1920] Richmond, Studies on the Biology of the Aquatic Hydrophilidce 15 



The larva is campodeiform; its head inclined (more primitive 

 genera), elevated (more specialized genera), or nearly horizontal (Helo- 

 phorus, Hydrochous); ocular areas in groups of five or six, distant or 

 aggregated, reddish, round or oval, convex or flattened; antennse three- 

 segmented, rarely four-segmented (full-grown larva of Hydrous triangu- 

 laris); epicranial suture present or absent; points of insertion of an- 

 tenna? situated nearer or farther from the externo-frontal angles than 

 those of the mandibles; labrum and crypeus well developed and distinct 

 (more primitive genera) or reduced and fused (more specialized genera) ; 

 mandibles with lacinia mobilis or sharply pointed and with inner teeth; 

 maxilla palpif orm or with stipes possessing a well-developed inner lobe ; 

 maxillary palpi three-segmented; labial palpi two-segmented; ligula 

 present or absent; gula well developed and attaining the occipital 

 opening or reduced and not attaining the occipital opening; labium 

 and maxillse inserted in a furrow on the under side of the head (more 

 primitive genera) or not inserted in a furrow (more specialized genera) ; 

 legs segmented and with claw-like tarsi, without tarsi (Sphceridium) , 

 or legs entirely absent (Cercyon) . 



Abdomen with nine well-developed segments and a reduced tenth 

 or eight well-developed segments and reduced ninth and tenth. The 

 body is usually slightly chitinizecl. The types of breathing overlap but 

 there is a marked tendency in each genus towards one of three types: 

 holopneustic, apneustic, or metapneustic. The spiracles (one pair of 

 mesothoracic and eight pairs of abdominal) may be well developed 

 bifore spiracles (Helophorus), annuliform spiracles (Ochthebius, Limnebius, 

 HydroEna), apparently absent (Hydroscapha) , or poorly developed bifore 

 spiracles (the remaining genera). Cerci absent (Hydroscapha) or 

 present as three-segmented or two-segmented appendages; reduced in 

 the higher forms. 



The pupa 1 is usually white in color except the eyes, which are red- 

 dish at first but change to nearly black before emergence. However, 

 Tropisternus lateralis and Hydrophilus obtusatus are greenish. The 

 anterior and posterior ends of the body are incurved and are not visible 

 from the dorsal side. The integument is smooth, except that it is sparsely 

 covered with styli. The supraorbital setae are usually present but there 

 is only one in Hydrous triangularis and none in Tropisternus glaber. 

 The pronotal styli vary in number and size. There are only sixteen in 



'The pupa of the most primitive forms has not been described. 



