12 



Illinois Natural History Survey Bulletin 



Vol. 23, Art. 1 



Fig. 11. — Dead River, in Dunes Park near Zion, Illinois. This river is more marsh than 

 stream. It is the only situation in the state in which large numbers of both individuals and spe- 

 cies of the large Phryganeidae have been found. 



leptocerid Triaenodes aba is common; Poly- 

 centroptis interruptus is also common. Ex- 

 cept for Phryganea sayi, these are known 

 only from similar nearby situations. Re- 

 stricted in Illinois to this locality are Fabria 

 inornata, Triaenodes baris and Polycentro- 

 pus remotus. 



A somewhat similar marsh area is located 

 along the edges of the Des Plaines River 

 near Rosecrans, just a few miles south of 

 the Wisconsin line. In this area we have 

 taken Triaenodes aba in considerable num- 

 bers, but only a few of the other species 

 common in the Dead River. 



DISTRIBUTION 



In the preceding pages an analysis is 

 given of the manner in which the various 

 species of Illinois caddis flies are distributed 

 in relation to habitats within the state. To 

 understand the faunal characteristics, how- 

 ever, this should be correlated with the 

 geographic distribution of the species in re- 

 lation to the entire continent. 



We have found that the geographic dis- 

 tribution of caddis fly species within Illinois 

 may give an erroneous impression of their 

 continental distribution. For instance, the 

 fact that Cheumatopsyche lasia occurs prin- 

 cipally in the central and northern parts of 

 Illinois might indicate that the species is 

 primarily northern; such is not the case, 

 most of the records for the species being 

 southwest of Illinois, and a few northwest, 

 with no records yet known from northeast 

 of Illinois, fig. 12. Similarly our Illinois 

 records of Phylocentropus placidus are from 



the extreme southern portion of the state, 

 whereas the main range of the species is to 

 the northeast. 



These circumstances have led to a study 

 of the continental range of as many species 

 as possible. The results are of considerable 

 interest because of the scarcity of caddis 

 fly records in past literature and because 

 of the demonstration that many caddis flies 

 have an extensive range. Since many species 

 are known from few records, the following 

 remarks apply to the better known. It is 

 difficult to determine accurately the center 

 of distribution of any caddis fly species on 

 the bases of existing records, because in 

 many areas in North America collecting for 

 this order has been very inadequate. The 

 present analysis attempts to give a picture 

 gleaned from available records. 



The Illinois species may be divided rough- 

 ly into two general categories. The first 



