Detailed consideration of the most efficient measures for controlling mosquitoes is to be found in Dr. Howard's Bulletin No. 88 of the Bureau of Entomology, "Preventive and remedial work against mosquitoes" or, in more summarized form, in Farmers' Bulletin No. 444. One of these should be obtained by any person interested in the problems of mosquito control and public health.
The Simuliidæ, or Black Flies
The Simuliidæ, or black flies, are small, dark, or black flies, with a stout body and a hump-back appearance. The antennæ are short but eleven-segmented, the wings broad, without scales or hairs, and with the anterior veins stout but the others very weak. The mouth-parts ([fig. 74]) are fitted for biting.
The larvæ of the Simuliidæ ([fig. 75]) are aquatic and, unlike those of mosquitoes, require a well ærated, or swiftly running water. Here they attach to stones, logs, or vegetation and feed upon various micro-organisms. They pupate in silken cocoons open at the top. Detailed life-histories have not been worked out for most of the species. We shall consider as typical that of Simulium pictipes, an inoffensive species widely distributed in the Eastern United States, which has been studied especially at Ithaca, N.Y. (Johannsen, 1903).
The eggs are deposited in a compact yellowish layer on the surface of rock, on the brinks of falls and rapids where the water is flowing swiftly. They are elongate ellipsoidal in shape, about .4 by .18 mm. As myriads of females deposit in the same place the egg patches may be conspicuous coatings of a foot or much more in diameter. When first laid they are enveloped in a yellowish white slime, which becomes darker, until finally it becomes black just before the emerging of the larvæ. The egg stage lasts a week.
The larvæ ([fig. 75]) are black, soft skinned, somewhat cylindrical in shape, enlarged at both ends and attenuated in the middle. The posterior half is much stouter than the anterior part and almost club-shaped. The head bears two large fan-shaped organs which aid in procuring food. Respiration is accomplished by means of three so-called blood gills which are pushed out from the dorsal part of the rectum. The larvæ occur in enormous numbers, in moss-like patches. If removed from their natural habitat and placed in quiet water they die within three or four hours. Fastened to the rock by means of a disk-like sucker at the caudal end of the body, they ordinarily assume an erect position. They move about on the surface of the rocks, to a limited extent, with a looping gait similar to that of a measuring worm, and a web is secreted which prevents their being washed away by the swiftly flowing water. They feed chiefly upon algæ and diatoms.