Part 18

In the _Hymenoptera_ Order itself, almost every genus has been ascertained to have its Ichneumon parasites. Not even the fortified habitations of the gall-flies (_Cynips_) can escape them, almost every species becoming their prey; a circumstance which puzzled not a little some of the older naturalists, when they at one time saw a fly not remarkable for its colours or brilliancy emerge from the curious moss-like _Bedeguar_ of the wild rose, and at another were struck by the appearance of one of those splendid minims of nature which almost dazzle the sight of the beholder[972]. Immunity, however, from this pest seems to have been granted to the _gregarious Hymenoptera_; at least none has yet been discovered to attack the ant, the wasp, the humble-bee, or the hive-bee; in which last, had there been one appropriated to it, it could never have escaped the notice of the Reaumurs and the Hubers. The _solitary_ bees, however, as we have seen above[973], do not escape; and _Epipona spinipes_, a _solitary_ wasp which feeds its own young with a number of green caterpillars[974], is itself, when a larva, though concealed in a deep burrow, the prey of the grub of an Ichneumon, which by means of a long ovipositor introduces its egg into its body[975]. Even these parasites, whose universal office it is in their first state to prey upon insects, are themselves subject to the same malady. Ichneumonidan devourers are kept in check by other Ichneumonidan devourers. These in some cases are so numerous as to destroy the tithe of the kinds they attack[976]. Thus an ever-watchful PROVIDENCE prevents these parasites from becoming so numerous as to annihilate in any place the species necessary for the maintenance of the general economy and proportion of animal and vegetable productions. Amongst the assailants of the _Hymenoptera_, none seem to have a more laborious task assigned them than those that pierce the various galls in which the larvæ of the _Cynips_ tribe are inclosed. To look at an oak-apple, we should think it a work of difficulty, requiring much sagacity and address, for one of our little flies to discover the several chambers lurking in its womb, and to direct their ovipositor to each of them. Its CREATOR, however, has enabled it instinctively to discover this, and furnished it with an appropriate elongated instrument, which will open a way to the deep and hidden cells in which the grubs reside, penetrate their bodies, and to each commit an egg. When it prepares to perforate the gall, the Ichneumon begins by depressing this organ, that it may extricate it from its sheath; it next elevates its body as high as possible, and bending the instrument till it becomes perpendicular to the body and to the gall, so as to touch the latter with its point, it then gradually plunges it in, till it is quite buried[977]. A very remarkable Hymenopterous parasite (_Leucospis_), which when unemployed turns its ovipositor over the back of its abdomen, so that its end points to its head, is said to deposit its eggs in the nest of the mason-bee, most probably in the larva: but the curious observations that are stated to have been made by M. Amédée Lepelletier upon its history have not yet been given to the public[978].

_Dipterous_ insects, likewise, do not escape from these pests of their Class: but few observations, however, have been recorded as to the species assailed by them. We learn from De Geer, that a gnat (_Cecidomyia Juniperi_), which forms galls upon the juniper is devoured by an external Ichneumon[979]; that which injures the wheat in the ear, whose ravages I formerly mentioned to you[980], affords food to three of these parasites,--one I lately mentioned as probably devouring its eggs; another pierces the glumes of the floret, where its destined prey is concealed; and the third enters it. I once placed a number of the larvæ of the gnat upon a sheet of paper, at no great distance from each other, and then set down one of these last Ichneumons in the midst of them. She began immediately to pace about, vibrating her antennæ very briskly: a larva was soon discovered, upon which she fixed herself, the motion of her antennæ increasing intensely; then bending her abdomen obliquely under her breast, she inserted her ovipositor, and while the egg was depositing these organs became perfectly motionless. The larva when pricked gave a violent wriggle. This operation was repeated with all that had not already received an egg, for only one is committed to each larva. I have often seen it mount one that was already pricked, but it soon discovered its mistake, and quitted it untouched[981]. The Hessian Fly also (_Cecidomyia Destructor_) related to the preceding, whose alarming ravages I formerly described to you[982], has a peculiar parasite attached to it, which keeps it in check. The only other Dipterous insects that I have seen mentioned as affording pabulum to an Ichneumon, are--one of the aphidivorous flies mentioned by De Geer, who does not note the species, to the larva of which the Ichneumon commits only a single egg, producing a grub that intirely devours its interior[983];--and two described by Scopoli, one, the larva of a _fly_ frequenting hemp; and the other, which feeds on a _Boletus_, that of a _gnat_[984].

The _Lepidoptera_, however, is the Order over the larvæ of which the Ichneumons reign with undisputed sway; attacking all indiscriminately, from the minute one that forms its labyrinth within the thickness of a leaf, to the giant caterpillar of the hawk-moth. The most useful of all, however, the silkworm, appears at least with us, exempted from this scourge. De Geer, out of fifteen larvæ that were mining between the two cuticles of a rose-leaf, belonging to the first tribe here alluded to, found that _fourteen_ were destroyed by one of these parasites, only one coming forth to display itself in all its brilliancy and miniature magnificence[985]. One of the most useful to us is that which destroys the clothes-moth, which the same writer also traced[986]. Another, equally serviceable, takes up its abode in the caterpillar that ravages our cabbages and brocoli (_Pontia Brassicæ_) which perish by hundreds from its attacks. As this falls frequently under our notice, it will not be uninteresting to give a fuller account of it. Reaumur has traced and related its whole history. One of these little flies that he observed, was so intent upon the business in which she was engaged, that she suffered him to watch her motions under a lens, without being discomposed. She pursued nearly the same plan of proceeding with that of the Ichneumon of the wheat-gnat just described; except that she repeated her operations frequently on the same caterpillar in different parts, alternately plunging in and extracting her ovipositor. She seemed to prefer the spot where the segments of the body are united, particularly where the eighth meets the ninth, and the ninth the tenth. When the fly had completed its work and quitted the caterpillar, Reaumur gave it food, and it did not seem less lively and vigorous than others of its kind; in less than a fortnight it assumed the pupa; and in four days the whole of its interior being devoured, it died: but its parasites, perhaps not finding a sufficient supply of nutriment in it, never came to perfection[987]. Sometimes, however, these little grubs arrive at maturity before the caterpillar has become a chrysalis, when they pierce the skin and begin to emerge. First appears a little white tubercle, which gradually elevates itself in a direction perpendicular to the body; while this is doing, a second appears in another place; and so on, till fifteen or sixteen are seen on each side, giving the caterpillar a very grotesque appearance. By the alternate contraction and relaxation of their bodies the grubs effect their complete liberation, which takes place with respect to the whole in less than half an hour. When entirely disengaged, they place themselves close to the sides of the caterpillar: even before this they begin spinning, and draw unequal threads in different directions, of which they form a cottony bed, which serves as the base of the separate cocoon of each individual, which they next construct of a beautiful silk thread of a lovely yellow, which, if it could be unwound and in sufficient quantity, would yield a silk unrivalled in lustre and fineness[988].

De Geer has recorded a very singular fact which deserves your notice. An Ichneumon, appropriated to one of the _Tortrices_, had deposited its eggs in two of their caterpillars; each produced a considerable number; but those that emerged from one were all _females_, and those from the other, _males_[989]. He observed a similar fact take place with _Misocampus Puparum_[990]. One might conjecture from this circumstance, that as in the queen-bee[991], so in these Ichneumons, the eggs producing the two sexes were arranged separately in the ovaries. Reaumur has related, that in one instance three or four males were produced to one female; and in another four or five females to one male[992].

But though the great majority of insects are subject to this _Scolechiasis_[993] in their larva state, yet sometimes they are not attacked by the _Ichneumon_ till they have become _pupæ_. Of this kind is one just mentioned (_M. Puparum_), which commits its eggs to the chrysalis of the butterfly of the nettle, _Vanessa Urticæ_: the moment this caterpillar quits its skin to assume that state, while it is yet soft they pierce it and confide to it their eggs[994]. De Geer and others have supposed that this same Ichneumon attacks the _Cocci_ and _Coccinellæ_[995]; but this probably is an erroneous supposition. _Cryptus Compunctor_ also attacks the pupæ of butterflies[996].

If we consider the great purpose of PROVIDENCE in giving being to this tribe of destroyers--the keeping of insects within their proper limits,--we may readily conceive that this purpose is more effectually answered by destroying them in their _preparatory_ than in their _ultimate_ state, since at that time the laying of their eggs and a future progeny could not so effectually be prevented;--this will account for there being few or no Ichneumons appropriated to them in their latter state.

The next tribe of insect parasites are to be found in the _Diptera_ Order. The species that has been particularly noticed as such is the _Tachina Larvarum_; its larva is polyphagous, laying its eggs _upon_ the bodies of caterpillars of different kinds. Sometimes a pair is placed on the first segment, sometimes on the head itself, and sometimes near the anus. These eggs are very hard, convex, of an oval figure, polished and shining like a mirror. They are fixed so firmly that if you attempt to remove them with a penknife the skin comes off with them. When hatched, they enter the body and feed on the interior, and, undergoing their metamorphosis within it, do not emerge till they enter their perfect state. The caterpillar thus attacked lives long enough to spin its cocoon, when it dies[997]. Sometimes, however, these animals quit their prey sooner. Reaumur saw a grub of one of the _Muscidæ_ come out of a caterpillar, and then become a pupa, which was so large that he wondered how it could have been contained in the animal it had quitted[998].

We have now done with those parasites that produce in insects the disease I have called _Scolechiasis_[999]: the rest, which belong to the _Aptera_ Order, will afford us examples both of _Phthiriasis_ and _Acariasis_[1000].

I begin with the _first_. Mr. Sheppard once brought me a specimen of a bird-louse (_Nirmus_) which he took upon a butterfly (_Vanessa Io_): and should such a capture be more than once repeated, it would afford a _certain_ instance of the _first_ of these diseases amongst insects;--but most probably the specimen in question had dropped from some bird upon the butterfly. The only remaining animal belonging to the apterous hexapods that is parasitic on insects, is by many supposed to be the larva of a giant-beetle (_Meloe Proscarabæus_). I have before alluded to this animal[1001], and shall now resume the subject. Gœdart, Frisch, and De Geer, observed that it deposited in the earth one or two considerable masses, containing an infinite number of very minute orange-coloured eggs adhering to each other, which in about a month were hatched, and produced a number of small hexapods distinguished by two pairs of anal setæ and a proleg, by means of which they could move readily upon glass, as I have myself seen: these little animals precisely corresponded with one found by the latter author upon _Eristalis intricarius_; and when that fly was placed amongst them, they immediately attached themselves to it, so as to leave no doubt of their identity[1002]. A congenerous species had been detected upon wild bees, and described by Linné under the name of _Pediculus Apis_. De Geer is so thoroughly to be depended upon for his veracity and accuracy of observation, that we cannot suppose there is any incorrectness in his statement. If the mass of eggs be, as he represents it, of the size of a hazel-nut, it must have been the product of a very large insect: in confirmation of this opinion it may be further observed, that the larva of the kindred genus _Cantharis_ agrees with it in having anal setæ, though it appears to differ in having only two conspicuous segments in the trunk[1003]. Those which infest wild bees make their first appearance upon _acrid_ plants, which the _Meloe_ likewise feeds upon; from whence with wonderful agility they leap upon the _Andrenæ_, &c. that visit these flowers. Strong, however, as all these facts appear, still we cannot help exclaiming with the illustrious Swede last named, Who could ever have imagined that the larva of this great _beetle_ would be found upon the body of _flies_,--and we may add, or _bees_? Who could ever imagine that it would feed like a _bird-louse_ and resemble it so closely? that in the insertion of its palpi it should exhibit a character _exclusively_ belonging to that tribe[1004]? Another circumstance seems to indicate that these hexapods at the time that they take their station in bees or flies are perfect insects--they do not vary in size, at least not materially. Where, we may also ask, if they are to become large beetles, where do they take their principal growth? It cannot be as parasites on the little bees or flies that they are usually found upon; they must soon desert them, and like their kindred blister-beetles, as is most probable, have recourse to vegetable food. What an anomaly _in rerum natura_! It is much to be wished that some skilful insect-anatomist would carefully dissect the _Meloe_; or perhaps by digging round the roots of the ranunculuses and other acrid plants the larva of that beetle might be discovered in a later stage of growth, and so this mystery be cleared up. I should observe here, that Scopoli has described three parasites as _Pediculi_; viz. _P. rostratus_, _coccineus_, and _Cerambycinus_; the first of which Fabricius has adopted under the name of _P. Gryllotalpæ_, but which are all evidently hexapod _Acarina_[1005].

_Acariasis_ seems a disease almost as universal amongst insects as _Scolechiasis_; with this difference however, that _Acari_ most commonly take their station upon them in their _perfect_ state. You have doubtless often observed the common dung-beetles (_Geotrupes_) covered on the underside of their body with small mites (_Gamasus Coleoptratorum_) which look as if they were engaged in suction--they are often so numerous that no part is uncovered; they also attack other beetles[1006], and are sometimes found on humble-bees. They are easily disturbed, run with great swiftness, and may often be seen in hotbeds and fermenting dung prowling in search of the stercorarious beetles. But the most remarkable insect of this kind is the _Uropoda vegetans_: it derives its nutriment from the insects it assails not by its _mouth_, but by means of a long _anal pedicle_ by which it is attached to them. De Geer found these in such numbers upon a species of _Leptura_, that its whole body was almost covered with them; they hung from the legs and antennæ in bunches, and gave the animal a most hideous and disgusting appearance. Under this load of vermin it could scarcely walk or move, and all its efforts to get rid of them were in vain: many were attached to its body and to each other by their anal pedicles, but others had cast them off and were walking about. When put into a glass with earth, they began to abandon their prey, so that in a few days it was quite freed from its plagues. He found that these parasites lived long in alcohol[1007].

If you inquire--How are these mites originally fixed by their pedicles? it seems most probable, that as the _Hemerobii_, when they lay their eggs, know how to place them upon a kind of footstalk, so the parent _Uropoda_ has the same power; and this pedicle appears to act the part of an umbilical chord, conveying nutriment to the fœtus not from a _placenta_, but from the body of the insect to which it is attached; till having thus attained a certain maturity of growth and structure, it disengages itself and becomes locomotive. Many eggs of the aquatic _Acarina_ (_Hydrachna_, &c.) are also furnished with a short pedicle by which they are fixed to _Dytisci_ and other water insects. De Geer found some of this description on the underside of the water-scorpion, so thickly set as to leave no void space: they were oval, of a very bright red, and of different sizes on different individuals; whence it was evident that they grow when thus fixed: when hatched or released--for perhaps they may be regarded as fœtuses in their _amnios_ rather than eggs--they cease to be parasitical. Let us admire on this occasion, (piously observes this great Entomologist,) the different and infinitely varied means by which the AUTHOR of Nature has endowed animals, particularly insects, for their propagation and preservation: for it is a most extraordinary sight to see eggs grow, and pump as it were their nutriment from the body of another living animal[1008]. As these mites are fixed to the _crust_ as well as its inosculations, they must have some means of forcing their nutriment through its pores.

Another insect, remarkable for its resemblance in some respects to the scorpion--called in this country the book-crab (_Chelifer cancroides_), from its being sometimes found in books--occasionally is parasitic upon flies, especially the common blue-bottle-fly (_Musca vomitoria_). They adhere to it very pertinaciously under the wings; and if you attempt to disturb them, they run backwards, forwards, or sideways, with equal facility.

Spiders also are infested by mites. Mr. Briggs once found a very small _Theridion_, to the thorax of which were attached four oblong bright scarlet mites, each of which was as large as the thorax itself. He afterwards met with another spider still smaller, attacked by two of these swoln parasites, one of which appeared to him nearly equal to the spider in size. This mite was probably either _Leptus Phalangii_, or _Astoma parasiticum_.

2. We now come to a perfectly distinct tribe of insect parasites, which belong to that section or order of intestinal _worms_ which Rudolph has denominated _Entozoa nematoidea_, and Lamarck _Vers rigidules_[1009]. To this tribe belong the _Gordius_ of Linné and the _Filaria_ of modern zoologists, which from the experiments and observations of De Geer, Dr. Matthey, &c. appear to have been too hastily separated, being really congenerous, and living indifferently in water and in the intestines of insects and other animals[1010]. To this genus belong the guinea-worm (_Gordius medinensis_[1011]), the _Furia infernalis_, and several others that are found in various vertebrate animals. These little worms have been discovered in insects of almost every Order; and their attack generally produces the death of the animal, though they appear not to devour those parts that are essential to life[1012]. I once took a specimen of _Pœcilus azureus_, and upon immersing it in boiling water I was surprised to see what at first I mistook for an intestine, thrust itself forth; but upon a nearer inspection, to my great surprise I found it was one of these worms, thicker than a horse-hair and of a brown colour. Mr. W. S. MacLeay also once found one in _Abax Striola_. It still remains in my specimen, making it appear as if it had a long tail. De Geer long ago found these worms in grasshoppers[1012]; but Dr. Matthey has given the fullest account of one which infested _Acrida viridissima_. A friend of his noticing one of these insects which had not strength enough to leap and could scarcely even walk, being struck with the circumstance, caught the animal, upon which its hind legs were immediately detached from it. His surprise was greatly increased when he saw issue from its body a cylindrical worm about two feet and a half in length. Upon being called, Dr. M. soon recognised it for a _Gordius_ or _Filaria_; and on his putting it into water, it moved in it with great velocity, twisting its long and slender body in all directions. Upon opening the body of the grasshopper, nothing appeared within it but the intestine shrunk up to a thread. A few days after, another was brought, which appeared in full vigour, but its abdomen was enormously distended, and from it another worm was extracted, which remained without motion rolled in a spiral direction: intending to preserve this in spirits of wine--as it had become flat he first immersed it in water, that it might recover if possible its cylindrical form. Upon immersion a movement took place in the animal, and it gradually recovered its plumpness; but it still remained without motion, as if dead, for nearly five days, when another living specimen being brought and placed with it, as soon as water was poured on them, the seemingly dead one began to show by a slight oscillation in its extremities that life was not extinct in it. Fresh water being poured upon it, at the end of the day it had recovered all its strength and agility. He afterwards often repeated the same experiment with a similar result[1013]. From this account it appears that the _Gordius_ or _Filaria_ has a property resembling that of the _Vibrio Tritici_, so well described and so beautifully figured by M. Bauer[1014], of apparently dying and being resuscitated by immersion in water. How long it can retain this property remains to be ascertained.