1920] Richmond, Studies on the Biology of the Aquatic Hydrophilidce 77 



of four just a little anterior of the middle, with the outer two set a little more forward 

 than the inner two. Metathoracic pterotheca? visible from above. Mesonotum and 

 metanotum smooth, with a transverse row of two styli, each just lateral of the scutel- 

 lum. Inner spur of metatibia not very prominent and blunt, no outer adjacent spur 

 present. Tarsi not ending in a spine but blunt and tibise not noticeably spinous on 

 the outer side, although all tibise have slight protuberances. First to seventh ab- 

 dominal tergites with a transverse row of six styli, the outer one on each side arising 

 from a tubercle; second to seventh pleurites each with a stylus; eighth tergite semi- 

 circular, the rounded posterior margin bearing two styli; ninth segment with a pair 

 of tapering, fleshy cerci, about as long as the ninth tergite (0.215 mm.), extending 

 caudally and slightly diverging. Externo-caudal angles of ninth sternite not promi- 

 nent and acute. A pair of appendages arise from between the eighth and ninth 

 sternites and extend caudally on either side of the median line to the base of the cerci 

 and closely appressed to the ninth segment. Spiracles well defined but small. 



9. Sphaeridiinse l 



Eggs laid in a mass with a slight covering of loosely applied silk or a round 

 blanket-like covering of closely applied silk (Phoznonotum) ; deposited in dung, damp 

 earth, or on damp leaves. Larva with head elevated; antennae with their points of 

 insertion situated farther from the externo-frontal angles of the head than those of 

 the mandibles; epicranial suture absent; ocular areas flat, round, small, varying in 

 size (first to third larger than fourth to sixth in Phcenonotum and Ccelostoma) and 

 closelv aggregated; labrum and clypeus reduced and united; with a small projection 

 usually unidentate but tridentate in Phcenonotum; antenna? with a more or less 

 prominent antennal appendage; mandibles sharply pointed distally, strongly curved 

 and with inner teeth, or without inner teeth and grooved on the inner side (Sphceri- 

 dium : stipes widened and depressed on the outer side in Sphceridium and Cercyon but 

 normal in Phcenonotum and Ccelostoma; stipes with many small setae on its inner and 

 outer -ides: second segment of labial palpus distinctly longer than the first; labium 

 and maxilla? inserted at the anterior margin of the under side of the head; the ventral 

 side of the head in Sphceridium and Cercyon with the median line impressed and with 

 a small pit in the middle; gula very much reduced and not attaining the occipital 

 opening: jugular sclerites fin front of procoxae) well developed; tarsi present (Ccelo- 

 stoma, Phcenonotum I, tarsi absent (Sphceridium) , or legs entirely wanting (Cercyon). 

 Eight complete abdominal segments; ninth and tenth reduced, forming a stigmatic 

 atrium. Spiracles are rudimentary and bifore. Type of breathing is pseudo-meta- 

 pneustic. Cerci reduced, two-segmented. The abdomen bears no chitinized plates 

 and is more or less truncate. The larvae show a tendency towards the scarabaeoid 

 type. 



III. PHYLOGENETIC CONSIDERATIONS 



Among the Hydrophilidse there has been an evolution of distinct 

 generic types. This is particularly evident among the larvae and ; in an 

 endeavor to show the general trend of adaptation, a phylogenetic tree 



!This subfamily requires very much additional study but the characters thus far known and those 

 taken from additional material at hand have been incorporated. The larvai of Phcenonotum and Coelo- 

 sioma differ materially from those of Sphosridium and Cercyon. 



