CHAPTER VIII. 162

PARASITES THAT ARE FREE WHEN OLD.

We are about to study in this chapter animals which seek for assistance from others while young, and are able to provide for themselves completely when they have grown old. We may compare the hosts which afford them shelter to _crèches_ which receive none except newborn infants. It is generally supposed that animals known under the name of parasites are such as require assistance from their neighbours during all the stages of their existence.[3] This is a mistake. There are very few among them which are not able to provide for themselves during some period of their development, and they then lead an independent life. We have mentioned a certain number of them in the preceding chapter, which only seek for external assistance when they are old; we bring together, on the contrary, in this chapter, those which require help at the commencement of their life, and live at large on their own industry when they have once made their entry into the world. There are even some among them which are richly endowed, 163 and one would never imagine that they would have recourse to strangers in order to bring up their progeny. All their young family is usually entrusted to the care of a nurse, who lives just long enough to bring them up; she gives them convenient shelter under her roof, and often bestows upon them the last drop of her blood.

When the young one has at last abandoned her first resting-place, she begins to think seriously of Hymen; she changes her dress and her mode of life, and seeks no more extraneous assistance till she lays her eggs. Among the animals brought up in this manner, the most remarkable are the Ichneumons, which have always attracted the notice of entomologists. These charming creatures, whose shape is delicately slender, whose transparent wings flutter with so much grace, have a less stormy youth than their boldness would induce us to suppose. As the cuckoo lays her eggs in the nest of a strange bird, the mother ichneumon deposits hers in a caterpillar full of health, by means of a long and thread-like ovipositor, so that the larvæ as soon as they are hatched, find themselves in a bath of blood and viscera, which serves them for food. The different organs palpitate under the teeth of these intruders, and the young larva grows and increases in size till it is hatched under the skin of its nurse: this skin is the cradle of the ichneumon.

The young ichneumon devours its nurse piecemeal, organ after organ; and for fear that death should supervene too quickly, the mother takes care to chloroform the victim beforehand to make her last longer. The method which many of them adopt to get rid of their young, reminds 164 us forcibly of the turning-box in which they used formerly to place children whom they wished to be brought up by public charity; with this difference, that young ichneumons are not only fed and taken care of by some good neighbour, but that her body itself serves them as food.

It has sometimes happened that entomologists, instead of finding beautiful butterflies produced from the caterpillars which they had reared, have had nothing hatched but a brood of ichneumons. Was it not natural then for them to dream of the transformation of species, when they saw issuing from the skin of a caterpillar, which is usually transformed into a beautiful chrysalis, a swarm of small winged flies which disperse with the rapidity of lightning? These ichneumons discover with astonishing ingenuity the caterpillar which can bring up their young, and they often reach it with their ovipositor in the midst of a fruit, or in the substance of a branch of a tree. Every one knows the _Anobium_ and other little beetles which attack wood, and live in the dark galleries which they excavate. The mother ichneumon knows perfectly how to discover the beetle which bores into our furniture, and winged ichneumons have often been seen to proceed from worm-eaten wood. It is not only caterpillars that are sought by ichneumons for the sake of their young; many kinds of larvæ of coleoptera and hemiptera, of aphides and weevils, are attacked by the mother ichneumons, which plunge their ovipositors between their articulations. These winged corsairs well know the weak points of their cuirass.

Ichneumons are therefore decidedly parasitical at this first period of their life. As they approach maturity, the time of which varies 165 more or less according to the species, each ichneumon takes his departure, seeks for booty on his own account, and passes through the last stages of his existence at full liberty in the open air. Nothing is more beautiful than this insect in the plenitude of its life. The species of the ichneumon are very numerous. Mons. Wesmael has devoted a part of his life to the study of these insects.

We often ask ourselves what can be the use of these little creatures--what good purpose can be effected by vermin which annoy everybody? Michelet replied to this question when he wrote "The Insect." "Birds," says the brilliant historian, "prefer to destroy those insects which are the most injurious." We may say the same of those which we are now considering. The most common caterpillar, and that which is the most dreaded on account of its great fecundity, is precisely that which is more eagerly sought by the greater number of ichneumons. No less than thirty-five kinds of these little assassins fall on certain species, to make them serve as a quarry to be given to their young ones. The _Bombyx pini_ is one of the most dangerous and destructive insects in our woods. The ichneumons would seem to take into consideration the too great fecundity of this moth, and instead of one species, as is often the case, thirty-five different species direct their attacks upon it. It would be indeed difficult for the mother to withdraw her young ones from the ovipositors of so many enemies, but there will be always enough of them remaining to keep up the balance in this little world; the greatness of the danger with respect to plants will be counterbalanced by the number of ichneumons which arrest the propagation of the caterpillars. These insects 166 contribute more effectually to the destruction of caterpillars than all the means employed by man. To arrest the Pyralis of the vine, its cultivators encourage the little Chalcis (_Chalcis minuta_); and it has lately been recommended to introduce the acarus which attacks the _Phylloxera_, in order to lessen the number of this new pest. Do not aphides also prevent the too rapid development of certain plants? and the black species which lives on Windsor beans has doubtless suggested to the gardener that he ought to cut off the head of the plant when the flowers appear.

Some other hymenoptera may be mentioned: for example, the _Evaniadæ_, the _Chalcididæ_, as well as the _Tachinariæ_, which are remarkable for this kind of life. At the moment when the mining hymenoptera introduce into their hiding-places the insects which they have seized, and which they destine for their young ones, the Tachinariæ introduce themselves by stealth, and lay their eggs on these provisions. Each kind of tachinariæ attaches itself to a particular insect. There is one essential difference between them and ichneumons, that the females of the latter perforate the skin of their victims with a pointed instrument, and cause their eggs to penetrate to the interior of the entrails; while the mother tachinæ, less cruel, are contented to lay their eggs on the surface of the skin, and leave to the larva the care of penetrating into the interior.

In the department of the Aube, not far from Lezignan, the Tithymalis (_Euphorbia helioscopa_) grows abundantly, and the natural guest of this plant is a Sphynx. While this sphynx is still a caterpillar, a dipterous tachinaria takes possession of it to feed her young ones. 167 For this purpose the fly establishes itself upon the back of the caterpillar, and mounted thus, without the caterpillar's suspecting the least in the world the danger that it runs, the fly inserts her larvæ to the number of ten or twelve. When she has thus deposited these, the fly goes to seek another caterpillar, like the cuckoo in search of a fresh nest every time that she lays an egg.

The young flies, left to themselves, pierce the skin of their host, and all take their place at the banquet, says Mons. Barthelemy.

After three moults the fly is completely developed, it devours the interior of the larvæ which has nourished it, pierces the skin, and the dead body of its host, which might have been its tomb, becomes, on the contrary, its cradle.

While not far off from the remains of its feast, its own skin hardens till it becomes a veritable shell, and the parasitical insect awakes, furnished with wings, ready to recommence, after a minute devoted to love, the circle in which pass the unvarying phases of its evolution.

The female of the _Scolia_ attacks the larva of the large scarabæus (_Oryctes nasicornis_), which is found in tan, and pierces it with its ovipositor at the same time that it deposits an egg in the body of the gigantic larva. The larva which will proceed from the egg will suck up the fluid parts of the Oryctes while on the grass, and the skin of its victim will serve in the spring as a cradle for its transformation into a nymph.

Scolietes also attack the large oryctes which destroys the cocoa-nut trees of the Seychelles Islands. It is the same with a large species found in Madagascar.

There are around us, even in the midst of our cities, insects known 168 under the name of Scolyti, which attracted much attention a few years ago. The trees by the side of the high roads, and even those of our boulevards, were attacked by them, and it was feared for a time that it would not be possible to arrest this new plague, which appeared simultaneously with the oidium of the vine and the parasite of the potato.

The boulevards of Brussels were planted with fine elms, and these trees were disappearing one after another. The seeds of this plague were also sown in France, in the environs of Paris. Mons. Eug. Robert had paid attention to it, and had announced to the Académie des Sciences a remedy to arrest the evil.

The regency of Brussels invited Mons. Eug. Robert to come and put in practice the means which he had recommended to destroy the scolyti; but, if I remember rightly, the death of the trees quickly followed that of the scolyti. Nature, instead of employing pitch to arrest this plague, has simpler and more expeditious means; these are, to bring forward an insect equally small, which multiplies sufficiently to keep the terrible Scolytus under. Such is the part which has devolved on the _Bracon iniator_. It simply lays its eggs in the bodies of the larvæ of the scolyti, and destroys them.

Wesmael has related a curious fact of this kind, concerning this enemy of our plantations. These little people can be well trusted to manage their own affairs. Each of these hymenoptera ascertains with an admirable instinct the place where the larvæ of the scolyti are to be found, and with its long flexible ovipositor darts an egg into the body of its victim.

It is not only caterpillars which are assailed by mortal enemies; the 169 eggs themselves are watched by some hymenoptera, which pierce the shell, and lay within it their own eggs. When the larvæ are hatched, the yolk and the young tissues of the legitimate proprietor serve as rations for the usurper.

In this manner, the _Ophioneuri_ live, in their larva state, in the egg of the _Pieris brassica_, the cabbage butterfly so abundant in our gardens; without this police establishment they would multiply immoderately, and our kitchen gardens would suffer still more from the ravages of these caterpillars.

It is in vain for insects to lay their eggs in the middle of fruits, or in the substance of a leaf or a branch; there will be always some hymenopterous insect which, guided by its marvellous instinct, will pierce them with its ovipositor, and reach them without their even perceiving it.

In the substance of those beautiful leaves of the water-lily which cover our ponds in summer, we often see a charming insect, known by the name of _Agrion virgo_, or damsel dragon-fly, a name given to it on account of its graceful attitudes and its elegant appearance. We observe this insect deposit its eggs with great prudence, fully persuaded that they are safe in the midst of the water; but the poor neuroptera reckons without its host. An hymenopterous insect, named _Polynema_, is there, watching every movement of the Agrion; and as soon as the latter has laid an egg, the Polynema darts down like a bird of prey on its victim, pierces it, and deposits its own egg in the interior. The egg of the wounded agrion will hatch a polynema. The cuckoo acts with less cruelty, since she is contented to lay her eggs 170 by the side of those which occupy the nest.

Remarkable examples of the refinement of cruelty and of gluttony are to be found in this little animal world. It is not enough that some among them feed on the entrails of their young neighbours; there are wasps which, in order to make the agony last longer, place by the side of the eggs which they lay, chloroformed flies, which wait patiently for the time when they can yield themselves up, still palpitating, to these young tyrants. The days, the hours, perhaps even the minutes, are scrupulously reckoned for the preparation of this living morsel. As the process of hatching proceeds, the repast acquires properties more and more adapted to the age of the young wasps.

The _Sphex_ is not less cruel. Some of the insects which are found in South America attack, not the young ones, but those which are grown up, and snatch spiders from their webs as slave-hunters carry off negroes from the wood; they garotte them, and cram them into narrow cells, after having chloroformed them to preserve them more effectually. These spiders, retaining enough life not to lose their nutritious qualities, become the easy prey of the larvæ of the Sphex. The mother of these hymenoptera takes care to deposit her eggs, as well as the living booty, in such a manner that the larvæ, at the moment of being hatched, live in abundance. These young larvæ, white and without feet, are dainty enough to reject any other kind of food. This is an act of cruelty which resembles that of the ichneumon, to which it may well be compared.

The _Platygasters_, another kind of hymenopterous insects, show their cruelty in a different manner; they live in the bodies of the larvæ 171 of _Cecidomyæ_ which are lodged in the rolled leaves of the Salix, and suck the blood of their victims.

Other insects, known by the name of _Meloïdeæ_, adopt a different plan. Their larvæ have been long known by the name of _bee-lice_; but they had not been recognized in the perfect state, as the larvæ did not resemble their parents.

These insects undergo four different moults before they become nymphs, and at each moult their appearance is completely changed. It may be easily understood that it was long before these little beings were recognized behind their masks.

This is the manner in which they ravage our flowerbeds. While they still wear the dress of larvæ, they cling to certain female hymenoptera which they know very well; and being fully assured that the door would be shut in their face if they presented themselves openly, they enter, on their neighbour's back, the galleries where their housekeeping is carried on, and at the instant that the female host lays an egg in a cell of honey, the young Meloë glides in with it, and allows itself to be shut in. During this time it continues its metamorphosis, lying in a lake of honey; it devours it all at its ease, caring nothing for the provision laid up for the hymenoptera which introduced it. It is a brigand who, having secreted himself in the carriage of a rich neighbour, introduces himself on his shoulders into his children's bed-chamber, assassinates them, and grows fat on the provisions destined for his victims.

"The _Sitaris_, the _Meloë_, and apparently other Meloedeæ, if not 172 all of them, are, when young, parasites of certain hymenoptera," says Mons. Fabri, who has watched with rare sagacity the obscure and interesting habits of these microscopic assassins.

The _Sitaris humeralis_ has a progressive development at first, a recurrent one afterwards, and then again it becomes progressive.

Aphides which are not yet full grown, and which arrest the exuberant vegetation of certain plants, are in their turn attacked by an insect which is by no means lukewarm in its proceedings. A small species of cynips (_Allotria victrix_) lays its eggs, like an ichneumon, in the body of a rose aphis, and multiplies rapidly at their expense. (Westwood).

There are certain flies which are not more delicate in their mode of life than the preceding insects. We allude to the _OEstri_. We give the representation of the species which attacks the horse.

Instead of making their attacks on those of their own class, the gadflies prefer to instal themselves on mammals and sometimes even on man. Fortunately their wants are not very great; they are contented with a little. Their presence can at most only cause some uneasiness, 173 or some trifling functional trouble.

The oestri are dipterous like ordinary flies; but instead of passing their youth on some waste organic matter, they live in the nostrils or the stomach of some hairy animal, and undergo all their metamorphoses in the interior of its body.

Thus they pass all their youth in a _crèche_; but when they have reached the adult state, they get their own living in freedom.

These oestri especially attack herbivorous mammals, and the terms _gastricola_, _cuticola_, and _cavicola_, sufficiently indicate the places which they inhabit; the first kind lodging in the stomach, the second frequenting the skin, and the third establishing themselves in some of the cavities of the body.

Dr. Livingstone doubtless alludes to some kinds of oestri when he mentioned the numerous intestinal worms which infest animals in Southern Africa:

"All the wild animals," says the celebrated traveller, "are subject to intestinal worms. I have observed bunches of a tape-like thread-worm and short worms of enlarged sizes in the rhinoceros. The zebras and elephants are seldom without them, and a thread-worm may often be seen under the peritoneum of these animals. Short red larvæ, which convey a stinging sensation to the hand, are seen clustering round the trachea of this animal, at the back of the throat; others are seen in the frontal sinus of antelopes; and curious flat leech-like worms are found in the stomachs of leches" (a new species of antelope).[4]

A species, peculiar to the horse in Europe, usually lives in its 174 stomach in summer; and when its development is complete, the winged insect follows the course of the food, and goes out from the anus to breathe the open air. The mother fly, excited by the sentiment of maternity, flies round the breast of the first horse that she meets, and lays her eggs there on some hairs which are not beyond reach of the animal's tongue. The horse wishing to get rid of these foreign bodies, licks them off, and thus they are introduced into the mouth, and from the tongue pass to the stomach. These eggs are hatched in the midst of the gastric juice, the larvæ leave them, and the young gadflies find in the juices of the stomach the milk which serves to nourish them. These larvæ pass through their metamorphoses in the stomach, and when the young fly has assumed its perfect form, with its delicate wings, its sucker, and its facetted eyes, it leaves the stomach, follows the path traced by the food, arrives some fine day at the rectum, presents itself at the place of exit, and takes its flight. Thus the fly can take its journey through the intestines on a portion of the digested food.

When she has once taken her flight she is very near the end of her life, and after a moment of love she gives up her place to others.

There is another gadfly which finds a _crèche_ in the sheep; but instead of lodging in its stomach, it instals itself in the nostrils, which are more easily reached. This second species goes through its evolutions in the vestibule.

This is the species which sometimes introduces itself into the body of man. Many instances of this have been known, and our late colleague Spring gave a very interesting account of one of them in the 175 bulletins of the Belgian Academy.

A gadfly found at Cayenne is distinguished by the name of the Macaco Worm; it belongs to the genus _Cuterebra_, and usually attacks the skin of oxen and dogs in South America. It is accidentally found sometimes on man. This is the _Cuterebra noxialis_. We here give the representation of it.

There is also a gadfly on the ox.

Professor Joly has devoted himself to zoological researches on OEstridæ in general. Professor Schroeder Vander Kolken, in Holland, and Mons. Brauer, in Austria, have studied them with great success.

The _Hippoboscus_ is a fly which is very greedy of blood, and attaches itself to horses and oxen, especially under the tail, in the parts where there is less hair. It sometimes also attacks man.

The Hippoboscus lives on the horse, and an allied species, of which a different genus has been formed, lives on bats (_Strebla vespertilionis_) in South America. Mons. Von Baër noticed hippobosci on the elan, during his residence in Königsberg.

Many other insects live and develop themselves at the expense of their nearest neighbours.

Travellers since Azara's time assure us that Uruguay contains but few 176 oxen and horses, because a fly exists in that country which lays its eggs in the navel of these animals at the moment of their birth. These animals, on the contrary, are abundant in Paraguay. In order to increase their number in Uruguay, it would be necessary to favour the multiplication of birds or insects which make war on these flies, either in the larval or the sexual state.

Diptera, known by the name of _Conops_, pass their first three changes in the soft parts of drone-bees. Dumeril had formerly suspected, from the curvature of the abdomen, that the Conops lays its eggs in the body of some other insect. Lachat and Victor Audouin have given an instance of this in the "Journal de Physique."

Thus the Conops, in its larval state, inhabits the abdomen of drones or other hymenoptera; the _Echinomyæ_ are developed within various lepidoptera when in the state of caterpillars or chrysalids; there are even some which live on flesh, and prefer that which is in a state of incipient putrefaction.

We may also speak, in this category, of animals which seek assistance, while young, from neighbours of whom they take advantage during their life, and utilize them even after their death; these are insects of various orders. They are in general more cruel than beasts of prey, which often contend on equal terms with their victims. Here we have an enemy which furtively introduces itself into its neighbour, who is nearly sucked dry before he suspects the danger to which he is exposed. He harbours unawares the assassin who is about to murder him. This is the refinement of cruelty.

The _Melophagus_ of the sheep is a wingless dipterous insect, like 177 the _Lipoptena_ of the stag. We give figures of these two curious insects.

The _Stratiome chameleon_ pays visits to flowers to seek for insects, on whose blood it feeds. Its very elongated larva lives in stagnant water.

We have now to mention in the following passages parasites much less cruel in general, and which receive with greater delicacy the hospitality which is afforded them. We refer to some worms which pass, not their youth, but their mature age in the body of a neighbour, and use their host not as a _crèche_, but as a lying-in hospital.

Their early youth is passed in freedom, but they soon give birth to a numerous progeny. The fate of the male is unknown; as to the female, she introduces herself in a microscopic state into the body of a neighbour, is developed there till she arrives at sexual maturity, and then quits her retreat to go and scatter her eggs.

It appears, however, that these females are obliged to seek assistance from insects; but before they enter this living asylum, the male, which is not yet known, ensures by his fecundation the preservation of the species.

We often find in summer in puddles of water, thin worms, which are 178 sometimes a foot long, resembling a violin string, and have for a long time puzzled naturalists. They are known by the name of _Gordius_, and have lately been very carefully studied, both with reference to their organization, to their mode of life, and their development. We give here the figure of a Gordius of the natural size. The _Mermis_, like the Gordius, passes its youth in the body of certain insects, and leaves its living cradle to scatter its eggs abroad. In this case, the embryos themselves go to seek for their host, and unlike the ichneumons, they use them with moderation. The life of the host is never compromised, and no functional disturbance is observed, notwithstanding the enormous size of the worm.

The Mermis is especially found after a heavy shower; some kinds of _Filaria_ are also more common when it rains. Under the title of "Notes on the Appearance of Worms after a Shower of Rain," I communicated to the Academy of Belgium some observations on these creatures, and these observations were recorded in the bulletins.

Some years ago they brought me one morning, after a shower of rain, a quantity of worms, four or five inches in length, very thin, and 179 twisted round each other, which had been collected in the morning, on the flower borders of several gardens within the city. It was thought that there had been a shower of worms in the night.

There was not one male worm among three hundred; all were full of eggs, and the young ones were already wriggling about within them.

Whence come they? said I, in my article. Have they fallen from the sky completely formed? It is evident that they have not been developed on the ground where they have been found; it is not less evident that they appeared suddenly on the borders. Did they come from within the bodies of certain insects which they have quitted, on account of the rain which had fallen? These worms, in fact, had completed their parasitical stage in the bodies of their hosts, and the great drought which had continued for many weeks prevented their resuming their first course of existence. It was the sudden emancipation of so many worms at once which had attracted the attention of gardeners: earwigs, cockchafers, and many other insects give them shelter during the time of this strange gestation.

It is known, by the observations of Siebold, that the eggs of the Mermis, laid during the winter, produce in the following spring embryos which live in damp earth. They immediately seek the larvæ of insects, perforate their skin, and develop themselves there without becoming encysted. After this, they again pass through the skin of their host, return to the damp earth, where they change their skin, are fecundated, and lay eggs. The larvæ of _Mermis albicans_ especially resort to caterpillars, or the larvæ of the coleoptera, 180 orthoptera, or diptera, and even to a mollusc, the _Succinea amphibia_.

Professor Meissner, and more especially Dr. Grenacher, professor at Göttingen, have made known to us the structure of the Gordius. The _Gordius bifurcus_ produces embryos at the end of a month; these embryos perforate their shell by means of their beak, become free in the damp earth, and introduce themselves through the skin into the perigastric cavity of certain larvæ. The sexual worm again becomes free. If we may believe Mons. Villot, who has made recent observations on the Mermis and the Gordius, the latter alone pass through complete metamorphoses; they assume three different forms, and change their habitation three times. Their first abode must be in the water, or in the larva of a dipterous insect, as a free embryo; the second in the larval state, in the intestines of a fish; and the third, like the first, in a sexual state.

To judge by some specimens of gordius brought from India, these curious parasites exist not in Europe only; they have been found in different parts of the world, and they lead everywhere the same kind of life.

They have been found in Calcutta in the _Hapale_; in the Philippine Islands in a _Mantis_, and the museum of Hamburg possesses some from Venezuela, which came from the body of a _Blatta_.

These worms, when they approach the adult and sexual age, lose their various external organs, and are so completely modified with respect to their organization, that at last they are merely a case for eggs. They are so entirely egg-cases, in which the digestive tube and the other organs disappear in proportion as the sexual organs are 181 developed, that many naturalists have taken these worms for a simple ovisac. This has also been the case with the _Nematobothrium_ of the fish known under the name of the eagle-fish; it has been taken by an eminent naturalist for a nest of psorospermiæ.

There are also worms which take refuge in plants, and live at their expense, as if they were in an insect. One of the most remarkable is that which attacks corn, and produces the disease known by the name of smut, the corn eel (_Anguillulina tritici_). It is a very small and thin cylindrical worm, which dries up completely with the grain of corn which has nourished it, and which can remain for an indefinite period without dying, in a state resembling dust. Every time that it is moistened, it resumes its activity. This return to life has been compared to a kind of resurrection.

Mons. Davaine has studied this worm with great care; he has made known the different phases of its development, and the manner in which it introduces itself into the plant and the grain. Needham, in his "New Discoveries made with the Microscope," (1747) gives a whole chapter to these microscopic eels.

The larvæ of the _Anguillula scandens_ are dried in the galls inhabited by the mother. As soon as these galls fall and grow moist, the larvæ revive, and abandon their cradle to live in freedom. Soon after this, they go in search of their plant, take it by storm, and penetrate into the tissues before the period of fecundation; having become sexual in the interval, these microscopic nematodes lay their eggs in a nest formed at the expense of the plant.

Another species lives in the _dipsacus_, in which also it produces 182 disease (_Anguillulina dipsaci_). It attacks the flowers, and remains on them without signs of life till the moment that they are moistened. The vinegar eel is another nematode worm which has some affinity with the preceding ones. It has been considered a _Rachitis_.

There exists also a river species; but have not different worms been confounded under this name? Many species live in brackish water, and these are remarkable for the presence of bristles on their heads, and by very distinct eyes.

[3] The discovery of a free bothriocephalus at the bottom of a ditch caused a great sensation in the world of naturalists some years ago. It was then thought that the parasite could not exist except in the body of an animal: they could only imagine it shut up as in the cells of a gaol.

[4] Missionary Travels in South Africa, p. 136.