INSECTS TAKEN AT REBECCA SHOAL FROM JUNE 26 TO JULY 18.
Those insects which migrated to Rebecca Shoal light-station proved to belong to four orders, Odonata, Neuroptera, Lepidoptera, and Dip tera. Of the first order only two appeared, one being a large dragon fly and the other a damsel-fly. A single Neuropteran, the golden-eyed lace-wing Chrysopa, was taken, while the only Lepidopteran was a moth, Syntomeida epilais Walker. The Diptera, as anticipated, were more numerous both as to species and individuals, and it is with these that the remainder of the paper will be mainly concerned.
On the next page is a table showing the weather conditions and the kinds and numbers of insects taken.
Considering first the occurrence of mosquitoes, the table on the fol lowing page shows that during all the time from June 26 to July 3, while the wind was blowing from the east or changing into the north, only a single mosquito was noted. This was taken on July 1 at 9 p. m., and was of the species Aides niger. It is probable that its point of departure was the Marquesas Keys, 24 miles to the east. A few others appeared later on east winds.
On July 2 the wind shifted into east-northeast and continued lightly from that direction during the night. At 5 a. m. of July 3 it was north, having shifted considerably during the night; at 10 a. m. it was north east, at 12 noon it was north and continued so during the rest of the day, freshening toward night. The weather was now clear and the moon full. No mosquitoes were noted during the day.
At 5h30n' on the morning of July 4 the wind was still in the north and continued so during the forenoon. Throughout the afternoon it blew from a few points west of north, again freshening toward night. At 8 a. m. the first specimen of what proved to be one of the two important mosquito migrations appeared at the station. This arrival, as may be calculated, was at least 27 hours after the wind had begun blowing from the northeast or north, but no more were noted until between 6 and 7°30m p. m., when 3 were taken, indicating that mosquitoes in the vicinity were increasing in number.
The wind was still blowing moderately from the north at 4k30' a. m. on July 5. That mosquitoes were still about was proved by the cap ture of a specimen at this early hour, just as day was breaking. An other was taken at 5h30m, and during the next 3 hours of the morning mosquitoes were so numerous that at times 2 or more could be cap tured with one sweep of the net; at 8h15m a. m. the count stood at 14 specimens. At intervals during the day others appeared, the last at 8 p. m., which made the total 21 for the day.
Now this marked increase in the number of mosquitoes encountered at the station, following a shift of 90° or more in the direction of the wind, from east to north by west, gains more significance as we note the interval of time elapsing between the inauguration of this change and the appearance of the insects. As already stated, at least 27 hours had passed from the time when, on the night of July 2 and 3, the wind was blowing from the vicinity of Cape Sable, 105 miles to the northeast on the southwest point of Florida, and the arrival of the first mosquito at Rebecca Shoal at 8 a. m. on July 4. That only 1 specimen was taken at this time indicates that comparatively few mosquitoes were blown out to sea from the southern capes. Not until toward night, after the wind had been blowing for over 37 hours, did the num bers begin to rise. The inference is that as the wind shifted—assuming that its direction held over the entire distance—points farther and farther north on the west coast of Florida contributed to the forced migration of mosquitoes. As will appear, the species taking part were only such as breed in salt marshes; therefore, those regions having such swamps would send out large swarms.
It will be seen that the body of the swarm encountered at Rebecca Shoal arrived on July 5, after the wind had been blowing from due north—the direction of Tampa Bay—for 48 hours or more. Obviously it can not be determined just how far up the coast these mosquitoes originated, but it is certainly possible that some of them may have come from points 150 miles to the north.