Insects at Rebecca Shoal and Tortugas. 207 



most isolated of the group as well as least frequently visited by man. 

 Although it has a fairly luxuriant growth of bay cedars and xerophytic 

 shrubs, there is not a drop of water on the key, and hence no possi- 

 bility of mosquitoes breeding. On July 20 and 21 I found Aedes 

 niger so numerous at dusk and daybreak that as many as 4 females 

 would be attempting to bite at one time. A walk through the grass at 

 sunset was sure to arouse several of the insects. As to the origin of the 

 mosquitoes found on East Key, two or three sources may have con- 

 tributed. We have seen that on east winds a few of the Aedes niger 

 migrated to the Rebecca Shoal light-station. Since East Key is only 

 12 miles farther west it seems likely that a few may also reach this 

 island from Marquesas, but, no doubt, the greater number came in the 

 migrations of July 5 and 11. It is known that they are capable of 

 living for some time in captivity, and so may well have survived on 

 plant juices from July 11 to 21. 



On July 23 an examination of the fresh-water reservoirs at Fort 

 Jefferson was undertaken to determine the extent to which mosquitoes 

 were breeding there. It was found that all except one of the tanks 

 had been treated a short time before with kerosene, but this one con- 

 tained large numbers of larvae and pupa?. Several hundred were 

 transferred in glass jars to Loggerhead Key, where some were pre- 

 served and the others allowed to mature. 



Since Stegomyia calopus used to breed in the reservoirs at Fort Jeffer- 

 son it was expected that at least a part of these would prove to be of that 

 species. But all larvae and adults reared from them proved to be 

 Culex pipiens Linnaeus. Not a single adult of this had been, or was 

 later, taken either at Rebecca Shoal or at Tortugas. It is probable 

 that the adults had been introduced by vessels visiting the fort and 

 had deposited eggs in this reservoir. That the imagoes emerging from 

 this source do not fly to other keys of the Tortugas 2 and 3 miles away 

 is not surprising; Culex pipiens does not fly far from its breeding-place. 



Unfortunately I did not succeed in determining the origin of the 

 specimens of Stegomyia calopus which were taken on Loggerhead Key. 

 The fact that the majority were males indicates that the breeding- 

 place was not far away; indeed, it is known that this species seldom 

 flies more than a few hundred yards. 



THE MIGRATION OF THE HOUSE-FLY. 



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, 1 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 



