allee] small crustaceans 71 



The thorax is plainly segmented, since the hard chitonous 

 covering is divided between each segment. The abdomen is 

 also segmented but this is not visible on the upper side, since 

 the segments are fused together and covered by a single plate. 

 The gills are located on the under or ventral side of the abdomen 

 and are covered by large plates which keep up a continual waving 

 motion. Two branched appendages called uropods extend be- 

 hind the abdomen. These are sensitive to touch and thus serve 

 the same purpose as the antennae. Hairs are scattered all over 

 the body and these are also sensitive. A dark median line is often 

 visible, extending the full length of the body. This is the ali- 

 mentary canal and its color is due to food that has been recently 

 taken. 



The isopods of the Chicago area are brown with a rather 

 faint color pattern. The adults vary in length from two-fifths 

 to two-thirds of an inch, depending upon the conditions in which 

 the animals have been living. There are only two species found 

 near Chicago. Their distinguishing characters follow : 



1. Extremely flattened wide isopods with the upper cover- 

 ing (carapace) extending far beyond the body proper and having 

 a well defined reddish color pattern on a darker background. 



Mancasellus Danielsii (Fig. 2). 



2. Less flattened, narrower isopods, with the carapace ex- 

 tending only a little beyond the body; color markings faint and 

 of a dull cream color on the smoke colored background. 



ASELLUS COMMUNIS (Fig. 1). 



Of the two species, Asellus communis is the more widely 

 distributed. This is true not only in the Chicago area but in all 

 of the northeastern states. The males of this species are dis- 

 tinctly larger than the females and have a different body form. 

 Usually the females are almost as broad at the anterior end of 

 the thorax as at the posterior end, while the males taper gradually 

 from the extreme posterior end to the head. This distinction is 

 not so marked in the other species. Asellus is abundant 

 in temporary and permanent ponds and in small streams 

 and is present in protected embayments in small lakes. 

 It is most abundant in the spring in the temporary road- 

 side ponds which have the bottom thickly covered with leaves. 

 In the early spring the isopods may be collected by the hundreds 

 from these ponds by dipping up the wet leaves with any close 

 meshed net and picking out the isopods with the fingers. Later in 

 the season as the ponds dry the Aselli retire under the leaves 



