The hind body springs from the lower part of the propodeum; it is usually of slender form, and its segmentation is very conspicuous. The females bear an ovipositor, which differs greatly in length according to the species, and is known in the case of one species to attain a length six times that of the whole of the rest of the body.[[468]] The egg is deposited by some species on the skin, by others within the body of the victim; it varies much in form and colour, some eggs being stalked and of peculiar shape. The larvae issuing from the eggs are legless maggots with a delicate integument of pallid white or creamy colour. If the eggs are laid on the surface of the body, the resulting larvae (except in the cases of the external parasites) soon bore into the interior of their victim, and disappear therein. The changes that take place in the lifetime of the larvae have been studied in only a few cases; but if we can judge from Ratzeburg's history[[469]] of the changes that take place in Anomalon, they are of great interest. From observation of the differences existing amongst a great number of larvae of A. circumflexum he distinguished four stages. It is of course impossible to follow directly the growth of one individual, because it is concealed in the interior of the caterpillar in which it lives, and to open this involves the death of both caterpillar and Ichneumon-larva. The life history must therefore be constructed from a great number of separate observations; and it is not ascertained that the four instars described by Ratzeburg represent the number of moults of the larva that actually take place. He, however, entertained no doubt that all the forms he observed were stages in the development of one species. In the earliest stage, when only one millimetre in length and about as thick as a horse-hair, the larva is free in the interior of the caterpillar's body, and has a small head armed only with a pair of mandibles. There are, in addition to the head, thirteen segments, and the last of these is an elongate tail forming nearly one-half the length of the creature. No trace of tracheae can be discovered. In the second stage the larva is still free, an elongate tracheal tube exists, the tail has diminished to half the length, the head has become much larger, and rudimentary antennae of one joint are visible; possibly stigmata are present at this stage, though they cannot afterwards be detected. In the third stage (Fig. 361, C) the larva is encysted, the head is large, the parts of the mouth are all developed, the tracheal system is extensive, and the caudal termination of the body is quite short; notwithstanding the extensive development of the tracheal system, no stigmata can be found. In the fourth stage the larva is still encysted, the tail has disappeared, the head and mouth parts are reduced in size and development, and the creature has now the appearance of a normal larva. The changes to pupa and perfect Insect take place within the body of the victim, in some cases, if not usually, after it has undergone its metamorphosis into a chrysalis. Very little information is extant as to the duration of the various stages, but it appears to be the rule that only one generation appears annually, though in some cases there are pretty certainly two.
It is very difficult to observe the act of oviposition; the Ichneumon-flies usually decline to notice caterpillars with which they are placed in confinement. Ratzeburg thinks they will only attack caterpillars that are in a deficient state of health or vitality. Occasionally we may by a happy chance observe the act in Insects at large, and from the records of observers it may be deduced with tolerable certainty that the sense of sight takes no part in the operation. Ratzeburg relates that he saw a Pimpla alight on a leaf of Rhus and thrust its ovipositor through the leaf. On looking to the under-side of the leaf he found that a cocoon of Bombyx neustria was concealed there in such a position that it could not have been seen by the Ichneumon.
Fig. 362.—Thalessa lunator. Oviposition. (After Riley.)
Among the most remarkable of the Ichneumon-flies are the Insects of the genera Rhyssa and Thalessa. These fine Insects have an ovipositor three or four inches in length, and are parasitic on species of the family Siricidae, which, as we have previously described, live in solid wood. In order therefore to deposit the egg in a suitable place, the wood must be pierced by the Ichneumon. The ovipositor is not only of extreme length, but is also furnished with serrations on its apical part, so that it forms a very effective boring apparatus. It is brought into use by being bent on itself over the back of the Insect (Fig. 362), so as to bring the tip vertically down on to the wood, through which it is then forced by a series of efforts; the sheaths do not enter the wood. The egg is laid anywhere in the burrow of the Sirex; the young larva seeks its prey, and lives on it as an external parasite (Fig. 342, D). Erne, however, states[[470]] that the young larva of Rhyssa persuasoria enters its victim, and remains within the latter till its death occurs. This happens when the young Rhyssa is two or three lines in length, and it then makes its exit from the interior of the body and gradually eats it up. Should the larva it has attacked be of large size, it of itself affords sufficient food for the completion of the growth of the Rhyssa. Should the Rhyssa, however, have attacked a small larva, this does not furnish it with sufficient food, and it consequently dies without seeking another larva. Erne says, indeed, that it will not eat another if offered to it, so that in order to rear the Rhyssa in captivity, the victim it has first attacked must always be given to it. The same observer states that the Rhyssa larva is sometimes transported by the Sirex deep into the wood, so that when it has completed its metamorphoses the Ichneumon-fly may find itself buried in solid wood to a depth of about two inches. In that case it excavates the wood with its mandibles, and should it fail to gain the exterior after three days of work, it dies. In the case of Thalessa it is stated that it sometimes bores into wood where there are no larvae, but Riley thinks this erroneous; it is, on the other hand, certain that the Insect after penetrating the wood is frequently unable to withdraw the ovipositor, and consequently dies.
Packard has recorded,[[471]] without mentioning the species, the oviposition of an Ichneumon of which the egg is deposited externally. It was placed on the head of the caterpillar, and speedily hatched; the young larva at once bored through the prothoracic segment of the victim, the head of the latter then became swollen, and covered the opening into the prothorax, made by the parasite.
Fig. 363.—Young larva of Paniscus in position of feeding on the skin of Mamestra. (After Newport.) a, The egg-shell.
The history of an Ichneumon larva that feeds as an external parasite has been sketched by De Geer and Newport. The observations of the latter[[472]] refer to Paniscus virgatus; he found small, shining, black bodies attached to the skin of the larva of a moth, Mamestra pisi; these were the eggs of the Ichneumon. They are furnished with a short peduncle, which is implanted in the skin of the victim; the egg, according to De Geer, being retained more firmly by the peduncle subsequently swelling, so as to form two knobs. The hatching takes place by the egg-shell splitting longitudinally, while from the split protrudes the little head of the destroying larva. This becomes fixed to the caterpillar, from which the nutriment is to be drawn; the Paniscus larva does not, however, leave the egg-shell, but, on the contrary, becomes adherent to it, so that the parasite is in this manner fixed by the two ends to its victim. In fifteen days the parasite was full-grown, and had become half an inch in length. At first no tracheae were to be seen, but these were detected after the second day. Moulting took place three times, and in a peculiar manner, very different from that described by Ratzeburg as occurring in the internal parasites (which, he states, change their very delicate skin by detaching it in almost imperceptible fragments). In the external parasite the skin remains entire, and is shuffled down to the extremity of the body, but cannot be completely detached owing to the anchoring of the posterior part of the body to the caterpillar; the cast skins thus remain as envelopes to the posterior part of the body. Newport states that if the mouth of the parasite be detached, it usually cannot again seize hold of the victim, and consequently perishes. It is a curious fact that more eggs than one caterpillar can support are habitually placed on it, and some of the resulting larvae of necessity perish during the period of growth. Poulton, who has recently made some additional observations on the development of Paniscus,[[473]] says that if three larvae are close together, it is the middle one that perishes, and suggests that this is due to some simple physical condition. From Newport's account it may be gathered that the Mamestra retains sufficient vitality to form its cocoon, and that the Paniscus larvae likewise construct their own cocoons within that of the Mamestra. In the case of Paniscus cephalotes feeding on Dicranura vinula, Poulton relates that the latter died after the twelfth day of attack. The parasites, having relaxed their hold on the victim just previous to this event, then thrust their heads into the dead body, and devoured the larva, leaving only a dried and empty integument. These larvae span a loose sort of web in which to undergo their metamorphosis. In a natural state, however, they form cocoons inside the cocoon of the Dicranura. The period passed in the pupal condition was about four weeks. This parasite only attacks the Lepidopterous larva during the last stage of its existence as a larva, but the eggs may be laid on the victim in an earlier stage; and in such case De Geer has stated, and Poulton has confirmed the observation, that though the larva sheds its skin it does not get rid of the eggs.
The little Ichneumons of the genus Pezomachus are quite destitute of wings and somewhat resemble ants; they are common Insects in Britain. Only the female sex is known, and it is believed that the winged Ichneumons assigned to the genus Hemiteles—of which no females are known—are the males of Pezomachus. Repeated efforts have been made to place this beyond doubt, but they have usually failed, for when a brood of these parasites is reared, the individuals generally prove to be either all Hemiteles or all Pezomachus. It is to be hoped that this interesting case will be fully elucidated.