Fig. 50.—Mimesa bicolor ♂. Britain.
The species of Passaloecus live in the burrows that they form in the stems of plants; Pemphredon lugubris frequents the decayed wood of the beech. The larva and pupa of the latter have been described by Verhoeff; no cocoon is formed for the metamorphosis. Both these genera provision their nests with Aphidae. This is also the case with Stigmus pendulus, but the burrows of this species form a complex system of diverticula proceeding from an irregular main channel formed in the pithy stems of bushes. Cemonus unicolor, according to Giraud, forms its burrows in bramble-stems, but it also takes advantage, for the purposes of nidification, of the abandoned galls of Cynips, and also of a peculiar swelling formed by a fly—Lipara lucens—on the common reed, Arundo phragmites. This species also makes use of Aphidae, and Verhoeff states that it has only an imperfect instinct as to the amount of food it stores.
Fig. 51.—Pemphredon lugubris ♀. Britain.
Sub-Fam. 10. Crabronides.—Pronotum short, front wing with one complete submarginal and two discoidal cells: hind body variable in form, pedicellate in some abnormal forms, but more usually not stalked.
The Crabronides (Vespa crabro, the hornet, is not of this sub-family) are wasp-like little Insects, with unusually robust and quadrangular head. They frequently have the hind tibiae more or less thickened, and the clypeus covered with metallic hair. It appears at present that they are specially attached to the temperate regions of the northern hemisphere, but this may possibly be in part due to their having escaped attention elsewhere. In Britain they form the most important part of the fossorial Hymenoptera, the genus Crabro (with numerous sub-genera) itself comprising thirty species. The males of some of the forms have the front tibiae and tarsi of most extraordinary shapes. They form burrows in dead wood, or in pithy stems, (occasionally in the earth of cliffs), and usually store them with Diptera as food for the larvae: the wings and dried portions of the bodies of the flies consumed by Crabronides are often exposed to view when portions of old wood are broken from trees.
Fig. 52.—Crabro cephalotes ♀. Britain.
The genus Oxybelus is included by some systematists, but with doubt, in this sub-family; if not placed here, it must form a distinct sub-family. It has the metathorax spinose, and the sub-marginal and first discoidal cells are not, or are scarcely, separated.