March, 1921] Macro-Famia of Mirror Lake 167 



Hydrophilus triangularis Say. Of this species, the largest 

 secured in the survey, only one specimen was found, in the 

 debris off shore at station 15. Hence this, too, must be consid- 

 ered rare here. 



Tropisternus nimbatus Say. This was fairly numerous but 

 decidedly less so than T. glaber. It was well distributed in the 

 small pond. 



Tropisternus glaber Herbst. This was found much more 

 common than its congener just mentioned, and was the most 

 abundant beetle in Mirror Lake. It compared favorably in 

 numbers with the commonest kinds of water striders, except 

 Trepobates pictiis. As is the case with other really abundant 

 forms, relatively more could have been secured and counted, 

 than the table indicates. They could not be as completely 

 collected nor would be quite as assiduously^ kept as rare forms. 



It was evidently present throughout the small pond and 

 a few were also found in the large. Like T. nimbatus it was also 

 found in the pool, station 17. But one place in the small pond, 

 station 12, it was much more abundant than anywhere else. 

 There the mud was not so deep and soft as on most of the 

 bottom, and it had a thin layer of loose, fluffy, brown material, 

 probably the so-called dust-fine detritus, which offered no 

 impediment to the scrambling, swimming combination sort of 

 movement across the bottom which these beetles indulged in. 



Philhydrus nebulosus Say. This beetle was found in sit- 

 uations frequented also by Helophorus. Like many other beetles 

 it was restricted to the small pond. It was common, and found 

 principally near the north and northeast shore. 



Philhydrus ochraceus Melsh. Of this related species only 

 one specimen was found, and it undoubtedly was rare. 



Cymbiodyta fimbriata Melsh. This beetle also frequented 

 situations in the small pond like those mentioned above. It 

 was fairly common. 



Creniphilus subcupreus Say. This tiny beetle was taken in 

 situations identical with the preceding form. It was very 

 common. The greatest number was obtained from submerged 

 vegetation near shore at station 3. 



Family Parnidce. 



Dryops lithophilus Germar. This was the one beetle of 

 this family found here. It has been found at Lake Mendota 



