in astonishing numbers, preying upon them in their larva state. The common sparrow, though proscribed as a most mischievous bird, destroys a vast number of insects. Bradley has calculated that a single pair having young to maintain, will destroy 3360 caterpillars in a week[526]. They also prey upon butterflies and other winged insects. The fly-catchers (_Muscicapa_) and the warblers (_Motacilla_), which include our sweetest songsters, are almost entirely supported by insects; so that were it not for these despised creatures we should be deprived of some of our greatest pleasures, and half the interest and delight of our vernal walks would be done away. Our groves would no longer be vocal; our little domestic favourites the red-breast and the wren would desert us; and the heavens would be depopulated.--We should lose too some of the most esteemed dainties of our tables, one of which, the wheat-ear, is said to be attracted to our downs by a particular insect[527]. Lastly, insects are the sole food of swallows, which are always on the wing hawking for them, and their flight is regulated by that of their prey. When the atmosphere is dry and clear and their small game flies high, they seek the skies; when moist and the insects are low or upon the ground, they descend and just skim the surface of the earth and waters; and thus by their flight are regarded as prognosticating fair or wet weather. I was last summer much interested and amused by observing the tender care and assiduity with which an old swallow supplied her young with this kind of food. My attention was called to a young brood, that having left their nest before they were strong enough to take wing, were stationed on the lead which covers a bow window in my house. The mother was perpetually going and returning, putting an insect into the mouth first of one and then of the others in succession, all fluttering and opening their mouths to receive her gift. She was scarcely ever more than a minute away, and continued her excursions as long as we had time to observe her. When the little ones were satisfied, they put their head under their wing and went to sleep. The number of insects caught by this tribe is inconceivable. But it is not in summer only that birds derive their food from the insect tribes: even in winter the pupæ of _Lepidoptera_, as Mr. White tells us, are the grand support of those that have a soft bill[528].

* * * * *

I shall close my list of the indirect benefits derived from insects, by adverting to the very singular apparent subserviency of some of them to the functions of certain vegetables.

You well know that some plants are gifted with the faculty of catching flies. These vegetable Muscicapæ, which have been enumerated by Dr. Barton of Philadelphia, who has published an ingenious paper on the subject[529], may be divided into three classes: First, those that entrap insects by the irritability of their stamina, which close upon them when touched. Under this head come _Apocynum androsæmifolium_, _Asclepias syriaca_ and _curassavica_, _Nerium Oleander_, and a grass described by Michaux under the name of _Leersia lenticularis_. The second class includes those which entrap them by some viscosity of the plant, as many species of _Rhododendron_, _Kalmia_, _Robinia_, _Silene_, _Lythrum_, _Populus balsamifera_, &c.[530] And under the third class will arrange those which ensnare by their leaves, whether from some irritability in them, as in _Dionæa_, _Drosera_, &c., or merely from their forming hollow vessels containing water, into which the flies are enticed either by their carrion-like odour, or the sweet fluid which many of them secrete near the faux, as in _Sarracenia_, _Nepenthes_, _Aquarium_, _Cephalotus_, &c., the tubular leaves of which are usually found stored with putrefying insects. In this last class may be placed the common _Dipsacus_ of this country, the connate leaves of which form a kind of basin round the stem, that retains rain-water in which many insects are drowned. To these a fourth class might be added, consisting of those plants whose flowers smelling like carrion (_Stapelia hirsuta_, &c.) entice flies to lay their eggs upon them, which thus perish.

The number of insects thus destroyed is prodigious. It is scarcely possible to find a flower of the _Muscicapæ Asclepiadeæ_ that has not entrapped its victim, and some of them in the United States closely cover hundreds of acres together.

What may be the precise use of this faculty is not so apparent. Dr. Barton doubts whether the flowers that catch insects, being only temporary organs, can derive any nutriment from them; and he does not think it probable that the leaves of _Dionæa_, &c., which are usually found in rich boggy soil, can have any need of additional stimulus. As nothing however is made in vain, there can be little doubt that these ensnared insects are subservient to some important purpose in the economy of the plants which are endowed with the faculty of taking them, though we may be ignorant what that purpose is; and an experiment of Mr. Knight's, nurseryman in King's Road, London, seems to prove that in the case of Dionæa, at least, the very end in view, contrary to Dr. Barton's supposition, is the supplying the leaves with animal manure; for he found that a plant upon whose leaves he laid fine filaments of raw beef, was much more luxuriant in its growth than others not so treated[531]. Possibly the air evolved from the putrefying insects with which _Sarracenia purpurea_ is sometimes so filled as to scent the atmosphere round it, may be in a similar manner favourable to its vegetation.

Most of the insects which are found in the tubular leaves of this and similar plants enter into them voluntarily; but Sir James Smith mentions a curious fact, from which it appears that in some cases they are deposited by other species. One of the gardeners of the Liverpool Botanic Garden observed an insect, from the description one of the _Crabronidæ_, which dragged several large flies to the _Sarracenia adunca_, and, having with some difficulty forced them under the lid or cover of its leaf, deposited them in its tubular part which was half filled with water: and on examination all the leaves were found crowded with dead or drowning flies[532]. What was the object of this singular manœuvre does not seem very obvious. At the first glance one might suppose that, having deposited an egg in the fly, it intended to avail itself of the tube of the leaf instead of a burrow. Yet we know of no such strange deviation from natural instinct, which would be the more remarkable because the insect was _European_, while the plant was American and growing in a hot-house. And at any rate it does not seem very likely that the insect would commit her egg to the tube without having previously examined it; in which case she must have discovered it to be half full of water, and consequently unfit for her purpose.--It is not so wonderful that many large flies should, as Professor Barton informs us, drop their eggs into the Ascidia furnished with dead carcases: and it seems very probable that Dytisci oviposit in them; for the Squilla which Rumphius found there was probably one of their larvæ, this being the old name for them[533].

However problematical the agency of insects caught by plants as to their _nutriment_, there can be no doubt that many species perform an important function with regard to their _impregnation_, which indeed without their aid would in some cases never take place at all. Thus, for the due fertilization of the common Barberry (_Berberis vulgaris_) it is necessary that the irritable stamens should be brought into contact with the pistil by the application of some stimulus to the base of the filament; but this would never take place were not insects attracted by the melliferous glands of the flower to insinuate themselves amongst the filaments, and thus, while seeking their own food, unknowingly fulfill the intentions of nature in another department[534].

The agency of these little operators is not less indispensable in the beautiful tribe of _Iris_. In these, as appears from the observations of Kölreuter, the true stigma is situated on the upper side of a transverse membrane (_arcus eminens_ of Haller) which is stretched across the middle of the under surface of the petal-like expansion or style-flag, the whole of which has been often improperly regarded as fulfilling the office of a stigma. Now as the anther is situated at the base of the style-flag which covers it, at a considerable distance from the stigma, and at the same time cut off from all access to it, by the intervening barrier formed by the _arcus eminens_, it is clear that but for some extraneous agency the pollen could never possibly arrive at the place of its destination. In this case the _humble-bee_ is the operator. Led by instinct, or, as the ingenious Sprengel supposes, by one of those honey marks (_Saftmaal_) or spots of a different colour from the rest of the corolla, which, according to him, are placed in many flowers expressly to guide insects to the nectaries, she pushes herself between the stiff style-flag and elastic petal, which last, while she is in the interior, presses her close to the anther, and thus causes her to brush off the pollen with her hairy back, which ultimately, though not at once, conveys it to the stigma. Having exhausted the nectar she retreats backwards; and in doing this, is indeed pressed by the petal to the _arcus eminens_; but it is only to its lower or negative surface, which cannot influence impregnation. She now takes her way to the second petal, and insinuating herself under its style-flag, her back comes into close contact with the true stigma, which is thus impregnated with the pollen of the first visited anther: and in this manner migrating from one part of the corolla to another, and from flower to flower, she fructifies one with pollen gathered in her search after honey in another.--Mr. Sprengel found, that not only are insects indispensable in fructifying the different species of Iris, but that some of them, as _I. Xiphium_, require the agency of the larger humble-bees, which alone are strong enough to force their way beneath the style-flag: and hence, as these insects are not so common as many others, this Iris is often barren, or bears imperfect seeds[535].

_Aristolochia Clematitis_, according to Professor Willdenow, is so formed, that the anthers of themselves cannot impregnate the stigma; but this important affair is devolved upon a particular species of gnat (_Cecidomyia pennicornis_). The throat of the flower is lined with dense hair, pointing downward so as to form a kind of funnel or entrance like that of some kinds of mouse-traps, through which the insects may easily enter but not return: several creep in, and, uneasy at their confinement, are constantly moving to and fro, and so deposit the pollen upon the stigma: but when the work entrusted to them is completed, and impregnation has taken place, the hair which prevented their escape shrinks, and adheres closely to the sides of the flower, and these little go-betweens of Flora at length leave their prison[536]. Sir James Smith supposes that it is for want of some insect of this kind that _Aristolochia Sipho_ never forms fruit in this country.

Equally important is the agency of insects in fructifying the plants of the Linnean classes _Monoecia_, _Dioecia_ and _Polygamia_, in which the stamens are in one blossom and the pistil in another. In exploring these for honey and pollen, which last is the food of several insects besides bees[537], it becomes involved in the hair, with which in many cases their bodies seem provided for this express purpose, and is conveyed to the germen requiring its fertilizing influence. Sprengel supposes that with this view some plants have particular insects appropriated to them, as to the dioecious nettle _Catheretes Urticæ_, to the toad-flax _Catheretes gravidus_, both minute beetles, &c. Whether the operations of _Cynips Psenes_ be of that advantage in fertilizing the fig, which the cultivators of that fruit in the East have long supposed, is doubted by Hasselquist and Olivier[538], both competent observers, who have been on the spot. Our own gardeners, however, will admit their obligations to bees in _setting_ their cucumbers and melons, to which they find the necessity of themselves conveying pollen from a male flower, when the early season of the year precludes the assistance of insects. Sprengel asserts, that apparently with a view to prevent hybrid mixtures, insects which derive their honey or pollen from different plants indiscriminately, will during a whole day confine their visits to that species on which they first fixed in the morning, provided there be a sufficient supply of it[539]; and the same observation was long since made with respect to bees by our countryman Dobbs[540].

Thus we see that the flowers which we vainly think are

"... born to blush unseen, And waste their fragrance on the desert air,"

though unvisited by the lord of the creation, who boasts that they were made for him, have nevertheless myriads of insect visitants and admirers, which, though they pilfer their sweets, contribute to their fertility.

I am, &c.

FOOTNOTES:

[457] Sparrman's _Voyage_, i. 367.

[458] The _Coprion_, _Cantharus_, and _Heliocantharus_ of the ancients was evidently this beetle, or one nearly related to it, which is described as rolling backwards large masses of dung, and attracted such general attention as to give rise to the proverb _Cantharus pilulam_. It should seem from the name, derived from a word signifying an ass, that the Grecian beetle made its pills of asses' dung; and this is confirmed by a passage in one of the plays of Aristophanes, the _Irene_, where a beetle of this kind is introduced, on which one of the characters rides to heaven to petition Jupiter for peace. The play begins with one domestic desiring another to feed the Cantharus with some bread, who afterwards orders his companion to give him another kind of bread made of _asses'_ dung.

[459] PLATE XXII. FIG. 4, 5.

[460] See Latr. _Gen._ i. 275.

[461] This property in the carrion insects may be turned to a good account by the comparative anatomist, who has only to flay the body of one of the smaller animals, anoint it with honey, and bury it in an ant-hill; and in a short time he will obtain a perfect skeleton, denudated of every fibril of muscle, though with the ligaments and cartilages untouched.

[462] Gleditsch, _Abhandlungen_, iii. 200.

[463] It is to be observed that in our cold climates, during the winter months, when excrement and putrescent animal matter are not so offensive, they are left to the action of the elements, insects being then torpid.

[464] Curtis _Brit. Ent. t._ 5.

[465] Surely Mr. Marsham's name for this genus, _Boletaria_, is much more proper than that of Fabricius, _Mycetophagus_ (Agaric-eater), since these insects seldom eat agarics.

[466] _Œcon. Nat. Amœn. Ac._ ii. 50. Stillingfleet's _Tracts_, 122.

[467] Maupertuis observes, that in Lapland he saw many birch-trees lying on the ground, which had probably been there for a very long time, with the bark entire, though the wood was decayed. Hence we may probably infer, that in that country there are few or none of the bark-boring insects.

[468] Latreille, _Observations nouvelles sur les Hyménoptères_. _Annal. de Mus._ 11.

[469] _Nat. Hist. of Carolina_, ii. 105.

[470] Reaum. vi. 282. St. Pierre's _Voyage_, 72.

[471] Bartram in _Philos. Trans._ xlvi. 126.

[472] The larvæ of some species of Coccinellæ feed, according to Prof. D. Reich, solely on the leaves of plants; as that of _C. hieroglyphica_, which eats the leaves of common heath (_Erica vulgaris_) after the manner of the larvæ of _Lepidoptera_. I suspect, however, that there is some mistake in this statement. _Der Gesellschaft naturf. Fr. in Berlin Mag._ &c. iii. 294.

[473] Latreille denominates this family, as he calls it, _Pupivora_: if by this he alludes to their devouring the young of insects, from the _classical_ meaning of the word _pupa_, the term is very proper; but this should be borne in mind, as the majority of readers would imagine it to refer to the _pupa state_ of _insects_, in which they are not so generally devoured by their parasites.

[474] Not having had it in my power to consult Dalman's work on the _Chalcidites_ of Latreille, referred to by that learned Entomologist in his _Familles Naturelles du Règne Animal_, I am not able to refer them to their proper genera.

[475] PLATE XVI. FIG. 1.

[476] Marsham in _Linn. Trans._ iii. 26.

[477] See above, p. 169-170.

[478] _Alysia Manducator_; and another species allied to _Alomyia Debellator_, which I have named _A. Stercorator_.

[479] De Geer, ii. 863.

[480] Ibid. 851-5.

[481] Reaum. ii. 419.

[482] De Geer, i. 196. vi. 14. 24.

[483] Reaum. ii. 440-4.

[484] _Linn. Trans._ xi. 86.

[485] Kirby's _Mon. Ap. Ang._ ii. 110-113.

[486] Rossi _Fn. Etrusc. Mant._

[487] Preys. _Bömisch. Insekt._ 59. 61.

[488] PLATE XVII. FIG. 13.

[489] _Entom. Helvétique_, ii. 158.

[490] In the former edition of this work (Vol. IV. p. 392), this tribe is denominated _Eupodina_; but as this seems too near to M. Latreille's _Eupoda_, belonging to a different tribe of beetles, we have substituted the above name, which means the same.

[491] One was taken at Aldeburgh in Suffolk by Dr. Crabbe, the celebrated poet; another by a young lady at Southwold, which is now in the cabinet of W. J. Hooker, esq.; and a third by a boy at Norwich, crawling up a wall, which was purchased of him by S. Wilkin, esq.

[492] Latr. _Hist. Nat._ x. 181.

[493] _Linn. Trans._ vi. 149. Kirby, _Ibid._ ix. 42. 23.

[494] The late R. Kittoe, Esq.

[495] p. 123.

[496] _Voyages_, i. 185.

[497] Percival's _Ceylon_, 307.

[498] Mr. Knight made the same observation in 1806, and supposes the scarcity of neuters arose from the want of males to impregnate the females. _Philos. Trans._ 1807, p. 243.

[499] St. Pierre, _Voy._ 72.

[500] Lesser, _L._ i. 263, note.

[501] Reaum. vi. 400. _t._ 36-38. PLATE XVI. FIG. 5. _a._

[502] Thiebaut de Berneaud's _Voyage to Elba_, p. 31.

[503]

"Even Tiger fell and sullen Bear Their likeness and their lineage spare. Man only mars kind nature's plan, And turns the fierce pursuit on Man!" Scott's _Rokeby_, canto iii. 1.

[504] Reaumur, ii. 413.

[505] De Geer, i. 533. iii. 361. v. 400. vi. 91.

[506] Rösel, iv. 96.

[507] Thunberg's _Travels_, ii. 66.

[508] De Geer, vii. 335.

[509] De Geer, vii. 180.

[510] Bingley, ii. 374.

[511] Bingley, iii. 27.

[512] Collinson in _Philos. Trans._ 1763.

[513] Sparrman, ii. 180.

[514] St. Pierre, _Voy._ 73.

[515] Reaum. vi. 479-487.

[516] Swamm. _Bib. Nat._ i. c. 4. 106. b.

[517] In Col. Venable's _Experienced Angler_, a vast number of insects are enumerated as good baits for fish, under the names of _Bob_, _Cadbait_, _Cankers_, _Caterpillars_, _Palmers_, _Gentles_, _Bark-worms_, _Oak-worms_, _Colewort-worms_, _Flag-worms_, _Green-flies_, _Ant-flies_, _Butterflies_, _Wasps_, _Hornets_, _Bees_, _Humble-bees_, _Grasshoppers_, _Dors_, _Beetles_, _a great brown fly_ that lives upon the oak like a _Scarabee_--(_Melolontha vulgaris_ or _Amphimalla solstitialis_?) and _flies_ (_i. e._ may-flies) of various sorts.

[518] Anderson's _Recreations in Agricult. &c._, iv. 478. Latr. _Hist. Nat._ xiv. 154.

[519] According to Mr. Heckewelder (_Trans. Am. Phil. Soc._ iv. 124.) _L. Excubitor_, called in America the nine-killer, from an idea that it transfixes nine individuals daily, treats in this manner _Grasshoppers_ only; while _L. Collurio_ would seem to restrict itself chiefly to _Geotrupes_, two of which Mr. Sheppard once observed transfixed in a hedge that he knew to be the residence of this bird. Kugellan even thinks that it impales only _G. vernalis_, which he has often found transfixed, but never _G. stercorarius_. (Schneid. _Mag._ 259.) I must remark, however, that I last summer observed two _humble-bees_ quite alive, impaled on the thorns of a hedge near my house, which had most probably been so placed by this species, _L. Excubitor_ being rarely found except in mountainous wilds. (Bewick's _Birds_, i. 61.) And Prof. Sander states that on opening this bird (_L. Collurio_) he has sometimes found in its stomach nothing but grasshoppers, and at others small beetles and other insects. _Naturforscher_ Stk. xviii. 234.

[520] Stillingfl. _Tracts_, 175. _Linn. Trans._ v. 105. note^b.

[521] Bingley, ii. 287-290.

[522] Sparrman, ii. 186.

[523] See above p. 208. note^b. and Bewick's _Birds._ i. Pref. xxii. 130.

[524] _Bib. Nat._ i. 126. b.

[525] _Travels_, i. 110.

[526] Reaum. ii. 408.

[527] Bingley, ii. 374.

[528] White's _Selborne_, i. 181.

[529] _Philos. Mag._ xxxix. 107.

[530] Small flies are sometimes found sticking to the glutinous stigma of some of the Orchideæ like birds on a limed twig: (Sprengel _Entdecktes Geheimniss_, 21--) and ants are not unfrequently detained in the milky juice which the touch of even their light feet causes to exude from the calyxes of the common garden lettuce. _Ann. of Bot._ ii. 590.

[531] _Elements of the Science of Botany_, 62.

[532] Smith's _Introduction to Botany_, 195.

[533] Mouffet, 319.

[534] Smith's _Tracts_, 165. Kölreuter, _Ann. of Bot._ ii. 9.

[535] Chr. Conr. Sprengel _Entdecktes Geheimniss, &c._ Berlin 1793, 4to. quoted in _Ann. of Bot._ i. 414.

[536] _Grundriss der Kräuterkunde_, 353. A writer however in the _Annual Medical Review_ (ii. 400.) doubts the accuracy of this fact, on the ground that he could never find _C. pennicornis_, though _A. Clematitis_ has produced fruit two years at Brompton. Meigen (Dipt. i. 100. e.) places this amongst his doubtful _Cecidomyiæ_. Fabricius considers it as a _Chironomus_.

[537] I have frequently observed _Dermestes flavescens_, Ent. Brit. (_Byturus_) eat both the petals and stamens of _Stellaria Holosteum_; and _Mordellæ_ will open the anthers with the securiform joints of their palpi to get at the pollen.

[538] Hasselquist's _Travels_, 253. Latr. _Hist. Nat._ xiii. 204.

[539] Willd. _Grundriss_, 352.

[540] _Phil. Trans._ xlvi. 536.