72 NATURE STUDY REVIEW [9 :3— Mar, 1913 



and find there enough moisture to keep alive until the following 

 rainy season. 



Isopods do not occur in young ponds that have sandy bot- 

 toms. But whenever the bottom has begun to be enmeshed by 

 interlacing roots, Aselli appear and once present they remain 

 until the pond is reduced to a mere vernal pool corresponding to 

 the roadside ditches just mentioned. They are most abundant, 

 however, in grassy bottomed ponds that contain water a greater 

 part of the year. In lakes they are limited to the pond-like em- 

 bayments where conditions are essentially the same as in a true 

 bond. 



The Aselli found in streams average from one-eighth to one- 

 sixth of an inch longer than those of ponds, and are more vigorous 

 in their reactions. They are most common in those streams hav- 

 ing a slight sewerage contamination but even then they do not 

 extend along the entire stream, but are most abundant in the pools 

 among the leaves or other debris that protects them from the 

 sweep of the current. They ar,e common in small brooks that 

 alternate pools and rapids. They also occur in streams that are 

 twenty feet or more across. Isopods are scarce in the larger 

 streams about Chicago. Asellus is also present in old base-leveled 

 rivers, but here pond plants grow along the margin and it is 

 obvious that the conditions approach those of ponds. Asellus 

 inhabits one other type of habitat, that is the spring-fed marsh. 

 In such a water cress marsh near Gary, 111., they are abundant 

 although smaller than those found in ponds. 



Mancasellus, the extremely flattened isopod, is found wher- 

 ever Asellus occurs except it is not present in the larger, rapid- 

 flowing streams. It is reported only from small grassy brooks 

 near the Naval Station at Great Lakes, 111. Neither is this form 

 found in the small summ,er-dry ponds of this locality. In general 

 it is more limited to the grassy regions of the ponds than is 

 Asellus. 



The breeding season of the isopods begins in the early spring 

 as soon as the ice leaves the water and continues until about the 

 middle of July. More than one brood is brought forth by the 

 same female in one season. Many adults die at the close of the 

 breeding season, although some of them live over another winter. 



The isopods are negative to direct sunlight ; that is, they avoid 

 it and collect in dark areas or in regions of faint light. Isopods 

 kept in aquaria will be found wandering around over the bottom 

 at night, while in the day time they retire under leaves and into 

 crannies. In ponds containing vegetation the isopods are nearer 



