Home › Volumes › Insects › Window-flies and the Stiletto-flies

Window-flies and the Stiletto-flies

(Families Scenopinidae and Therevidae.)

The Scenopinid flies, which Comstock called the windowflies for the reason that they are quite commonly seen upon the windows of houses, are small, active, shining black flies of which we have a half-dozen species in North America, the commonest being Scenopinus fenestralis Linn., which is common to both Europe and North America. Its specific name, fenestralis, is due to its window-loving habit. The larvae of these flies resemble those of the following family, and are long and very slender, white in color and with apparently many joints to the body. They are frequently found under carpets and in decaying wood; also in woolen blankets, and Riley has stated that he found one in human expectoration. This, however, was probably accidental. The manager of a storage warehouse noticed many of these slender, white larvae under carpets sent in by his customers for storage. He was worried at their number since he supposed that their presence might indicate the advent of some new kind of carpet moth. He was assured, however, that they were considered as predatory in habit, and that they feed upon clothes moths and other insects found in such places, such as book-lice. Nowhere, however,does there appear to be any record of any definite observations on this point. One observer tells me that he tried to decide this question, but that the insect intended for prey turned out to be more aggressive and ate up the Scenopinus larva. They are apparently always especially abundant, as I am informed by Mr. Chittenden, in the sweepings in feed stores and the flies are always to be found around the windows in such establishments. The probability is very strong that they feed upon such small, soft-bodied insects as flour-mites and book-lice. Mr. Pergande tells me that he has seen them eat the pupae of one of the little stored-grain beetles (Silvanus surinamensis) and also disabled house-flies which he had offered them, as well as their comrades of their own species.

The family Therevidae comprises a group of rather small and rather slender flies, frequently of variegated color, and looking something like robber-flies. They have been called stiletto-flies. The flies themselves prey upon other insects, principally other flies. They are not as active as the true robber-flies, nor are they as strong, and as a result they choose weaker prey and have the habit of lying in wait upon leaves and bushes and even upon the ground for their prey to come near them instead of flying about actively in search of it as do the robber-flies. Their larvae are very curious and are found in rotting wood or in earth which is full of vegetable mold, and seem to feed upon both decaying animal and vegetable matter. They have been found feeding upon dead caterpillars and pupae. They are very long and slender, and have apparently twenty segments to the body including the head. This appearance, however, is deceptive, and is due to a seeming division of the anterior segments.