The Caddis Flies, 

 or Trichoptera, 

 of Illinois 



HERBERT H. ROSS 



INTRODUCTION 



THE caddis flies, or Trichoptera, are 

 for the most part medium-sized to 

 small insects resembling moths in gen- 

 eral appearance. Their larvae are aquatic 

 in habit and caterpillar-like in appearance. 

 The order Trichoptera contains over 750 

 species, ranking about seventh among the 

 insect orders. For Illinois, we have now 

 recorded 184 species, the largest known list 

 for any state. It must be remembered, how- 

 ever, that Illinois does not have the same 

 wealth of diverse aquatic situations as some 

 other states, the lists of which will be great- 

 ly increased with intensive collecting. 



In 1931, when this project was started, 

 the only available listing of Illinois species 

 was contained in Dr. Cornelius Betten's 

 then unpublished manuscript of the Trichop- 

 tera of New York. In this. Dr. Betten list- 

 ed not only published records but also the 

 results of his own collecting in the vicinity 

 of Lake Forest, Illinois. This list enumer- 



ated 37 species for Illinois. Since that time 

 we have added 146 species to the list, show- 

 ing how poorly the caddis fly fauna of the 

 entire midwestern and central states was 

 then known. 



There is no doubt that additional species 

 will be discovered in the state with continued 

 collecting. For this reason, species known 

 from nearby points have been included in 

 the keys. In addition, as an added pre- 

 caution to anticipate future discoveries, all 

 genera known to occur in the Great Plains 

 area and eastward have been included in the 

 keys to genera. 



Immature stages are known for 120 spe- 

 cies treated in this report. There are so 

 many additional species and genera, especial- 

 ly in the western states, for which the im- 

 mature stages are unknown that the treat- 

 ment given here will have to be considered 

 as only provisional in certain families for the 

 continent as a whole. 



BIOLOGY 



The bundle of sticks crawling about in 

 the water, green worms under stones in the 

 stream, swarms of "flies" around the lights 

 along river and lake — these are forms of 

 caddis flies familiar to the general insect 

 collector. They are but a few isolated 

 phenomena, however, in a picture of life 

 histories and interrelationships varied in 

 pattern and interesting in detail. 



Life Cycle 



In general the life history of caddis flies 

 follows this pattern: The eggs are laid near 

 or in the water, each soon hatching into 



a worm called a larva, which lives in the 

 water and may build a case of sticks, sand 

 grains and other small objects. When full 

 grown, this larva makes a cocoon in which 

 it changes into a transformation stage called 

 a pupa. The adult structures (e.g., wings 

 and genitalia) develop within the pupa. 

 When the adult structures are fully de- 

 veloped within it, the pupa cuts its way 

 out of the cocoon, swims to the surface, 

 crawls out of the water and attaches itself 

 firmly to a stick, stone or other object. The 

 adult then bursts the pupal skin, wriggles 

 and crawls out of it and flies away free. 

 Mating flights follow; a period ensues for 



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