The course of life of these flies—if all be truth that is spoken of them—is, indeed, very extraordinary, for during the first or winged stage of their adult life they live on birds, but migrate from them to some quadruped—as, say, a deer—as soon as they find themselves within easy reach of it, and then, as having reached their final place of abode, do away with their wings. Thus, being too lazy or lethargic to fly themselves, they choose rather to stand indebted to another being for a power which they no doubt once possessed in perfection, and which they are still quite capable of exercising. What the larval stage of these flies is, whether they lay their eggs upon their first or last habitation—or on both, and if not, where or in what manner the larva passes its life, I do not know, and as my authority, who should be up to date, holds his peace upon the matter, I conclude that it is not yet made out. Possibly the grub is a vegetable feeder, or possibly, again, it is as fatal to some other insect as is that of a little fly with a big name—_Apocephalus pergandei_ to wit—to ants. The victim here chosen—if there be not others also—is a black tree-climbing ant, common in Pennsylvania. As it runs over the ground or up and down the trunks of trees, the fly darts after it on tiny wings, intent on laying her egg upon its neck. The ant tries to elude her endeavours, but _Apocephalus_—or Sit-o’-my-head—has a mission to fulfil, and will take no denial. The egg is laid, it cannot be detached, and, when hatched, the issuing grub bores, with enthusiasm, into the head of the ant. Coming to the brain he has nothing to do but to eat it, and he does so until the whole cavity of the skull has become an empty chamber, except for his own presence there. The movements of the ant during this process—of its feelings we have no record—have become more and more erratic, and it feels itself less and less capable of performing its duties as a member of an active and industrious community. At length it falls down, and not long afterwards its head falls off, giving to the maggot inside it its first opportunity of looking out into the world through the window of the neck-hole. Hitherto its life, however easy and pleasant, has been of a sedentary nature, but now it can enjoy the pleasures of a walk, and moves about something after the manner of a snail, dragging its cephalic shell behind it. But this active state is not of long duration. The time of change is at hand, and snug within the ant’s head and its own last larval skin, which, as is the way with fly caterpillars, serves it in lieu of a cocoon, the fortunate little creature turns into a chrysalis, and dreams away its time till, on some sunny day, it issues from its cradle a happy, active fly, feeling strangely attracted by ants.