This Insect provisions its cell with small caterpillars to the number of twenty or upwards (Fig. 28, A.) The egg is deposited before the nest is stocked with food; it is suspended in such a manner that the suspensory thread allows the egg to reach well down towards the bottom of the cell. The caterpillars placed as food in the nest are all curled up, each forming a ring approximately adapted to the calibre of the cell. Fabre believes these caterpillars to be partly stupefied by stinging, but the act has not been observed either by himself, Réaumur, or Dufour. The first caterpillar is eaten by the wasp-larva from its point of suspension; after this first meal has been made the larva is supposed to undergo a change of skin; it then abandons the assistance of the suspensory thread, taking up a position in the vacant chamber at the end of the cell and drawing the caterpillars to itself one by one. This arrangement permits the caterpillars to be consumed in the order in which they were placed in the cell, so that the one that is weakest on account of its longer period of starvation is first devoured. Fabre thinks all the above points are essential to the successful development of this wasp-larva, the suspension protecting the egg and the young larva from destruction by pressure or movement of the caterpillars, while the position of the larva when it leaves the thread and takes its place on the floor of the cell ensures its consuming the food in the order of introduction; besides this the caterpillars used are of a proper size and of a species the individuals of which have the habit of rolling themselves up in a ring; while, as the calibre of the tube is but small, they are unable to straighten themselves and move about, so that their consumption in proper order is assured. Some interesting points in the habits of an allied species, O. (Pterocheilus) spinipes have been observed by Verhoeff; the facts as regards the construction and provisioning of the cell are almost the same as in O. reniformis. The species of Odynerus are very subject to the attacks of parasites, and are, it is well known, destroyed to an enormous extent by Chrysididae. Verhoeff says that the wasp in question supplied food much infested by entoparasites; further, that a fly, Argyromoeba sinuata, takes advantage of the habit of the Odynerus of leaving its nest open during the process of provisioning, and deposits also an egg in the nest; the Odynerus seems, however, to have no power of discovering the fact, or more probably has no knowledge of its meaning, and so concludes the work of closing the cell in the usual way; the egg of the Argyromoeba hatches, and the maggot produced feeds on the caterpillars the wasp intended for its own offspring. Verhoeff observed that the egg of the wasp-larva is destroyed, but he does not know whether this was done by the mother Argyromoeba or by the larva hatched from her egg. Fabre's observations on allied species of Diptera render it, however, highly probable that the destruction is effected by the young fly-larva and not by the mother-fly.
Mr. R. C. L. Perkins once observed several individuals of our British O. callosus forming their nests in a clay bank, and provisioning them with larvae, nearly all of which were parasitised, and that to such an extent as to be evident both to the eye and the touch. In a few days after the wasps' eggs were laid, swarms of the minute parasites emerged and left no food for the Odynerus. Curiously, as it would seem, certain of the parasitised and stored-up larvae attempted (as parasitised larvae not infrequently do), to pupate. From which, as Mr. Perkins remarks, we may infer that (owing to distortion) the act of paralysing by the wasp had been ineffectual. Mr. Perkins has also observed that some of the numerous species of Hawaiian Odynerus make a single mud-cell, very like the pot of an Eumenes, but cylindrical instead of spherical. This little vessel is often placed in a leaf that a spider curls up; young molluscs of the genus Achatinella also avail themselves of this shelter, so that a curious colony is formed, consisting of the Odynerus in its pot, of masses of the young spiders, and of the little molluscs.
Horne has recorded that the East Indian O. punctum is fond of availing itself of holes in door-posts where large screws have been; after the hole has been filled with provisions, the orifice is covered over level with the surface of the wood so that it eludes human observation. It is nevertheless discovered by an Ichneumon-fly which pierces the covering with its ovipositor and deposits an egg within.
The genus Abispa is peculiar to Australia and includes some very fine solitary wasps, having somewhat the appearance of very large Odynerus: these Insects construct a beautiful nest with a projecting funnel-shaped entrance, and of so large a size that it might pass for the habitation of a colony of social wasps; it appears, however, that this large nest is really formed by a single female.
The species of the genus Rhygchium are also of insecticide habits, and appear to prefer the stems of pithy plants as the nidus for the development of the generation that is to follow them. Lichtenstein says that a female of the European R. oculatum forms fifteen to twenty cells in such a situation, and destroys 150 to 200 caterpillars, and he suggests that, as it is easy to encourage these wasps to nest in a suitable spot, we should utilise them to free our gardens from caterpillars, as we do cats to clear the mice from our apartments.
The East Indian R. carnaticum seems to have very similar habits to its European congener, adapting for its use the hollow stems of bamboos. Horne has recorded a case in which a female of this species took possession of a stem in which a bee, Megachile lanata, had already constructed two cells; it first formed a partition of mud over the spot occupied by the bee, this partition being similar to that which it makes use of for separating the spaces intended for its own young. This species stores caterpillars for the benefit of its larvae, and this is also the case with another Eastern species, R. nitidulum. This latter Insect, however, does not nidificate in the stems of plants, but constructs clay cells similar to those of Eumenes, and fixes them firmly to wood. Rhygchium brunneum is said by Sir Richard Owen to obliterate hieroglyphic inscriptions in Egypt by its habit of building mud nests amongst them. An individual of this wasp was found by Dr. Birch when unrolling a mummy—"There being every reason to believe that the Insect had remained in the position in which it was found ever since the last rites were paid to the ancient Egyptian."
Fam. 2. Vespidae—Social Wasps.
Claws of the feet simple, neither toothed nor bifid, middle tibiae with two spurs at the tip. Insects living in societies, forming a common dwelling of a papery or card-like material; each generation consists of males and females and of workers—imperfect females—that assist the reproductive female by carrying on the industrial occupations.
The anterior wing possesses four submarginal cells, as in the Eumenidae. The attention of entomologists has been more directed to the habits and architecture than to the taxonomy of these Insects, so that the external structure of the Insects themselves has not been so minutely or extensively scrutinised as is desirable; de Saussure, the most important authority, bases his classification of the Insects themselves on the nature of the nests they form. These habitations consist of an envelope, protecting cells similar in form to the comb of the honey-bee, but there is this important difference between the two, that while the bee forms its comb of wax that it secretes, the wasps make use of paper or card that they form from fragments of vegetable tissue,—more particularly woody fibre—amalgamated by means of cement secreted by glands; the vegetable fragments are obtained by means of the mandibles, the front legs playing a much less important part in the economy of the Vespidæ than they do in that of the bees and fossorial Hymenoptera.
In most of the nests of Vespidæ the comb is placed in stages or stories one above the other, and separated by an intervening space, but in many cases there is only one mass of comb. It is the rule that, when the cells of the comb are only partially formed, eggs are deposited in them, and that the larva resulting from the egg is fed and tended by the mother, or by her assistants, the workers; as the larvae grow, the cells are increased in correspondence with the size of the larva; the subsequent metamorphosis to pupa and imago taking place in the cells after they have been entirely closed. The food supplied is of a varied nature according to the species, being either animal or vegetable, or both.