At Tortugas house-flies are numerous at all times during the season when the Biological Laboratory is open. An attempt was made to determine whether or not they breed there. Since all sewage and refuse is at once disposed of in the sea, it was thought possible that Musca domestica might be breeding in dead land-crabs. In order to test this source 100 house-flies were inclosed within a glass jar con taining 3 inches of sand, 2 large ghost crabs, Ocypoda arenaria, and a small dish of water. The jar was tightly closed with fine bolting-cloth to prevent the entrance of ants. Apparently no eggs were deposited, for no maggots developed.
When it was found that there was little likelihood of houseflies breeding at Tortugas, attention was turned to the question as to whether it would be reasonable to expect that all Musca d,omestica found there had either been introduced upon vessels—and many cer tainly are—or migrated with the aid of the winds. Now, to one who had formed his estimate of the numbers of house-flies present on Loggerhead Key by observations made in the laboratories, such a proposition might well seem unreasonable, but as a matter of fact the flies are practically limited to the laboratories and to the lighthouse buildings.
I found hardly a half dozen specimens in the bushy and untenanted parts of the island. True to their name, the houseflies seek the build ings occupied by man. It is probable that the Musca domestica on Loggerhead Key during the latter part of July numbered not over 5,000. Calculating the number which may be expected to have been attracted to the island on the southerly wind from Cuba on July 11, by multi plying 25 taken at that time on Rebecca Shoal light-station by the relatively larger size of Loggerhead—say 2,000—and we have 50,000 flies, a number greatly in excess of those present. Hence it seems entirely within the range of possibility that Musca domestica does not breed on Loggerhead or the other Tortugas Keys, but that the few thousand specimens found there migrate from the mainland or from larger islands. Account should also be taken of the few flies which may reach Tortugas from the east; as high as 5 per day were often borne to Rebecca Shoal on easterly winds.