Insect guide > Flies > Moth Flies

Moth Flies

(Family Psychodidae)
There are certain very small, weak flies which look like little moths, from which fact they have been termed "moth-flies", which are frequently found upon windows and upon the under surfaces of leaves, and which have broad wings, rather thick antennae, and which are densely clothed with hairs, even the surface of the wings being hairy. These are the flies of the family Psychodidae. They are so small and so fragile that they are difficult to preserve, and though there are probably very many species only comparatively few have been described.

The arrangement of the wing veins in these flies differs from that of all other flies, and possibly represents the lowest or most generalized type in the Diptera, although there is good reason to believe that perhaps the Tipulidae more nearly represent the primordial fly.

In larval habits they are interesting and variable. Some of them live in dry cow dung; others on fallen leaves immersed in the water of pools or small streams, while others live in rapidly running water, and others are found in rotten potatoes. The larvae are remarkable from the fact that they have both tracheal gills and open spiracles, so that they can theoretically both breathe air and use the oxygen in the water.

One of the European forms (Pericoma canescens) has been carefully studied by Miall and Walker (Trans. Ent. Soc. London, 1895), but the larvae of none of the American forms were known until Kellogg discovered the larvae of Pericoma californiensis in the streams of the Sierra Morena Mountains near Stanford University, California. With Kellogg's larva no tracheal gills were found but they may have been retracted. On the under side of the larva are curious sucking discs, through which it attaches itself to objects under the water, a structure which seems to be necessary in order to prevent the larva from being carried down the stream.

They were found on the stones of the stream bed, not usually submerged, but always at the very verge of the water, sometimes submerged, sometimes above the water surface, but always wetted by the current or spray. They look something like a sow-bug or pill-bug (Oniscus) in shape, but are narrower.