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Evidence that Odonata and Lepidoptera Migrate to Tortugas

species, july, breed, flies and time

EVIDENCE THAT ODONATA AND LEPIDOPTERA MIGRATE TO TORTUGAS.

Mention has just been made of a dragon-fly which visited Rebecca Shoal station. At times these insects are numerous at Tortugas, both on Loggerhead and East Keys, where vegetation is abundant. In July 1914 within an hour I captured a score at East Key. On the i sland, whose extent is about 6 acres, there were at that time at least 100, but during the two days spent there the past season only 2 were seen, 1 of them being captured. At Loggerhead on July 19 a single individual was the reward of a day's lookout, and only 3 were seen there until the morning of July 30, when, as we were closing the labora tories preparatory to leaving the island, the air over the bay-cedar bushes near the buildings was alive with dragonflies. I counted a score in sight at one time.

Now, since there is no possible breeding-place for Odonata upon any of the Tortugas Islands, it is plain that the great variation in numbers must be due to their migratory habits; for it is not likely that boats or other agencies are responsible for their introduction. As an example of the tenacity with which dragonflies follow vessels upon which they may chance to be carried from shore, the case may be cited of a large specimen observed flying about the superstructure of the steamship Concho just before it left the dock at Key West. This insect, or one of its species, was noted on the following day while the boat was passing through the Straits of Florida. It was still on board when, toward the end of the voyage, the vessel was within 100 miles of New York.

Occasionally single specimens of Lepidoptera appear at Tortugas as if brought involuntarily from distant haunts. On July 28 a Sphingid, Sesia tantalus Linnaeus, was captured while feeding on lilies near the laboratory. While certain species apparently breed on Loggerhead and East Keys, others are so rare as to indicate that they are immi grants. Dr. Mayer is authority for the statement that one of the larger NoctuidEe often comes to Tortugas on southerly winds from Cuba.

Do SARCOPHAG1D/E BREED AT TORTUGAS ? Conspicuous by their numbers on the open sandy shores and among the vegetation closely bordering them at Tortugas are two species of Sarcophagithe or flesh-flies. Mr. R. R. Parker has identified the larger as Sarothromyia femoralis Schiner, and the smaller as Sarcopha gula occidua Fabricius. Like most of the insects inhabiting these white beaches where the light is intense, both species are much lighter in color than ordinary members of the family. Their flight is also peculiar in

so far as I am familiar with the group. As one walks along the beach these flies dart into the air only to alight a few feet in advance, repeat ing these short flights almost as a series of hops if disturbed by the progress of the intruder. I was unable to cause an extended flight. The possibility was suggested that their powers of flight had been modi fied after dwelling for some time on these isolated keys. If such were true then certainly they must be breeding there, for their numbers are considerable.

An experiment was performed in order to ascertain whether or not these Sarcophagids would breed in dead land-crabs. On July 19 a glass jar containing 3 inches of sand and a freshly killed ghost crab, Ocypoda arenaria, whose carapace measured 1 inches, was set under bay cedars near the beach and several specimens representing both species of flies were inclosed by means of a cover of fine bolting cloth. Within the next 2 days all the adults died and none was seen inside the jar until the morning of July 29, when three imagoes had emerged. On examination 101 pupae were found near the bottom of the sand; the majority were of the larger species. IV is thus evident that, if 104 maggots of a fly the size of the house-fly can develop to maturity within a single small crab, it is not necessary to look to migrations to keep up the stock of adults at Tortugas. To be sure, the large number of ants there may be relied upon to keep the flies within bounds; very few dead crabs are left long above ground. But years ago a former keeper of Loggerhead Light, Mr. George Billbury, called Dr. Mayer's attention to the fact that certain flies were to be noted about the entrances of crab burrows in the sand. As he surmised at that time, it is very possible that these flies may occasionally breed in crabs which, for some reason, die in their burrows.

Summary.

1. Large numbers of mosquitoes and house-flies are carried by northerly and southerly winds to Rebecca Shoal light-station and the Tortugai Islands from Florida and Cuba.

2. Easterly winds bring a few of these, as well as smaller numbers of blow-flies, horse-flies, and gnats from islands east on the Florida Reef.

3. Occasionally Odonata, Neuroptera, and Lepidoptera are carried by the winds to these parts of the reef.

4. Sarcophagithe breed in land crabs at Tortugas.