160 Journal New York Entomological Society. tVoi. xx. 



The mesosternuni (ins^ Figs, ii and 12) is hidden beneath the 

 intercoxal projections of the metasternum and prosternum. It is quite 

 solidly fused with pro- and metasterna. The episternum (cp. Figs. 

 II and 12) is a broad, irregular sclerite. It does not reach the coxal 

 cavity. The epimercn (cm) is nearly quadrate in outline and extends 

 to the coxal cavity. 



The metasternum is very broad behind, narrowed in front where 

 it exends between the middle coxae and is solidly fused with the 

 prosternum. The antecoxal sclerite is triangular in outline, the apex 

 projecting between the posterior coxje. The episternum (cp, Figs. 

 II and 12) is a long, narrow sclerite, wide in front, narrowed behind 

 and lies almost parallel to the median plane of the body. It extends 

 from the meso-episternum to the posterior coxa. The epimeron is a 

 thin, narrow sclerite lying directly laterad of the episternum. 



The anterior and middle coxae are globular. The posterior coxae 

 are in the form of flat plates, meeting in the middle line and covering 

 the first three to five abdominal segments. They extend laterally to 

 the pleurse of the elytra. At the anterior outer end of each is a 

 somewhat rounded knob which fits closely into a socket-like depression 

 of the elytron. (Figs. 15 and 17.) 



The anterior and middle femora are club-shaped, slightly flattened 

 so as to increase the striking surface in swimming. The posterior 

 femora are dumbbell-shaped, the proximal end being the larger (Figs. 

 15 and 17). The anterior and middle femora and tarsi of all the 

 legs are furnished with long swimming hairs. The first three seg- 

 ments of the tarsi of the anterior and middle pairs of legs of the male 

 are somewhat broadened at the distal ends and the under sides pro- 

 vided with hairs for clinging (Fig. 18). 



Each elytron is provided with ten rows of punctures which vary 

 considerably in depth and size in the various species. In the greater 

 number of species the elytra are marked with black spots or lines 

 which are very characteristic of this family. (See photographs, Plate 

 XV.) These markings, however, usually exhibit considerable range 

 of variation within the same species. 



A discussion of the habits, life-histories, etc., follows in the second 

 part of this paper. 



The three genera may be readily separated by the following key. 



