Fabre has sketched the habits of a species of Eumenes, probably E. pomiformis. This Eumenes constructs with clay a small vase-like earthenware vessel, in the walls of which small stones are embedded (like Fig. 28, B). This it fills with food for the young. The food consists of caterpillars to the number of fourteen or sixteen for each nest. These caterpillars are believed to be stung by the parent-wasp (as is the case in the fossorial Hymenoptera), but complete evidence of this does not seem to be extant, and if it be so, the stinging does not completely deprive the caterpillars of the capacity of movement, for they possess the power of using their mandibles and of making strokes, or kicking with the posterior part of the body. It is clear that if the delicate egg of the Eumenes or the delicate larva that issues from it were placed in the midst of a mass of this kind, it would probably suffer destruction; therefore, to prevent this, the egg is not placed among the caterpillars, but is suspended from the dome covering the nest by a delicate thread rivalling in fineness the web of the spider, and being above the mass of food it is safe. When the young larva leaves the egg it still makes use of the shell as its habitation, and eats its first meals from the vantage-point of this suspension; although the mass of the food grows less by consumption, the little larva is still enabled to reach it by the fact that the egg-shell splits up to a sort of ribbon, and thus adds to the length of the suspensory thread, of which it is the terminal portion. Finally the heap of caterpillars shrinks so much that it cannot be reached by the larva even with the aid of the augmented length of the suspensory thread; by this time, however, the little creature has so much increased in size and strength that it is able to take its place amongst the food without danger of being crushed by the mass, and it afterwards completes its metamorphosis in the usual manner.
Fig. 28—Nidification of solitary wasps: section through nest, A, of Odynerus reniformis; B, of Eumenes arbustorum. a, The suspended egg of the wasp; b, the stored caterpillars. (After André.)
It is known that other species of Eumenes construct vase-like nests; E. unguiculata, however, according to an imperfect account given by Perris, makes with earth a closed nest of irregular shape, containing three cells in one mass. The saliva of these builders has the power of acting as a cement, and of forming with the clay a very impenetrable material. One species, E. coarctata, L. of this genus occurs in Britain. The clay nests (Fig. 29) of this Insect are often attached to the twigs of shrubs, while those of the two species previously mentioned are usually placed on objects that offer a large surface for fixing the foundations to, such as walls. According to Goureau the larva of this species forms in one corner of its little abode, separated by a partition, a sort of dust-heap in which it accumulates the various débris resulting from the consumption of its stores.
Eumenes conica, according to Horne, constructs in Hindostan clay-nests with very delicate walls. This species provisions its nest with ten or twelve green caterpillars; on one occasion this observer took from one cell eight green caterpillars and one black. It is much attacked by parasites owing, it is thought, to the delicacy of the walls of the cells, which are easily pierced; from one group of five cells two specimens only of the Eumenes were reared.
Fig. 29—Nest of Eumenes coarctata: A, the nest attached to wood; B, detached, showing the larva. a, the larva; b, the partition of the cell. (After André.)
Odynerus, with numerous sub-genera, the names of which are often used as those of distinct genera, includes the larger part of the solitary wasps; it is very widely distributed over the earth, and is represented by many peculiar species even in the isolated Archipelago of Hawaii; in Britain we have about fifteen species of the genus. The Odynerus are less accomplished architects than the species of Eumenes, and usually play the more humble parts of adapters and repairers; they live either in holes in walls, or in posts or other woodwork, or in burrows in the earth, or in stems of plants. Several species of the sub-genus Hoplopus have the remarkable habit of constructing burrows in sandy ground, and forming at their entry a curvate, freely projecting tube placed at right angles to the main burrow, and formed of the grains of sand brought out by the Insect during excavation and cemented together. The habits of one such species were described by Réaumur, of another by Dufour; and recently Fabre has added to the accounts of these naturalists some important information drawn from his own observations on O. reniformis.
Fig. 30.—Odynerus antilope ♀. Britain.