80 Bulletin American Museum of Natural History [Vol. XLII 



Ganglbauer divided the family into five subfamilies: Helophorinse; 

 Hydrseninse; Sphercheinse; Hydrophilinse; and Sphseridiinse. It will 

 be seen that four more have been considered here. Recent authors have 

 laid stress upon larval characters in the determination of subfamilies. 

 Limnebiini was raised to subfamily rank by d'Orchymont, 1913, and 

 Hydroscaphidse was placed under the Hydrophilidse, as Hydroscaphinse, 

 by Boving, 1916. If the larval characters are to be accepted, surely the 

 Hydrobiini and Hydrochoini are worthy of subfamily rank. The struc- 

 tures which characterize the immature stages of these subfamilies are 

 listed on pages 27 and 42 respectively. The adult characters offer differ- 

 ences as well. The Hydrobiinse differ strikingly from the Hydrophilinae 

 in general form, the latter being very much better adapted to water life. 

 The Hydrochoinse possess only seven-segmented antennas (the last 

 three segments pubescent) and six abdominal sternites, the second to 

 fifth furrowed and the sixth unchitinized but prominent. The Hydrseni- 

 nse, with which Ganglbauer linked them, possess eight- to nine-seg- 

 mented antennse (the last five segments pubescent) and six or seven 

 smooth, chitinized, abdominal sternites. 



An examination of the phylogenetic tree will show three branches 

 near the base. The one to the left shows the trend of the Hydroscaphinse, 

 Sperchinse and Hydrophilinae. In these subfamilies, we find the tendency 

 to develop gills at first and then finally lose them. The branch at the 

 right shows the divergence of the Helophorinse, 1 as represented by Helo 

 phorus, from the primitive types of the Limnebiinse 1 and Hydrseninse. 

 Ganglbauer considered Helophorinse more primitive than those which 

 have been placed nearer the base but the only character which could 

 possibly suggest a more generalized larva is its three-segmented cerci. 

 In its other structures it is very much more specialized. 



On the main branch, Limnebiinse and Hydrseninse are near the base 

 and lead toward the Hydrobiinse, with the Hydrochoinse representing 

 the stepping-stone. The latter subfamily clearly bridges the gap between 

 the land forms and the water forms. Berosus, in its adaptation to water, 

 is clearly the most advanced of the Hydrobiinse. The pleural gills are 

 remarkably well developed and only a slight indication of a former 

 stigmatic atrium is discernible. 



The branch with Hydrobius, Helocombus (?), Cymbiodyta and Helo- 

 chares shows a tendency to a sublinear form and the gradual reduction of 

 the number of inner teeth on the mandibles. It terminates in Philydrus, 



1 The larva of Helophorus closely resembles that of Hister, while the larva? of the Limnebiina? are 

 strikingly similar to those small Staphylinida?, belonging to the Taehyporini, and to Choleva, Liodes, 

 and Agathidium of the Silphidae. 



