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.
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].