Part 17

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.

i. As insects pass often no small portion of their life in a state of torpidity, in which they remain chiefly without motion, it will not seem wonderful, should any partial moisture accidentally accumulate upon them, that it affords a seed plot for certain minute fungi to come up and grow in. Persoon observes with regard to his genus _Isaria_, that one species grows upon the _larvæ_ of insects (_I. truncata_), and another upon _pupæ_ (_I. crassa_[942]):--as he does not say upon _dead_ larvæ and pupæ, as upon a former occasion[943], perhaps in these cases these plants may constitute an insect disease; but I lay no stress upon it, and only mention the circumstance here as connected with the history of these animals. Mr. Dickson has described a _Sphæria_ under the name of _entomorhiza_ that grows upon _dead_ larvæ; it has a slender long stipes and spherical granulated head: on the pupa of a species of _Cicada_ in my cabinet, another kind of _Sphæria_, with a twisted thickish stipes and oblong head, springs up in the space between the eyes. I observed something similar but longer, in the grub of some large beetle in M. Du Fresne's museum at Paris; and I have a memorandum of having noticed something of the kind on the rostrum of a _Calandra_. Bees and humble-bees have been sometimes thought to have some species of _mucor_ or other _Fungilli_ occasionally growing upon them; but Mr. Brown is of opinion that _stamina_ which they have filched from flowers have been mistaken for these _Fungilli_, since he has detected those of _Orchideæ_ in some of this tribe, and upon a beetle shown to him by Mr. MacLeay, one which he knew to be the stamen of an _Aristolochia_. I once observed a bunch of what I mistook for a singular _mucor_ that adorned the vertex of a humble-bee, between the antennæ, which doubtless were of the same description; and I even saw one upon its wing. Upon a former occasion I mentioned a parallel circumstance with respect to a species of _Xylocopa_[944].

ii. The _animal_ parasites that infest insects are either themselves _insects_; or _worms_.

1. Their _insect_ infesters, as far as we know at present, are confined to the Orders _Strepsiptera_, _Hymenoptera_, _Diptera_, and _Aptera_: they attack them sometimes in their _egg_ state, most frequently when they are larvæ, occasionally when pupæ, and very rarely in their perfect state. Upon many of these I have formerly enlarged[945], and I shall now add such further circumstances as I then omitted. The _Strepsiptera_ Order, as at present known, consists only of two genera, _Stylops_ and _Xenos_; the first being appropriated to the imago of _Andrena_, a kind of _bee_, and the latter to that of the _wasps_. Their eggs appear to be deposited in the abdomen of these insects in which they feed, till having attained their full growth they perforate the membrane that connects its segments; and at the proper time their pupa-case bursts, they emerge, and take their flight. Sometimes four or five infest a single bee. Whether the latter dies upon their quitting it I have not been able to ascertain, but from their flying, when the little parasite is very near leaving them, with their usual activity, it should seem that this disease is not mortal; but it probably prevents their breeding: I do not recollect observing the exuviæ of one in a _male_ bee[946].

The great body of insect parasites, however, belong to the _Hymenoptera_ Order, and chiefly to the Linnean genus _Ichneumon_. The insects of this order have been denominated _Principes_, because of the wonderful instincts of ants, wasps, bees, and other gregarious tribes that belong to it; and they merit a name of honour not less for the benefits that they confer upon mankind, by keeping within their proper limits the various insect-destroyers of the produce of the globe. It deserves notice that when these latter increase to a degree to occasion alarm, their parasites are observed to increase in a much greater, so as to prevent the great majority of them from breeding[947]. Though these benefactors of the human race constitute numerous genera, at present not well ascertained, I shall speak of most of them under the common name of _Ichneumon_.

The appearance of these little four-winged flies puzzled much the earlier naturalists:--that a caterpillar usually turning to a _moth_ or _butterfly_ should give birth to myriads of _flies_, was one of those deep mysteries of nature which they knew not how to fathom[948]: even the penetrating genius of our great Ray, though he ultimately ascertained the real fact[949], was at one time here quite at fault; for he seems at first to have thought, when from any defect or weakness nature could not bring a caterpillar to a butterfly, in order that her aim might not be entirely defeated, that she stopped short, and formed them into more imperfect animals[950].

Before I detail more particularly the proceedings of Ichneumons, I shall make a few general remarks upon them. The structure of the instrument by which they are enabled to deposit their eggs in their appropriate station has been before sufficiently described[951]; it is long or short according to the situation and circumstances of the larva which receives them: if this lives in the open air, and the access to it is easy, it is usually _short_ and retracted within the body; but if it lies concealed in deep holes or cavities, or shuns all approach, it is often very long. Thus in _Pimpla Manifestator_, which commits its eggs to the grub of a wild bee inhabiting the bottom of deep holes bored in posts and rails, the ovipositor is nearly an inch and half in length, and in some extra-European species three inches. How the egg is propelled so as to pass in safety from the oviduct, along this extended and very slender instrument to the grub for which it is destined, has not been certainly ascertained: but from an observation of Reaumur's[952] it should seem that it is aided in its passage by some fluid ejected at the same time with it, or is so lubricated as to slide easily without being displaced. The flies we are speaking of, by some authors are called _Muscæ vibrantes_, because when searching for the destined nidus of their eggs their antennæ vibrate incessantly, and it is by the use of these wonderful organs that they discover it wherever it lurks. Bergman observed that _Fœnus Jaculator_ searches for the latent grub of certain bees and other _Hymenoptera_ with its antennæ[953]: and from Mr. Marsham we learn that _Pimpla Manifestator_, before it inserts its ovipositor in the nest of the grub of _Chelostoma maxillosa_, explores it first with one antenna and then with the other, plunging them all the while intensely quivering up to the very root[954]. With respect to their _size_, Ichneumons vary greatly; some being so extremely minute as to be invisible to the naked eye, unless moving upon glass; while others, as to their _length_, emulate the giants amongst insects. The former, unless appropriated to the eggs themselves, usually commit many eggs to a single larva, while the latter are directed by their instinct to introduce into them only one. Some of the former description are endowed with the faculty of leaping[955]. The food of Ichneumons, and indeed of other internal parasites, is chiefly the _epiploon_ or fat of the larva, but they never touch any vital organ; so that it continues to feed, and probably more voraciously, grow, cast its skin, and often it changes to a chrysalis, although at the same time inhabited by an army of these little devourers.

Ichneumons, as far as has been at present ascertained, are parasitic upon other insects chiefly in their _three_ first states, a solitary instance only having been observed of their inhabiting an _imago_; but from their first exclusion as eggs from the ovary till their assumption of that state they give them no rest. I shall therefore first treat of those that infest the _eggs_; next those appropriated to _larvæ_; and lastly those that devour _pupæ_.

Vallisnieri appears to have been the first naturalist who discovered that Ichneumons were appropriated to the _eggs_ of other insects. He observed one proceed from those of the emperor-moth (_Saturnia Spini_): finding two holes in each egg, one larger than the other, he conjectured that one was made when it entered, and the other when it emerged. In this case the egg of the Ichneumon must be fixed on the outside of the egg it was to feed upon; though some appear to pierce it with their ovipositor, and consequently introduce their egg within: for he says afterwards; "I have seen with my own eyes a certain kind of wild flies deposit their eggs _upon_ other eggs, and _bore_ and pierce others with an aculeus--by which they have introduced the egg[956]." Count Zinanni, a correspondent of Reaumur's, saw an Ichneumon pierce the eggs with her ovipositor repeatedly; which in about fifteen days were filled with the pupa, and in six more produced the imago[957]. _I. Ovulorum_ L. is the only _known_ species of egg-devourers; but most likely there are many, varying in size, according to the size of the egg they inhabit. Probably _I. Atomus_ L., and _I. Punctum_ Shaw, are of this description[958]. It is wonderful what a number these little flies destroy:--out of a mass of more than _sixty_ eggs which was brought to De Geer, not _one_ had escaped the Ichneumon[959]. But the most extraordinary thing is, that even these little creatures we are told are destroyed by another still more minute[960].

Though the animals we are speaking of usually destroy only a _single_ egg, yet some appear not so to confine themselves. Geoffrey informs us that the larva of one of the Ichneumons whose females are without wings (_Cryptus_) devours the eggs of the nests of spiders, and from its size--it is nearly a quarter of an inch long--it must require several of them to bring it to maturity[961]. One of those also which destroys the gnat infesting the wheat (_I. inserens_) appears to devour them in their _egg_ state, and could not be brought to perfection by the food that a single one would furnish[962].

The Ichneumons that are parasitic upon _larvæ_ are the most numerous of all. Some of them are deposited by the parent fly on the _outside_ of their prey, and others introduced into its _interior_. _Ophion luteum_ is one of the former tribe; it plants its eggs in the skin of the caterpillar of the puss-moth (_Cerura Vinula_). Each egg is furnished with a footstalk terminating in a bulb[963], which is so deeply and firmly fixed that it is impossible to extract it without detaching a portion of the animal with it, and even when the caterpillar changes its skin it is not displaced. After it is hatched, the grub, while feeding, keeps its posterior extremity in the egg-shell, to which it adheres so pertinaciously, that it is scarcely possible to disengage it without crushing it. It fixes itself by its mandibles to the skin of the caterpillar, and keeps constantly sucking the contents of its body till it dies: sometimes nine or ten of these larvæ inhabit a single caterpillar[964]. Reaumur has given an account of other external Ichneumons. Upon one caterpillar that he examined, they were so numerous as to render the poor animal quite a spectacle, and they underwent their metamorphosis attached to it[965]. One species of this description avenges the cause of insects upon their most pitiless foes, the all-devouring spider--for in the midst of her toils and lines of circumvallation it makes her its prey. De Geer, meeting one day with a young spider of a common kind, observed with surprise, engaged in sucking it, a small white grub, which was firmly attached to the abdomen near the trunk. Putting it by in a glass, after some days he examined it again; when he observed that it had spun the outline of a vertical web, had stretched threads from the top to the bottom of the glass and from one side to the other, and had also spun the radii that meet in the centre, and this was all;--but what was remarkable, the larva that had fed upon it was suspended in the centre of this web, where it was engaged in spinning its own cocoon, while the spider, exhausted by this last effort, had fallen dead to the bottom of the glass. It cannot be asserted positively that this suspension of the larva of the Ichneumon in the centre of the web _always_ takes place; but if it does, as seems most probable, it shows that this little parasite is endowed with an instinct which causes it so to act upon the spider as may induce it to spin a web so nicely timed as to be sufficiently complete at the period of its death and of the change of the Ichneumon, for the latter to cast it down and assume its station[966].

But the great bulk of the parasitic Hymenopterous devourers of larvæ have their assigned station _within_ the body. As Entomologists in breeding insects have paid their principal attention to _Lepidoptera_, it necessarily follows that their Ichneumon infesters must be most generally known; but doubtless the larvæ of the other Orders are not wholly liberated from this scourge: they also require to be kept within due limits, and have their appropriate parasites. Some, however, in most of them have been detected: of which I shall now proceed to state to you the most interesting examples, beginning with the _Coleoptera_.

_Alysia Manducator_[967], remarkable for having mandibulæ that do not close, and toothed at the end, usually attends masses of dung, both of man and cattle, probably for the purpose of depositing its eggs in some of the _Coleopterous_ larvæ that inhabit it. Mr. Stephens, one of the most accurate observers as well as one of the best Entomologists of the present day, informs me that he once captured three specimens of _Timarcha tenebricosa_, from each of which forty or fifty minute Ichneumons emerged. An insect also of this Order, that is a great benefactor to mankind, as a destroyer of the plant-lice,--I mean the lady-bird (_Coccinella_), in its larva state is itself subject to the attack, as we learn from De Geer, of one of these small parasites[968]. He detected them also in that of two species of weevils: and in the pupa of some large grub of a beetle inhabiting the wood of the elm, perhaps that of the stag-beetle, he found the pupa of one of those Ichneumons that have an exserted ovipositor[969]. Doubtless, did we know their history, we should find that numberless species have their internal assailants belonging to this tribe.

_Orthopterous_ larvæ seem not to have been yet announced as affording a pabulum to these animals: but the late Dr. Arnold, whose tact for observation with regard to the manners and economy of insects has rendered his loss irreparable, discovered that the remarkable parasitic genus _Evania_ was appropriated to the all-devouring _Blatta_. Whether it attacked it in its egg or larva state I have not been informed. This little benefactor is here extremely rare, at least in the _country_; perhaps in _towns_, where the cock-roach abounds, it may be more common.

The observations of naturalists have chiefly been confined to the _Hemipterous_ genus _Aphis_; but these early attracted their notice. Leeuwenhoek has given a particular and entertaining account of the proceedings of _I. Aphidum_. As soon as the little flies approached their prey, they bent their abdomen, which is rather long, between their legs, so that the anus projected beyond the head; then with their ovipositor they pierced the body of the Aphis, at the same time carefully avoiding all contact with it in every other part: whenever they succeeded in their attempt, a tremulous motion of the abdomen succeeded. Only a single egg is committed to one Aphis: when hatched, the latter becomes very smooth and appears swelled; it is, however, full of life, and moves when touched. Those that are thus pricked separate themselves from their sound companions, and take their station on the _underside_ of a leaf. After some days the inclosed grub pierces the belly of the Aphis, and attaches the margin of the orifice to the leaf by silken threads; upon this it dies, becomes white, and resembles a brilliant bead or pearl[970]. De Geer observed also an Ichneumon on the Coccus of the elm, _I. Coccorum_[971].

Amongst the _Neuropterous_ tribes likewise, probably the _Ichneumonidæ_ commit their usual ravages; but their exploits, as far as I recollect, have met with no historian. I have a small species related to _Chelonus_, which a memorandum made when I took it tells me was obtained from _Æshna viatica_; yet I do not remember ever tracing that species to its final change, so that I must have taken this Ichneumon from the _perfect insect_. It suffices, however, to prove that this tribe is also exposed to the attack of these parasites. Where larvæ and pupæ are _aquatic_, it seems probable, if any attack is made upon them, that it must take place after they have quitted the water.