THE MIGRATION OF THE The migratory powers of Musca domestica were clearly demonstrated by the occurrence of specimens at Rebecca Shoal. On my arrival at the station, June 26, I found 6 house-flies; 2 more were taken on the following day. As previously explained, there was no possible breeding place for Musca domestica on the lighthouse; all refuse is immediately thrown into the sea. So it was plain that these flies had either been introduced by boats or had flown across the sea. That boats are responsible for the presence of a few flies at the station was shown when the small auxiliary catboat brought one of the assistant keepers from Key West; 3 house-flies were found upon it as it was raised from the water to the platform. Mr. Roberts, the assistant, stated that the flies had annoyed him during the trip from the Marquesas Keys to Rebecca Shoal that day.
No more specimens were taken during the next 8 days, as will appear from the table on page 202. It is important to note that not even with the swarm of mosquitoes which arrived on July 5 were any house flies noted. But on July 6, following the sudden change of the wind from north to east late on the preceding day, the lighthouse by con trast seemed fairly alive withMusca domestica; 25 were captured during the day-3 between 6 and 7h30m a. m. ; 1 each at 9h20m, 940m, 10h55m, 11h30m, 11h32m; 10 between 11h33m and 1h15m; 2 at 2h10m p. m.; 1 at 3 p. m.; 2 at 4h20m; 1 at 5h15m, and the last at 5h25m.
I am at a loss to account for the flight of house-flies from the east, since no such numbers had come to the station on the east winds which prevailed from June 26 to July 2. Had they arrived along with the mosquitoes on July 5 the obvious assumption would have been that they had come from the coast of Florida, but the fact that none appeared until 12 hours after the wind had veered into the east seems rather to point to Marquesas or some island farther east along the reef as their source. That it would have been possible, as far as distance is concerned, for the flies to have been carried from Key West 46 miles to the east, will appear after a statement of the conditions under which the second notable migration of Musca domestica appeared at Rebecca Shoal, but the reason for their leaving an eastern point in such large numbers at this time is still hidden; possibly the sudden change in the direction of the wind took them unawares.
During the 7th, 8th, and 9th of July the wind continued to blow lightly from the east. On the 7th of July, 5 flies were taken, 2 of them at 5h30m a. m.; 1 at 6 and 2 at 12h30m p. m. July 8 brought 5 more, 1 at 940m a. m.; 2 at 12h30m; 1 at 2h30m, and 1 at 5h20m. A specimen was captured at 5b10m a. m. on July 9, another at 11h30m, and a third at 6 p. m. ; none appeared on July 10 while the wind was shifting toward the south, but on the memorable 11th, along with 37 mosquitoes, came 18 house-flies from Cuba. It is interesting to note that, whereas the mosquitoes were captured at all hours of the day as they reached the station, these flies were all taken between 9h50m and 11h42m a. m., 1 hour and 52 minutes. A possible explanation suggests itself. It will be remembered that no Mu-sca domestica accompanied the mosquitoes from the coast of Florida on the north wind of July 4 and 5, but the very shortest distance in that direction is 105 miles. Now, the distance from Rebecca Shoal to the coast of Cuba is 10 miles less or about 95 miles. These facts, taken with the sudden appearance and disappear ance of the flies on July 11, suggests that 95 miles is about the limit to the distance over which they were able to sustain themselves with the wind blowing moderately. That the wind was not the only factor that caused the cessation of their appearance is plain from the fact that it neither changed its direction nor lessened its rate until several hours later.
On July 12 the number of Musca domestica fell to normal, 5 being taken. During the succeeding days of high east winds very few appeared at the lighthouse, 1 on July 13, 2 on the 14th, 1 on the 15th, and the last on the 17th.