184: Illinois State Laboratory of Natural History. 



DIPTERA. 



From an aquatic point of view, the immature Diptera 

 divide according to their habits into three pjreat groups. 

 The first is the perfectly aquatic Orthorhapha — the 

 J//cw-orthorhapha, we may say. This group is represent- 

 ed by such typical fan'iilies as Culicidse, Chironomidae, 

 Siinuliidse, and Blepharoceridae. It contains genera rich 

 in species and individuals and constitutes one of the most 

 prominent features of the aquatic fauna. The larvae 

 are provided with a well-developed head, are usually 

 fully aquatic, and subsist, as a rule, on minute organic 

 objects. The second group includes the remaining Ortho- 

 rhapha, such as the nematocerous families Tipulidae and 

 Psychodidae and the brachycerous Stratiomyiidae and 

 Tabanidae. A large part of the larvae of this group are 

 semiaquatic, crawlers rather than swimmers, most at 

 home in or upon wet shores of mud or sand, amongst mar- 

 ginal vegetation, or burrowing in the mud of the bottom, 

 finding in these situations a varied diet. When in open 

 water they float at the surface and are apparently out 

 of their element. The head is chitinous, usually incom- 

 plete, not including the brain, and more or less immersed 

 in the prothorax. The third and last group is the great 

 army of cyclorhaphous Diptera, of which a very consider- 

 able number of scattered forms are more or less perfectly 

 at home in the water or in wet places. Their food is 

 usually decaying animal or vegetable matter. The head 

 is incomplete, and membranous above. The families here 

 treated are those which contain the larger forms of the 

 second group. 



