MIGRATION OF INSECTS TO REBECCA SHOAL STATION AND THE TORTUGAS ISLANDS, WITH SPECIAL REFERENCE TO MOSQUITOES AND FLIES.
For the purpose of determining whether mosquitoes can, either by their own powers of flight or with the aid of favorable winds, migrate across the sea for considerable distances, the writer spent the period from June 26 to July 18, 1917, upon the Rebecca Shoal light-station. The work was undertaken under the auspices of the Carnegie Institu tion of Washington, Hon. G. R. Putnam, Commissioner of Lighthouses, having kindly granted permission to study at Rebecca Shoal light house. Moreover Dr. L. 0. Howard has kindly made timely sugges tions respecting classification, but he is in no sense responsible for statements expressed herein. Except Stegomyia, the names of mos quitoes used are from Howard, Dyar and Knab, 1917.
The circumstances which suggested the desirability of such investi gations were the repeated experiences of Dr. Mayer and other scientists at Tortugas, Florida, in connection with the occurrence there of mos quitoes. It appeared that these insects were abundant on Loggerhead Key only after northerly winds of several hours' duration—in other words, under conditions favorable to their migration from the mainland of Florida. Owing to the care which would be necessary in order to prevent absolutely the breeding of mosquitoes on the several keys of the Tortugas Group, particularly at Fort Jefferson on Garden Key, it was thought advisable to make observations at the nearest point to Tortugas where this difficulty could be overcome.
Rebecca Shoal light-station was chosen (1) because of its isolation from the mainland and other keys, and (2) because of its freedom from all except easily controllable breeding-places for mosquitoes. The lighthouse stands in 12 feet of water upon Rebecca Shoal, a small and entirely submerged part of the Florida Reef, 12 miles east of the Dry Tortugas and 48 miles west of Key West. The nearest point upon the mainland of Florida is Cape Sable, 105 miles northeast. Havana lies 95 miles to the south on the Cuban coast, while Cape San Antonio, at the western end of Cuba, bears southwest from Rebecca Shoal at a distance of 230 miles. Cardenas Bay lies 135 miles to the southeast.
Between Rebecca Shoal and Key West several keys of the Florida Reef intervene. Most important in relation to insect migration is the Marquesas group. These keys are at the same time the largest and the nearest to Rebecca Shoal-24 miles east—and are largely covered with mangrove swamps which furnish breeding-places for myriads of mosquitoes. Their relation to the occurrence of these insects at the light-station will be considered later. In succession, passing eastward, lie Boca Grande, 6 miles distant from Marquesas; Man Key, Woman Key, Crawfish Key; then Mullet, Barracouta, Cottrell, and Snipe Keys at the entrance of Key West Harbor. It thus appears that any insect which reaches Rebecca light-station by flight alone must cover either 12 miles from Tortugas on the west or 24 miles from Marquesas on the east, or at least 105 miles from the mainland on the east and north, or 90 miles or more from Cuba.
The station consists of a wooden dwelling 25 feet square and 26 feet high, supported 34 feet above mean water-level upon an iron frame understructure. A 10-foot platform surrounds the base of the dwelling on all sides. The first story contains a single room 6 feet in height, which is occupied by four large wooden tanks for the storing of rain water, and by a paint-closet and work-bench. The keeper's room, a kitchen, and pantry open into a small hall on the second floor. From here a stairway leads to the third floor, which is divided into a lamp and oil room, two assistants' rooms, and a hall. A ladder gives access to the lantern, light, and its mechanism at the top of the station. The lantern is surrounded by a narrow balcony. The diagonals of the station run north and south and east and west. At a level 8 feet above the water are two small boat-landings, one each on the southeast and northwest sides.