Fig. 39—Methoca ichneumonides. A, Male; B, female. Britain.
The Thynnides are by some entomologists not separated from the Mutillides; but the distinction in the structure of the thorax of the females is very striking. In the Thynnides the nervuration of the wing appears always to extend to the outer margin, and in the Mutillides not to do so. This family is represented in Britain by a single very rare Insect, Methoca ichneumonides: to the unskilled observer the female would appear to be without doubt an ant. This Insect is by some considered as the type of a family distinct from the Thynnides proper. Thynnides are numerous in Australia. Very little is really known as to their habits, though it has been stated that they are parasitic on Lepidoptera, Bakewell having obtained specimens from subterranean cocoons of that Order. Those who are interested in differences between the sexes of one species should examine the extraordinary examples of that phenomenon presented by the Thynnides; the dissimilarity throughout the group—which is now of considerable extent—being so extreme that no entomologist would from simple inspection believe the two sexes to have any connection; but the fact that they are so connected has been demonstrated beyond doubt. In very few cases, however, have the sexes been matched, so that at present males are no doubt standing in the lists of Hymenoptera as one species and their females as other species.
Sub-Fam. 3. Scoliides.—Pronotum reaching back to the tegulae; legs stout; intermediate tibiae with one apical spur; both sexes winged; the nervures not extending to the posterior (i.e. distal) margin.
This group includes some of the largest and most powerful of the Aculeate Hymenoptera. Its members are usually hairy Insects with thick legs, the colour being black, more or less variegated with bands or spots of red or yellow; the hind body is elongate, has only a very short pedicel, and in the male is usually terminated by three projecting spines. The pronotum is of variable dimensions, but its front angles are always co-adapted with the points of insertion of the front wings. The nervuration of the front wings is confined to the basal part, the extensive apical or outer area possessing no nervures. There is frequently a great difference in the size of the two sexes of the same species, the female being very much larger than the other sex. The larvae, so far as is known, devour those of Lamellicorn Coleoptera.
Fig. 40.—Scolia haemorrhoidalis ♀. Europe.
Fabre has investigated the habits of some of the species of Scoliides found in France, and has informed us that their means of subsistence consists of larvae of the larger Lamellicorn beetles, Cetonia, Oryctes, Anoxia, and Euchlora; these beetles belong to very different divisions of the Lamellicornia, but they have in common the fact that their larvae are of subterranean habits, living in the earth or in accumulations of débris in which there is a large proportion of vegetable matter or roots. The female Scolia penetrates into the ground in order to find the Lamellicorn larvae necessary as food for its progeny. Scolia bifasciata attacks the larvae of several species of Cetonia, and S. (Colpa) interrupta chooses the larvae of the chafers Anoxia villosa and A. matutinalis. The mother Scolia enters the ground in August or September, and having found a suitable larva stings it and deposits an egg on the ventral surface of the prey; the paralysed larva is left where it was found, no attempt being made to place it in a special receptacle. The egg is placed on the ventral surface, well behind the feet, under a mass of matter in the alimentary canal. Shortly after being hatched the young destroyer penetrates with its head the skin of the victim, and in this position commences to feed; it is necessary that it should obtain its food without killing the Cetonia larva, for it cannot prosper on decaying food, so that if the Cetonia larva die the Scolia larva likewise perishes; the latter, accordingly, does not withdraw its head from the interior of the victim, but remains always in the same position, as it grows larger extending its head forwards into the front part of the interior of its victim; the internal organs of the latter are consumed in a systematic order so as to delay bringing about its death till the last moment, and thus all the interior of the Cetonia larva is appropriated till nothing remains but an empty skin. By a series of experiments, Fabre showed how essential it is that this apparently revolting operation should be carried on with all details strictly en règle. If the head of the Scolia larva be taken out from the victim and applied to another part of the body of the Cetonia, the result is that it cannot eat; even if it be replaced in the original situation, after being taken away, it frequently happens that the Cetonia larva dies, its death involving also that of the destroyer. It is necessary, too, that the victim should be paralysed, for if an intact Cetonia larva be taken and bound down in such a position that it cannot move, and if a small orifice in its skin be made in the proper spot and a young Scolia larva be placed on it, the little parasite will avail itself of the opportunity and commence to feed on the larva provided for it, but the latter will speedily die, and the Scolia necessarily perishes with it. Thus both the paralysis of the victim and the special mode of eating are essential to the life of the Scolia. The operation of stinging the larva so as to produce the necessary paralysis, or rather insensibility, is a difficult one, and requires great skill and patience. The Cetonia larva is of large size, and must be pierced in one particular spot; in order to reach this the Scolia mounts on its victim, and is frequently dislodged by its struggles; sooner or later, however, the proper position is obtained by the wasp, and the larva is then stung in the exact spot necessary to allow the sting (and the poison introduced by it) to reach the most important of the nervous ganglia that control the movements of the body, this spot being, in the case of the Cetonia, the line of demarcation between the pro- and meso-thorax, on the middle line of the ventral surface of the body. The Scolia gives but one sting to the victim, and this it will not administer until it can do so exactly in the proper place. This practice of devouring the victim slowly, without killing it till all is eaten, is very widely spread in the Hymenoptera, and it is satisfactory to find that we may infer from Fabre's observations that it is not so horrible as it would at first appear; for it is probable that the stinging prevents decomposition of the victim, not by reason, as some have supposed, of the poison injected by the wasp having an antiseptic effect, but rather by means of destroying sensibility, so that the creature does not die from the pain, as it is believed it did in certain cases where Fabre induced the young Scolia larva to feed on a victim that had not been stung. We may here remark that very little exact information exists as to the operation of stinging. Fabre attaches great importance to the sting being inflicted on a nerve-ganglion. Whether a sting that did not reach this part might not have a sufficient effect appears, however, doubtful.[[46]]
A remarkable form of Scoliides, with wings of smaller size than usual and deeply divided, has been described by Saunders under the name Pseudomeria graeca. Still more remarkable is Komarovia victoriosa found in Central Asia; in this Insect the male retains the appearance of a slender, pallid Scolia, but the female differs totally in form, and has the peculiar wings so reduced in size as to be useless for flight.
Sub-Fam. 4. Sapygides.—Closely allied to the Scoliides, but possessing slender legs and antennae; also the first abdominal segment is less disconnected from the second, so that the outline is less interrupted; the eyes are deeply emarginate; the hind body is not spinose at the apex.