14 NORTH AMERICAN DIPTERA. 



vein wanting ; the third vein arises from the first usually at 

 such an angle that its first section (to the anterior cross-vein) 

 has the appearance of a cross-vein ; fourth vein always, the 

 third and fifth either furcate or not ; sixth vein never furcate, 

 sometimes rudimentary ; seventh vein usually short, often 

 rudimentary or entirely wanting ; discal and posterior basal 

 cells always wanting. 



The family Mycetophilidae, commonly known as fungus- 

 gnats, comprises seven or eight hundred known species of 

 small or minute flies, the best known of which are, perhaps, 

 the various dark-winged species of Sciara so common about 

 gardens. In the following description of the immature stages 

 I draw largely from Osten Sacken. 



The larvae have a distinct horny head ; horny, flat, lamelli- 

 form mandibles ; maxillae with a large coriaceous inner lobe 

 and a horny outside piece, with a circular excision at the tip, 

 the labium small, horny, almost rudimentary; the antennae 

 are usually small or rudimentary, the ocelli are either wanting 

 or seen in a small pellucid spot below each antennae. The 

 body is subcylindrical, more or less elongated, fleshy, whitish 

 or yellowish, and composed of twelve segments. It is smooth, 

 without hairs or bristles, except those on the ventral side. It 

 is generally transparent, showing distinctly the intestinal 

 canal and the trachea. There are eight pairs of stigmata, one 

 on the first segment, and seven on the first seven abdominal 

 segments, the last two having none. The locomotory organs 

 consist of more or less apparent transverse swellings on the 

 under side of the ventral segments, sometimes furnished with 

 minute bristles or spines. 



The larvae present some of the most singular habits among 

 all diptera. They are gregarious, and live in or upon decaying 

 matter. Most of the species seem to prefer fungus or fungoid 

 substances, whence comes the common name of fungus-gnats 

 applied to the mature insect. The larvae of Sciara are found 

 among decaying leaves, in vegetable mold, in cow-dung, under 



