The nervuration of Trypoxylon is very peculiar, and differs from that of the widely-distributed genus Pison, though according to Kohl's views the two may be correctly associated to form this sub-family. The species of Trypoxylon are apparently rather fond of human propinquity, and build clay- or mud-nests in or near houses. T. albitarse has this habit, and is well known in Southern Brazil under the name of "Marimbouda da casa"; this Insect, like Pelopaeus, stores its nest with spiders, and Peckholt has remarked that however great may be the number of spiders placed by the mother-wasp in a cell, they are all consumed by the larva, none ever being found in the cell after the perfect Insect escapes therefrom. The European T. figulus forms a nest either in bramble-stems or in sandy soil or walls; it makes use of spiders as provisions.
Sub-Fam. 5. Astatides.—Eyes very large in the male, meeting broadly on the vertex; two spurs on the middle tibia.
Fig. 46—Astata boops, male. Britain.
We have two species of the genus Astata in Britain: one of them—A. boops—is known to form burrows in the ground, each of which contains only a single cell; this, it appears, is usually provisioned with bugs of the genus Pentatoma, Insects remarkable for their strong and offensive odour. St. Fargeau records that this species also makes use of a small cockroach for forming the food-store: thus exhibiting an unique catholicity in the toleration of the disagreeable; almost the only point of connection between bugs and cockroaches being their disagreeable character. According to Smith, Oxybelus, another genus of Fossores, is also used. Authorities are far from agreement as to the validity and relations of the sub-family Astatides. It consists only of the widely-distributed genus Astata, with which the North American Diploplectron (with one species) is doubtfully associated.
Sub-Fam. 6. Bembecides.—Labrum frequently elongate; wing-nervures extending very near to the outer margin; marginal cell of front wing not appendiculate; mandibles not emarginate externally; hind body stout, not pedicellate.
The elongation of the labrum, though one of the most trustworthy of the characters of the Bembecides, cannot be altogether relied on owing to the variation it presents both in this and the allied sub-families. The Bembecides carry their prey to their young tucked underneath their own bodies and hugged to the breast; they affect loose, sandy soils for nidification; make use, in the great majority of the cases where the habits are known, of Diptera for provisions, and give these dead to the young; making repeated visits to supply fresh food to the progeny, which notwithstanding this fact, are distributed in isolated burrows.
Fig. 47.—Bembex rostrata ♂. Europe.
One of the most interesting of Fabre's studies of the instincts of Hymenoptera is devoted to Bembex rostrata. The Bembecides have the habit of forming their nests in the ground in wide expanses of sand, and of covering them up, they leave them so that there appears to be absolutely nothing by which the exact position of the nest can be traced; nevertheless the Bembex flies direct to it without any hesitation. How necessary it is to these Insects to possess this faculty of finding their nests will be understood when we recall that the Bembex does not provision its nest once and for all, but supplies the young at first with only insufficient food, and has therefore to return at daily, or other intervals, with a fresh store of provisions. The burrow is made in the sand by means of the fore-legs; these work with such rapidity and skill that a constant stream of sand flows out behind the Insect while it is engaged in the act of excavation. The nest or cell in which the larva is to live, is formed by this process of digging; but no fastening together of the material occurs, nor does any expedient seem to be resorted to, other than that of making a way through the sand by clearing out all the pieces of stick or stone that might diminish facility of access. The cell being formed, the Bembex leaves the spot in search of prey, and when it has secured a victim in the shape of a two-winged fly, it returns therewith to the burrow, and the booty is placed therein, an egg being deposited on it. The wasp then leaves the burrow, disguising, however, the spot where it is situate, and flies away; to proceed possibly with the formation of other burrows.[[53]] In the course of twenty-four hours the egg hatches, and the larva in two or three days completely devours the stock provided for it. The mother-wasp then returns with another fly—this time probably a larger one—penetrates rapidly to the bottom of the burrow, and again retreats, leaving the second stock of provisions for the benefit of the greedy larva. These visits of supply are repeated with increased frequency, as the appetite of the larva for the benefit of which they are made increases with its growth. During the fourteen or fifteen days that form this portion of the life-cycle, the single larva is supplied with no less than fifty to eighty flies for food. To furnish this quantum, numerous visits are made to each burrow, and as the mother Bembex has several burrows—though how many does not appear to be known—her industry at this time must be very great. All the while, too, a great danger has to be avoided, for there is an enemy that sees in the booty brought by the Bembex to its young, a rich store for its own progeny. This enemy is a feeble, two-winged fly of the family Tachinidae and the genus Miltogramma; it hangs about the neighbourhood of the nests, and sooner or later finds its opportunity of descending on the prey the Bembex is carrying, choosing for its purpose a moment when the Bembex makes a brief delay just at the mouth of the burrow; then down comes the Miltogramma and lays one, two, or three eggs on some portion of the booty that may be projecting from beneath the body of the wasp. This latter carries in the food for its own young, but thus introduces to the latter the source of its destruction, for the Miltogramma larvae eat up the supply of food intended for the Bembex larvae, and if there be not enough of this provender they satisfy their voracity by eating the Bembex larva itself. It is a remarkable fact that notwithstanding the presence of these strange larvae in the nest the mother Bembex continues to bring food at proper intervals, and, what is stranger still, makes no effort to rid the nest of the intruders: returning to the burrow with a supply of food she finds therein not only her legitimate offspring, a single tenant, but several others, strangers, it may be to the number of twelve; although she would have no difficulty in freeing the nest from this band of little brigands, she makes no attempt to do so, but continues to bring the supplies. In doing so she is fulfilling her duty; what matters it that she is nourishing the enemies of her race? Both race and enemies have existed for long, perhaps for untold periods of time, why then should she disturb herself, or deviate from her accustomed range of duties? Some of us will see in such proceedings only gross stupidity, while others may look on them as sublime toleration.