Wasps, Social and Solitary

by George W. Peckham; E. G. Peckham

There is one thought that must strike even a casual observer at the sight of the hordes of parasites that hover over a Bembex colony:—

“The buzzing flies, a persevering train,
Incessant swarm, and chased return again.”

Why do not these wasps, fly-catchers as they are by profession, kill the worthless wretches that infest their homes, thriving abundantly on the fruits of their labor, a continual menace to the life and safety of their offspring? To the uninitiated it would seem that these flies might serve as food for the wasp larvæ quite as well as any of the dozen species that they actually take; but even if the wasp-mother believes that they possess indigestible qualities, it would be much less trouble to kill them and throw them away than to be perpetually chasing them to a little distance only to see them return as soon as she gives her attention to anything else. Whatever the reason for it may be, the relation between the wasps and the flies is certainly most curious and puzzling. Fabre’s explanation is that since this miserable little fly has its own part to play in nature, Bembex must respect it, thus preserving harmony in the world of living things. The idea is perfectly in accord with his own theories, but we find ourselves quite unable to accept it.

There can be no doubt that the parasites are a grave danger to Bembex. She suffers from them far more than any other wasp that we are familiar with, her mode of feeding the young rendering her peculiarly susceptible to their attacks. Of the ten or twelve nests that we opened only one was free from them, the others containing from two to five lively maggots nearly as large as the wasp larvæ, which were sharing the food brought in by the mother. Fabre, who has studied the question thoroughly, has found as many as ten parasitic larvæ in one nest. He has also noticed that where the parasites are most numerous the wasp-larva is proportionately small and emaciated, reaching only one half or one third of its normal size. When it attempts to spin its cocoon it has not strength enough to do so, and thus perishes miserably among the pupæ of the interlopers, which have the advantage of developing more rapidly. He has proved, by experiments upon nests transported to his study, that although the invaders preserve friendly relations with the rightful owner of the nest so long as food is abundant, they nevertheless, at the first suggestion of scarcity, fall upon the wasp larva and ruthlessly devour it. This “black action” he has seen with his own eyes. In view of this base ingratitude, we are more than ever impressed with the troubles of the poor Bembex mother, as she tries to feed a dozen mouths where she has bargained for only one.

We several times saw a fly follow a wasp into her nest, remaining within for half a minute, and it is probable that they go in to lay their eggs. According to Fabre, it is the habit of the flies that are parasitic upon the half-dozen species of Bembex that he has studied to seize the moment at which the fly projects from under the abdomen of the wasp as she enters the nest; and he has even known them to lay two or three eggs on one fly in the instant of time that its body was exposed.

Fabre took a partly grown Bembex larva from the nest, where it was surrounded by the remains of twenty flies. He fed it generously, and it ate sixty-two more, making a total of eighty-two in the eight days that passed before the spinning of the cocoon. Our experiments in this line gave similar results. We took charge of a partly grown larva on the afternoon of August tenth, and between that date and August fifteenth, when it spun its cocoon, it ate forty-two house flies besides a big Tabanus.

Fabre thinks that under natural conditions the mother does not give the larva all it can eat at one time, but provides it with what she considers a reasonable amount of food, and keeps anything that she catches beyond this out of its reach. He draws his conclusion from the fact that he has found several flies in the tunnel leading to the nest, while the larva had as many more close to it. It would certainly be convenient for Bembex to have a reserve of this kind in case of rainy weather, but the forethought implied in such an action seems to require a higher degree of intelligence than can be claimed for her.

In one nest we found a single fly with a long cylindrical egg attached to the left side of the thorax just at the origin of the third leg. In another, which we had seen made and provisioned, we found, six days later, a larva which we judged to be four days old. Assuming that the egg was laid on the first day, it must have taken it about two days to hatch. Other nests gave us larvæ in all stages of development, surrounded by the remains of Diptera, among which Syrphus, Tabanus, and Musca were represented.

In regard to the condition of the flies captured by Bembex, we have never seen the crushing of the thorax, which is noted by both Wesenberg and Fabre. Indeed, the flies that we found were not always dead, since in two instances they responded readily to stimulation. Similar results have been obtained by Mr. S. W. Dunning of Hartford, Connecticut.