Entomology of the Illinois River. 185 



In the following keys 1 have freely used the valuable 

 generalizations of Dr. Brauer,* modified by the studies 

 of the wing of the imago recently published by Professor 

 J. H. Comstoek.t 



. KEYS TO AQriATIC FAMILIES OF DIPTERA. 



Characters used. — Orthorhaphous larvae show a great 

 diversity of external appearance and structure, (due to 

 their environment,) which masks their true affinities ; and 

 therefore a key based, as the present one is, on the more 

 evident characters, must be more or less artificial. 



The true legs of the dipterous larva having disap- 

 peared, they are replaced as occasion requires by vari- 

 ously developed false feet on the prothorax or on the 

 anterior or posterior abdominal segments. The head may 

 be perfectly developed as in other orders [Fig. 16], or it 

 may be imperfect in structure [Fig. 51], not including 

 the brain ganglia, and strongly retracted into the 

 thorax, as in all the families herein reported on except 

 the Ptychopteridae. Respiration may take place by 

 tracheal gills, usually filamentous or branching; or 

 through the skin; or the insect may take in air at the 

 surface through posterior terminal [Fig. 15] or anterior 

 thoracic [Fig.l9] spiracles. These are often elevated or 

 variously fringed in order that connection with the air 

 may be readily secured and maintained, and are usuallv 

 chitinous and finely sculptured. 



The pupa may be soft and enclosed in the last larval 

 skin, which becomes hard and usually barrel-shaped ; or 

 it may be exposed and hard-shelled, with the appen- 

 dages visible, resembling a lepidopterous pupa, but 

 distinguished by the development of the tiioracic spira- 

 cles. The feet are folded beneath, the tai-si pointing 

 t)ackwards and in contact along the middle line beneath; 

 the anterior and middle pairs each individually in ron- 



♦"Die Zweiflugler des Kaiseil. Museums zu Wien." 

 i"Manua,l for the Study of Insects," pp. 413-489. 



