Insects at Rebecca Shoal and Tortugas. 209 



miles. These facts, taken with the sudden appearance and disappear- 

 ance of the flies on July 1 1 , 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. 



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 house-flies 

 breeding at Tortugas, attention was turned to the question as to 

 whether it would be reasonable to expect that all Musca domestica 

 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 house-flies 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 tune 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. 



