822 



ICHNEUMONIDjE. 



Having placed a number of the grubs of this mis- 

 chievous insect upon paper, he introduced a female 

 Ichneumon amongst them : she immediately began 

 to pace about, vibrating her antennae very briskly, 

 whereby she soon discovered one of the larvae, upon 

 which she immediately fixed herself, her antennae 

 vibrating very intensely ; then bending her abdomen 

 beneath her breast, she inserted her ovipositor, and 

 whilst the egg was depositing, the antennae became 

 perfectly motionless. The larvae when pricked gave 

 a violent wriggle ; this operation was repeated with 

 all which had not already received an egg, for only 

 one is committed to each larva. Mr. Kirby noticed 

 that when it mounted one which had already re- 

 ceived an egg.it soon discovered its error, and quitted 

 it untouched. (Linn. Trans., Vol. iv., p. 236.) 



In the preceding instances the larva of the Ichneu- 

 mon, when hatched, is solitary, but others are grega- 

 rious ; the same Ichneumon depositing several eggs 

 in the body of a single caterpillar ; this is particularly 

 the case with the little Ichneumons (Microgaster g/o- 

 meratus], which prey upon the caterpillars which so 

 often devour our cabbage, and which are the offspring 

 of the common white butterfly (Pontia rapce). These 

 caterpillars when full grown creep up the sides of 

 houses to the corners of window-frames, or such like 

 places, where they are ordinarily transformed to a 

 greenish chrysalis, which may often be seen sus- 

 pended against the wall ; but occasionally a great 

 number of small white grubs are observed to burst out 

 of these caterpillars previous to their becoming chry- 

 salides, (which operation, of course, causes the death 

 of the caterpillar) which grubs immediately encase 

 themselves in little oval silken cocoons attached to- 

 gether round the exuviae of their victim, and which 

 resemble a mass of yellow silk. Sometimes, instead 

 of bursting out of the caterpillar, they remain enclosed 

 in it until it undergoes its transformation to the chry- 

 salis state, out of which the Ichneumons burst forth in 

 their perfect state. This variation depends upon the 

 species of parasite as well as upon the growth of the 

 caterpillar at the period when the eggs of the Ichneu- 

 mon are deposited in its body ; and here we cannot 

 but admire the admirable instinct of these parasitic 

 grubs. Had they as soon as hatched attacked the 

 vital organs of the caterpillar, of course their supply 

 of food would have been destroyed, and they must be 

 left to perish, since they are not provided with feet. 

 Instead of doing this, however, they confine them- 

 selves to the fatty parts of the caterpillar ; and it is 

 not until these parasitic grubs have nearly attained 

 their full size, that they attack the vital organs of their 

 victim ; in order to escape from the body of which, 

 they pierce its skin, and immediately commence the 

 spinning of a cocoon, some bursting out on one side 

 and some on the other ; here they immediately com- 

 mence the spinning of a loose covering drawing 

 the silk out of the spinneret of their lower lip in the 

 same way as ordinary caterpillars ; these threads they 

 cross in every direction, forming of them a support 

 for their cocoons, which differ but slightly from those 

 of the silk-worm as regards their consistence, being 

 of a yellow or white colour, according to the species, 

 and so quickly do they complete this operation, that 

 in the course of half an hour each has encased itself 

 in its cocoon. The stems of grasses sometimes bear 

 the cocoons of these insects, and occasionally, but 

 very rarely, a small case is found in the interior of 

 bee-hives, from which an Ichneumon has been pro- 



duced, which has probably fed upon the caterpillars 

 which devour the wax (Galleria cereana). Sometimes 

 these cocoons are banded with different colours, which 

 Reaumur endeavoured to account for by supposing 

 that the nature of the food of the grub would influ- 

 ence the colour of the silk first spun ; but this would 

 not account sufficiently for the regular bands often to 

 be seen in the centre of the cocoon. Thus the first 

 silk spun by the Ichneumon la'rva, and which forms the 

 external envelopes, may be white ; and the second, 

 forming the inner coating, brown ; but if the larva 

 employs this second kind of silk in defending the two 

 ends and the middle of the cocoon, the parts not thus 

 thickened will remain white ; and this is what takes 

 place, as may be proved by stripping off the brown 

 part, which is much more solid than the other part 

 the silk thus employed is very fine and shining, which 

 gives the cocoons a polished appearance. Some 

 species do not, however, form cocoons ; but, on the 

 contrary, remain enclosed within the skin of their vic- 

 tim, which, of course, serves them for a coveriisjr 

 during the inactive period of their pupa state. 



Reaumur discovered a cocoon upon the oak, of a 

 remarkable construction ; its form indeed resembled 

 that of the others, and it had a white central bund, 

 but it was suspended by a long thread to a leaf or 

 twig; the most singular circumstance connected with 

 it however was, that when detached from the leaf, 

 and laid in a box or upon the hand, it leaped to a 

 considerable height, varying from half an inch to 

 three or four inches. This motion is effected by the 

 inclosed larva making a sudden spring' in the same 

 manner as we have described in our account of the 

 motions of the cheesehopper ; the sudden unfolding 

 of the body, bringing the head and tail into violent 

 contact with the inside of the cocoon, so as to impart 

 to it a motion operating upon the surface upon which 

 it is placed. But wherefore is it necessary that this 

 larva inclosed in a firm cocoon should possess this 

 faculty of leaping? We may indeed suppose with 

 Reaumur that as the ordinary situation of the cocoon 

 is to be suspended in the air, it may be necessary, in 

 order to regain its position, in case it should be dis- 

 placed by the wind and lodged on the surface of an 

 adjacent leaf. 



It is a circumstance of great interest in an econo- 

 mical view of natural history, that when any particular 

 species of insect detrimental to our vegetable produce 

 increases in numbers so as to be more than ordinarily 

 destructive, its parasi'tes are observed to increase in 

 a much greater degree, so as to prevent the too great 

 multiplication of the former. 



These insects are very ramble in size ; thus some 

 of the British species are amongst the largest of our 

 native hymenopterous insects, and are destined to 

 check the hawkmoths and other large lepidoptera ; 

 others, however, are so exceedingly minute as to be 

 isible only when moving upon a window against the 

 light. The latter, whose parasitic habits are identical 

 with the Ichneumons, belong systematically to the 

 family Proclotrupidee, and are appropriated to the 

 eggs of many insects. Thus Vallisnieri observed, " 

 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 ;" and Linnaeus has accordingly 

 given the name of Ichneumon ovulorum to one of these 

 species (or rather to two, since his descriptions are evi- 

 dently drawn from two distinct species) ; and out of 



