LETTER XLIV.

_DISEASES OF INSECTS._

Having laid before you what observations I thought might sufficiently explain all the principal features of the Anatomy of insects both external and internal, you will next expect to be informed whether, like the higher animals, they are subject to have the admirable order observable in their frame interrupted by _Disease_; and you will perhaps imagine, from the multiplicity of their organs and vessels, that they must be peculiarly exposed to derangements of the vital and other functions. That they have their diseases is certain; but, except in the case of their appropriate parasitic assailants, which is a part of their economy, it does not appear that their maladies are more numerous and frequent than those of other animals. The same ALMIGHTY POWER which endowed them with so complex a structure, generally upholds them in health during their destined career, until they have fulfilled the purpose of their creation, when _they die and return again to their dust_[918].

But perhaps I may seem to you as making too great a parade about these little insignificant creatures if I assign a separate letter to the consideration of their _diseases_: but when you recollect that Aristotle has a chapter on this subject[919], and that the learned Willdenow has devoted a distinct portion of his excellent introductory work on Botany to the diseases of Plants[920],--you will perhaps be of a different mind: indeed, some facts I shall have to communicate are so remarkable and interesting, that I am sure, when you have read this letter, you will not think the subject one that deserves to be slighted.

Insect diseases may, I think, be divided into two great classes; those resulting, namely, from some accidental _external_ injury or _internal_ derangement, and those produced by _parasitic_ assailants.

I. Under the _first_ head we may begin with _wounds_, _fractures_, _mutilations_, and other _extraneous_ causes of disease. To these--insects are peculiarly subject; and though they are not, like the _Crustacea_ and _Arachnida_[921] and some other invertebrate animals, endowed with the power of _reproducing_ a mutilated limb, yet their wounds appear to heal very rapidly, and at the time they are inflicted to produce little pain[922]. But if those important members, their _antennæ_, are mutilated, insects seem to suffer a kind of derangement; the great organ of their communication with each other, and in various respects with the external world, being removed, all their instincts at once fail them. I formerly related how the amputation of these affects the _queen-bee_[923]. A similar result, as Huber tells us[924], follows, when the same experiment is repeated on the _workers_ or _drones_: they immediately become unable to take any further part in the labours of the hive; they can no longer guide themselves except in the light; if they petition one of their fellow-citizens for honey, they are unable to direct their tongue to its mouth to receive it; they remain near the entrance of the hive, and when the light is intercepted they rush out of it to return no more.

Insects occasionally are subject to _tumours_ or a preternatural enlargement of their parts and organs. The antennæ of bees sometimes swell at their extremity so as to resemble the bud of a flower ready to open, becoming at the same time very yellow, as does the fore part of the head[925]. I once saw a specimen of a _Hydrobius_--agreeing with _H. fuscipes_ in every other respect even to the most minute punctum--which had a large tumour on each side of the _prothorax_, evidently accidental, occasioned probably by the stoppage of the pores by which the superfluous moisture and air escape when it undergoes its last change. The converse of this I have observed to take place sometimes in the same part of _Geotrupes foveatus_, the ordinary lateral _foveæ_ becoming very considerably enlarged;--this was the case with the specimen from which Mr. Marsham made his description of that insect. The species is, however, very distinct in other respects, and may always be known by its small size. It happens now and then also, that these tumours represent _blisters_. I saw one once on one elytrum of a beetle and not on the other. Those of _Serropalpus_ (as Mr. MacLeay, on the authority of M. Clairville, informs me) are particularly subject to this disease. But, of all the organs, the wings are most exposed to derangements of this kind. De Geer, in a specimen of _Pieris Cratægi_ just excluded from the chrysalis, observed that one of these was distended by a considerable quantity of extravasated green fluid--two or three large drops following an incision. This disease appeared to arise from the lower membrane not adhering to the upper; so that the nervures--which are rather longitudinal channels, being open below, than tubes--were not closed to confine the fluid to its proper course. The malady, which might be called a dropsy of the wing, carried off the insect the day after its exclusion[926]. Reaumur observed that the wings of some flies were affected by an _air_-dropsy, as he calls it, which appeared to arise from the air escaping from its natural channels, and thus separating, the two membranes that form the wing, and filling the cavity produced by their separation[927].

Sometimes also _monstrosities_ are to be met with in these animals, or variations from a symmetrical structure in organs that are pairs. I have a beetle in which the terminal joint of one of the maxillary palpi is short, ovate, and acute; and that of the other, long, semiovate, and rather obtuse. A specimen of _Blaps mortisaga_ in my cabinet, taken by Mr. Denny, besides the terminal mucro of the _elytra_, has a long diverging lateral one. Goeze had the larva of a _Semblis_ brought to him in which one of the two fore-legs, though perfect in all its parts, was only half the length of the other[928]; which he regarded as a reproduction, but it seems rather a malformation. Müller mentions a most extraordinary fact of one of the _Noctuidæ_, which when disclosed from the pupa retained the head of the larva[929]. One of the most remarkable instances of this kind that have fallen under my own observation, may be seen in a specimen of _Chrysomela hæmoptera_ in the cabinet of our friend Curtis; in which one of the thighs produces a double tibia, but only one of these is furnished with a tarsus.

The diseases of insects which arise from some _internal_ cause are not very numerous. The first that I shall mention is a kind of _vertigo_. "Ants have also their maladies," says M. P. Huber: "I have noticed one extremely singular; the individuals attacked by it lose their power of guiding themselves in a straight line, they can walk only by turning round in a circle of small diameter and always in the same direction. A virgin female shut up in one of my glasses was seized on a sudden with this distemper; she described a circle of an inch in diameter, and made about a thousand turns in an hour, or not quite seventeen in a minute. She continued constantly turning round for seven days, and when I visited her in the night I found her still in motion. I gave her honey--and I think that she ate some of it." He observed that some workers were attacked by a similar disease: one of these, however, had the power of walking from time to time in a straight line; when placed upon its head it continued its gyrations[930]. Similar motions of a little moth, mentioned on a former occasion[931], may perhaps have been produced by the same cause. Bees are also subject to vertigo, which has been attributed to their eating poisonous honey[932]--but may not this disease in all these cases arise from some derangement of the nervous system? One of the ants which was so affected had lost one of its antennæ; but as this was not the case with the others, no great stress is to be laid upon the circumstance. Huber does not inform us whether those attacked by this disease recovered or not.

I have observed more than once, that the _flesh-fly_ and some others of the same tribe are subject in particular seasons to a kind of _convulsions_. When thus attacked, they kick and struggle, and seem unable to fly. Sometimes they lie upon their backs without motion, but if a finger be placed near them their convulsive motions are renewed. When thrown into the air, instead of flying, they fall to the ground. Had this distemper occurred earlier or later in the year I should have attributed it to the benumbing effects of cold; but as my observations were made one year (1816) in _May_, and in another (1811) in the latter end of _June_, this could scarcely be the case. In the year last mentioned I observed that many flies died under its influence. In wet seasons this tribe is subject to another disease, which proves fatal to many of them, and indeed to other _Diptera_. A white crust appears to be formed upon the abdomen both above and below, of a granular appearance, much resembling fine moist sugar. On the back of that part this crust does not cover the margins of the segments, which gives it the appearance of white bands; so that deceived by it, I have often at first flattered myself that I had met with some new species. The under-side of the abdomen is wholly covered by it, divided in the middle into two longitudinal masses, the anal segment being bare. De Geer has noticed this or a similar disease, which, when flies are attacked by it, causes the abdomen to swell so as even to burst, and the segments become dislocated. Upon opening the abdomen it is found filled with a white unctuous substance, which often accumulates (as above described) on its external surface[933]. Dr. Host says that in this disease when the animal is dead, the wings, which were before incumbent, become extended, and its almost invisible pubescence grows into long hairs[934]. De Geer seems to think that these flies are thus affected in consequence of having eaten some poisonous food[935]; but I rather suspect, as I have observed it become prevalent chiefly in wet seasons, that it arises from a superabundance of the nutritive fluid, or of the fat, so that it seems to be a kind of _plethora_. I once observed a fly fixed to a pane of glass, round which was a semicircle of what appeared to be merely vapour, whose radius was nearly three-fourths of an inch. Taking it for an aqueous fluid that had transpired from the dead animal, I paid no further attention to it at that time: but observing from day to day that the moisture did not evaporate, after two or three months had elapsed, I had the curiosity to examine it more closely, and, upon scraping some of it off with a penknife, I found it was a white substance of a fatty nature. In this case, then, the fat must have exploded on all sides with considerable violence from half the body or the abdomen. Probably this was a more intense degree of plethora. When I examined this appearance the fly had fallen off, and I could not find it.

Mr. Sheppard once brought me a panicle of grass, the glumes of which were rough with hairs, or small bristles, to which several specimens of a fly related to _Xylota pipiens_ adhered by their proboscis. At first I thought that having been entrapped by the bristles, and unable to extricate themselves, they had perished from want of food; but since when touched they readily dropped from the glumes, some other cause, perhaps disease, probably occasioned this singular suspension of themselves.

The maladies to which _bees_ and _silkworms_ are subject are more interesting to us than those of _flies_, on account of their utility as _cultivated_ insects. One of the worst distempers which attacks the first of these animals is a kind of _looseness_ or _dysentery_: this happens early in the year, when they are fed with too much honey without any portion of bee-bread[936], and sometimes destroys whole hives. Their excrements, instead of a yellowish red, then become black, and the odour they emit is insupportable; the bees no longer observe their usual neatness, inducing them to leave the hive when they void their excrements, but they defile it, their cells, and each other. Several remedies have been prescribed for this disease. To prevent it, a syrup made by an equal mixture of good wine and honey is recommended; and as a cure, to place in the hive combs containing cells filled with bee-bread[937]. But one of the worst maladies to which these useful animals are subject, is that called by Schirach _Faux Couvain_. It originates with the larvæ; and is caused either by their being fed with unwholesome food, or when the queen, as sometimes happens, lays her eggs so that the head of the grub is not in a proper position for emerging from the cell when the period for its disclosure is arrived:--the consequence is, that in both cases it dies and becomes putrid, which sometimes produces a real pestilence in a hive. The remedy for this evil is to cut away the infected combs, and to make the bees undergo a fast of two days[938]. The hive should be cleaned and fumigated, by burning under it aromatic plants.

The cultivators of the _silkworm_ in France have given names to several diseases to which that animal is subject. One is called _La Rouge_, and is supposed to be occasioned either by too great heat, or by too sudden a transition from cold to heat. It takes place when the caterpillar is first hatched; which lives perhaps, but in a very sickly state, till it should spin its cocoon and assume the pupa, when it expires. Another degree of the same disease is called _Les Harpions_ or _Passis_. A second distemper of this animal is _Des Vaches_, _Le Gras_ or _La Saune_: this is a mortal disease, supposed to be of a putrid nature, and produced by mephitic air; it shows itself after the second moult, but rarely after the subsequent ones. When a caterpillar is first attacked, changing the air may prove a remedy; but when the disease has made progress, it is best to burn or bury them, since if the poultry pick them up they might be poisoned by them. A third disease of silkworms is called _Les Morts Blancs_, or _Tripes_, which is also occasioned by impure air, when the leaves the animal feeds upon are heaped so as to produce fermentation. The caterpillars attacked by it die suddenly, and preserve after their death the semblance of life and health. Too great heat, whether artificial or natural, occasions _La Touffe_, a fourth, which, when the heat continues long, destroys all those that are arrived at their last stage of existence in their larva state. Black points scattered over different parts of the body, or livid and blackish spots in the vicinity of the spiracles, followed by a yellowish or reddish tint, are symptoms of a fifth malady, called _La Muscardine_. After this the animal soon dies, and becomes mouldy, but does not stink. This disease is not contagious, and is thought to be caused by a moist heat, attended by pernicious exhalations. _La Luzette_, _Luisette_, or _Clairène_, is another malady, which shows itself most commonly after the _fourth_ moult. It seems to arise from some original defect in the _egg_. The caterpillars attacked by it may be known by their clear red and afterwards dirty white colour; their body becomes transparent, and the matter of silk exudes in drops from their spinnerets; consequently, though as voracious as the rest, they are never able to construct a cocoon, and should be destroyed. _Les Dragées_ is the name given to cocoons which include a larva that never becomes a pupa. The cause of this disorder has not been ascertained, and whole broods are sometimes subject to it, which, as in the last, seems to imply some defect in the _eggs_. But as the caterpillar spins its cocoon, and the silk is as good as usual, it is a malady of no great importance. Lastly, sometimes the mulberry leaves have a gummy rather acrid secretion, which purges the silkworms; their excrement is no longer solid; they become weak and languid; and if the secretion is abundant, their transpiration is impeded, and at the time of moulting they are become so feeble as to be unable to cast their skin[939].

In the case of many caterpillars of _Lepidoptera_ that died, Bonnet found by dissection that the disease was remotely occasioned by a _diarrhea_, which taking place immediately before they became pupæ, prevented the inner membrane of their intestines from being rejected, as it would have been if no extraordinary cause had prevented it, attached to the hard excrement. He found this membrane converted into a jelly occupying great part of the stomach, which he conjectured was the proximate cause of their death[940].

To conclude this head--_spiders_ are reputed to be subject to the _stone_: I do not say _Calculus in Vesica_; but we are informed by Lesser that Dr. John Franck having shut up fourteen spiders in a glass with some valerian root, one of them voided an ash-coloured calculus with small black dots[941].

II. I now come to that class of diseases which appears to prevail almost universally amongst insects--I mean those resulting from the attack of _parasitic_ enemies. Thus millions and millions annually perish before they have arrived at their perfect state. Diseases of this kind proceed either from _vegetable_ or _animal_ parasites. I shall begin with the first, which will not occupy us long.