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.