1920] Richmond, Studies on the Biology of the Aquatic Hydrophilidce 35 



Tropisterntjs Solier 



Although the adults appear to be closely allied to Hydrous, the differ- 

 ences found in the immature stages tend to strengthen the validity of this 

 genus. The adults differ from Hydrous principally in their smaller size 

 and in the structure of the maxillary palpi. They are the most common 

 members of the Hydrophilinse and may be taken in almost any pool 

 or lagoon. Although more active than most of the water scavengers 

 the adults are easily captured by hand. T. glaber, T. mixtus, and T. 

 lateralis are the representatives in this region. The former two strongly 

 resemble each other but T. lateralis can easily be distinguished on 

 account of the narrow yellow margin of the prothorax and elytra. T. 

 glaber is by far the most abundant species. The genus is limited to 

 America in its distribution. 



Duges, 1884, was the first to give the characters of the immature 

 stages. His descriptions of the larva and pupa of T. lateralis, collected 

 in Mexico, were carefully done but his figures are not very helpful. 

 Wickham, 1893, added the larva and pupa of T. glaber. The egg-case 

 has apparently been unmentioned in literature. This seems strange as 

 the cases, at least those of T. glaber, are very common at the commence- 

 ment of the warm summer days. 



Tropisternus glaber Herbst 

 Plate VII 



The egg-cases cannot be mistaken; the horny mast is peculiar to 

 the subfamily to which this genus belongs; the case is not as long as the 

 naked case of Hydrous and is not wrapped up in a leaf as is the case of 

 Hydrophilus. Instead, it is fastened to a leaf, blade of grass, or any kind 

 of vegetation present at the surface of the water. 



The cap covers one end of the case and the mast is firmly attached 

 to it. There are from eleven to twenty -two eggs contained within and 

 the time of incubation is six to seven days. The main dates of develop- 

 ment of an individual observed in 1914* were as follows: 



May 25, egg deposited in the case; May 30, larva emerged; July 

 1, burrowed; July 10, pupated; July 14, adult emerged. 



The young larvae were placed in an aquarium where they fed readily 

 on entomostracans and small tadpoles. The different molts were not 

 observed. Only a few larvae reached maturity, as the weaker individuals 

 were quickly eaten by the stronger ones. During the latter part of June 

 and the first few weeks in July many full-grown larvae were taken out-of- 



