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 Jan., 1919 PARASITISM OF'NESTLING BIRDS BY FLY LARVAE 35 eral others which were being prepared for dissection were placed in a fixing fluid for six hours, then washed in a 50 percent alcohol solution and placed in one of 80 percent..They were still alive two days later and it was necessary to resort to a stronger fixing fluid ((]ilson's) to prepare them for dissection. Other' lar- vae were placed in a very strong insect powder, but they remained alive in it for two or three days. As soon as the larvae pupated, the pupae were placed beneath inverted tumblers. After the flies emerged from the pupae they were kept in a large, narrow-mesh cage and carefully studied. It may be of interest to state that the flies (about 1500), practically without exception, emerged from the pupae be- tween seven o'clock in the morning and two o'clock in the afternoon. Upon emerging they were of a slightly lighter hue than the adult of our Common t-/ouse-fly (Musca domestica), their wings being shriveled up, but after about an hour or so these straightened out and the young flies assumed the dark blue, me- tallic lustre of adult Protocalliphora azurea. Various kinds of food were placed befor. e these flies, such as milk, crushed fruit, cheese, and meat in various forms. The flies readily ate the milk and fruit, especially if the latter was placed on the cage wire instead of the cage floor, but they were rather indifferent to the meat and the c'heese. Although some of 'the flies were kept in the cage for six or seven weeks, none of them, to my knowledge, deposited eggs or maggots. One day I noticed a number of very small, bee-like insects, since identified as Nasonia brevicornis by Professor Brues of Harvard University, flitting about in one of the inverted tumblers. I wondered where they had come from, but thinking that they had perhaps got into the tumbler accidentally, I let them es- cape. To my surprise I found some twenty or thirty more of them under the same tumbler the following day. Upon investigation, I found one or two small holes of about the size made by an ordinary stick-pin in several of the pupae. I could easily tell by the weight of the latter that they were empty. This gave me a clue. I opened a number of the pupae and there found these little insects, more commonly known as Chalcid Flies, in several stages of development: as white, in- active maggots; as creamy, pink-eyed larvae, already showing their insect. form; and as full-grown insects which came swarming out as soon as the fly pupae were opened. I counted the Chalcid Fly larvae from a dozen pupae and found them to vary in number from about fifteen to twenty-five per Muscid pupa. In all these cases the embryo fly had been completely devoured. Some forty or fifty of these Chalcid Flies were then transferred to an in- verted aquarium jar below which a hundred Muscid pupae were placed. The Chalcid Flies seemed to be perfectly at home among these pupae, crawling about among them as do bumble-bees among their honey-combs. Within a few weeks hundreds upon hundreds of young Chalcid Flies emerged from the lIuscid pu- pae; less than a dozen Protocalliphora hatched, the remaining ones having been parasitized by the Nasonia. In some of the infested birds' nests, I had noticed a number of small grubs, similar in size and form to those found in almonds. As in the case of the Chal- cid Flies and the larvae of the Protocalliphora, I paid no attention to them at the beginning, but when they occurred repeatedly, I began to suspect that they might have some relation to one or both of the other insects. I therefore col- leeted some fifty or sixty of these grubs and placed them among a large number of Muscid pupae, many of which were parasitized by Chalcid Flies, and watched