The species of the genus Cerceris are numerous in Europe, and several of them are known to make burrows in the ground, and store them with beetles for the benefit of the future larvae. The beetles chosen differ in family according to the species of Cerceris; but it appears from the observations of Fabre and Dufour that one kind of Cerceris never in its selection goes out of the limits of a particular family of beetles, but, curiously enough, will take Insects most dissimilar in form and colour provided they belong to the proper family. This choice, so wide in one direction and so limited in another, seems to point to the existence of some sense, of the nature of which we are unaware, that determines the selection made by the Insect. In the case of our British species of Cerceris, Smith observed C. arenaria carrying to its nest Curculionidae of very diverse forms; while C. labiata used a beetle—Haltica tabida—of the family Chrysomelidae.
Fig. 49.—Philanthus triangulum ♂. Britain.
The beetles, after being caught, are stung in the chief articulation of the body, that, namely, between the pro- and mesothorax. Cerceris bupresticida confines itself exclusively to beetles of the family Buprestidae. It was by observations on this Insect that Dufour first discovered the fact that the Insects stored up do not decay: he thought, however, that this was due to the liquid injected by the wasp exercising some antiseptic power; but the observations of Fabre have shown that the preservation in a fresh state is due to life not being extinguished; the stillness, almost as if of death, being due to the destruction of the functional activity of the nerve centres that govern the movements of the limbs.
It has long been known that some species of Cerceris prey on bees of the genus Halictus, and Marchal has recently described in detail the proceedings of C. ornata. This Insect catches a Halictus on the wing, and, holding its neck with the mandibles, bends her body beneath it, and paralyses it by a sting administered at the front articulation of the neck. The Halictus is subsequently more completely stunned or bruised by a process of kneading by means of the mandibles of the Cerceris. Marchal attaches great importance to this "malaxation"; indeed, he is of opinion that it takes as great a part in producing or prolonging the paralysis as the stinging does. Whether the malaxation would be sufficient of itself to produce the paralysis he could not decide, for it appears to be impossible to induce the Cerceris to undertake the kneading until after it has reduced the Halictus to quietude by stinging.
Fabre made some very interesting observations on Cerceris tuberculata, their object being to obtain some definite facts as to the power of these Insects to find their way home when removed to a distance. He captured twelve examples of the female, marked each individual on the thorax with a spot of white paint, placed it in a paper roll, and then put all the rolls, with their prisoners, in a box; in this they were removed to a distance of two kilometres from the home and then released. He visited the home five hours afterwards, and was speedily able to assure himself that at any rate four out of the twelve had returned to the spot from whence they had been transported, and he entertained no doubt that others he did not wait to capture had been equally successful in home-finding. He then commenced a second experiment by capturing nine examples, marking each with two spots on the thorax, and confining them in a dark box. They were then transported to the town of Carpentras, a distance of three kilometres, and released in the public street, "in the centre of a populous quarter," from their dark prison. Each Cerceris on being released rose vertically between the houses to a sufficient height, and then at once passed over the roofs in a southerly direction—the direction of home. After some hours he went back to the homes of the little wasps, but could not find that any of them had then returned; the next day he went again, and found that at any rate five of the Cerceris liberated the previous day were then at home. This record is of considerable interest owing to two facts, viz. that it is not considered that the Cerceris as a rule extends its range far from home, and that the specimens were liberated in a public street, and took the direction of home at once.
Philanthus apivorus is one of the best known of the members of this sub-family owing to its habit of using the domestic honey-bee as the food for its offspring. In many respects its habits resemble those of Cerceris ornata, except that the Philanthus apparently kills the bee at once, while in the case of the Cerceris, the Halictus it entombs does not perish for several days. The honey-bee, when attacked by the Philanthus, seems to be almost incapable of defending itself, for it appears to have no power of finding with its sting the weak places in the armour of its assailant. According to Fabre, it has no idea of the Philanthus being the enemy of its race, and associates with its destroyer on amicable terms previous to the attack being made on it. The Philanthus stings the bee on the under-surface of the mentum; afterwards the poor bee is subjected to a violent process of kneading, by which the honey is forced from it, and this the destroyer greedily imbibes. The bee is then carried to the nest of the Philanthus. This is a burrow in the ground; it is of unusual depth—about a yard according to Fabre—and at its termination are placed the cells for the reception of the young; in one of these cells the bee is placed, and an egg laid on it: as the food in this case is really dead, not merely in a state of anæsthesia, the Philanthus does not complete the store of food for its larvae all at once, but waits until the latter has consumed its first stock, and then the mother-wasp supplies a fresh store of food. In this case, therefore, as in Bembex, the mother really tends the offspring.
Sub-Fam. 9. Mimesides.—Small Insects with pedicellate hind body, the pedicel not cylindric; mandibles not excised externally; inner margin of eyes not concave; middle tibia with one spur; wings with two, or three, submarginal cells.
Mimesides is here considered to include the Pemphredonides of some authors. Mimesides proper comprises but few forms, and those known are small Insects. Psen concolor and P. atratus form their nests in hollow stems, and the former provisions its nest with Homopterous Insects of the family Psyllidae. Little information exists as to their habits; but Verhoeff states that the species of Psen—like members of the Pemphredoninae—do not form cocoons.
The Pemphredonine subdivision includes numerous small and obscure Insects found chiefly in Europe and North America (Fig. 51, P. lugubris); they resemble the smaller black species of Crabronides, and are distinguished from them chiefly by the existence of at least two complete, submarginal cells on the anterior wing instead of one.