BIOLOGY OF PHYSA 15 



were found in the stomachs of frog and fish, and are known 

 to be the food of horsefly larvae (Tabanidae). There are cer- 

 tainly snails enough in the creek to justify the extraordinary 

 abundance of horseflies in this vicinity." Forbes (1888 p. 482) 

 says: "The delicate pond snails {Succinea, Lymnaea, and 

 Physa) were taken chiefly by the smaller mollusk-eating fishes, — 

 a few of them also by the catfishes and the suckers." Chrysemys 

 marginata eats Physa greedily and there is little doubt that 

 the water birds are formidable enemies. 



Attention should be called to the places in the river system 

 where young snails alone were found. It will be recalled that 

 they are found living upon submerged water weeds in the bottom 

 of the creek, long grasses that trailed into the deeper waters 

 of the streams, and upon weeds in slack water in the Huron 

 river. It is thought, perhaps, that either the eggs of the snail 

 or the snail that afterwards laid the eggs, may have been caught 

 in these plants while being transported down stream. It may 

 be, that if they escape their enemies and are not swept away 

 by the current they may live on in this region of the stream, 

 where the conditions are favorable. Judging from the condi- 

 tions under which the colonies of adult Physa are living, it does 

 not seem probable that the present habitat of these young 

 snails will prove at all lasting. 



Summing up the conditions that are common to all the most 

 favorable Physa habitats of the river system proper we have 

 the following: Shallow, well aerated water; a firm substratum 

 of rocks or weeds, full sunlight; relatively few enemies, the 

 presence of some water plants, such as Cladophora or Cerato- 

 phylltiin, and a protection from a strong current. 



IV. I.ake-Pond-Swamp Series. 



I. Lakes. Owing to the large number of lakes in the imme- 

 diate vicinity of Ann Arbor, a much more comprehensive study 

 of them was made than was possible with the river system. 

 These lakes are all of glacial origin, varying greatly in size, 

 shape and in their different stages of extinction. In the present 

 study of lakes it was early learned that Physa is found only 

 in the littoral zone, and in consequence the factors which affect 

 shores and shore margins alone will find place. Physa was also 

 sought in the shallow marginal waters, beach pools and open 



