272 W, C. ALLEE 



1. MATERIAL 



The isopods used in this study have almost all been Asellus 

 communis Say. In a very few cases trials have been made with 

 Mancasellus danielsii Rich. Asellus communis is the common 

 fresh water isopod. Richardson ('05, p. 420) gives its distribu- 

 tion as extending from Massachusetts and Connecticut on the 

 east to Illinois and Louisiana on the west. Near Chicago it is 

 found throughout the year in the older ponds of the series at the 

 south end of Lake Michigan (Allee, '11, p. 126; Shelford '11 a, 

 maps) and in young streams, particularly those which have per- 

 manent pools and temporary riffles. Most of the stream isopods 

 used in the course of this work were taken from the County Line 

 Creek near Glencoe, Illinois (Shelford '11, maps pp. 14; 17). In 

 general their stream distribution parallels that of the horned dace, 

 Semotilus atromaculatus (Shelford '11, p. 17). In the spring 

 these isopods are very abundant in the small temporary ponds 

 especially where there is a thick covering of leaves over the bottom 

 which guards against too severe desiccation in the dry periods. 

 The Mancasellus danielsii have never been taken from a stream or 

 from the small summer-dry ponds in this vicinit}^ They are 

 limited mainly to the series of ppnds mentioned above, although 

 some have been taken from a spring fed, watercress marsh at 

 Cary, Illinois. In the Chicago area they have never been found 

 in a place not containing A. communis. 



In early spring the isopods are usually found along the margins 

 of the ponds, later in the season they are more common in deeper 

 water (Allee, '11). They crawl around over the vegetation and 

 bottom and are almost never seen swimming. There is a dis- 

 tinct daily movement that is more pronounced in the deeper 

 water. Here they are more numerous at the surface during times 

 of dim light and retire to the bottom when exposed to bright sun- 

 light. If the pond dries, they burrow into the mud and are thus 

 able to withstand droughts extending over several months. In 

 the streams, they are usually found in protected positions, often 

 hiding among a bunch of leaves or other debris. In streams with 

 rocky beds they may occur under stones. 



