339 



Leech and Chandler: Coleoptera 



Fig. 13:37. Hydrophilidoe, larvae, a, Serosos peregrinus; b, He/ophorus sp.; c, Hydrochus sp.; 

 d, Cercyon sp.; e, Loccob/us sp.; f, Paracymus sp.; g, Anacaena sp. (o-c, e-g, Richmond, 1920; 

 a 1 , Bi'ving and Henriksen, 1938). 



25. Apex of abdomen not covered by elytra; scutellum an 

 elongate triangle; elytra with sutural striae; 1st ab- 

 dominal sternite not longitudinally carinate at middle; 

 antennae 8-segmented Sphaeridium Fabricius 1775 



— Elytra covering apex of abdomen; scutellum triangular, 

 but short; elytra with sutural striae or not; 1st abdom- 

 inal sternite carinate or not; antennae 9-segmented 

 26 



26. First abdominal sternite longitudinally carinate at 

 middle (fig. 13:35c); sutural striae distinct 



Dactylosternum Wollaston 1854 



— First abdominal sternite not carinate at middle (fig. 



13:356); sutural striae absent 



Phaenonotum Sharp 1882 



27. Elytra with well- formed epipleurae in basal half or 

 more; prosternum carinate at middle, or rarely elevated, 

 flat or tectiform (fig. 13:32a). CERCYONINI 28 



— Elytra inflexed tightly against sides of body, without 

 distinct epipleurae except at extreme base; prosternal 

 process always elevated at middle to plane of meso- 

 sternal protuberance as a flat plate, not carinate. 

 MEGASTERNINI 31 



28. Mesosternal elevation appearing to be a continuation 

 of mid-metasternum, the 2 in broad contact; elytra 

 never with strongly costate intervals 29 



— Mesosternal elevation discrete, contacting metaster- 

 num at a single narrow point or separated by a hiatus 

 30 



29. Mesosternal elevation a broad, flat or slightly con- 

 cave plate, pentagonal, the anterior end pointed .... 



Pelosoma Mulsant 1844 



— Mesosternal elevation narrow, almost carinate along 

 middle, sides steep, anteriorly arrowheadlike 



Omicrus Sharp 1879 



30. Middle of prosternum differentiated from sides, forming 

 a plate like a low roof, with fine but not acute median 

 longitudinal carina o sternum Sharp 1882 



— Middle of prosternum carinate but not differentiated 



from sides as a roof-shaped plate like area 



Cercyon Leach 18 17 



31. Lateral margins of pronotum nearly evenly arcuate 

 as usual; front tibiae entire, or with outer edge abruptly 



notched, narrowed at apical two-thirds 32 



— Lateral margins of pronotum deeply angulate at middle; 

 front tibiae entire Crypt oplewum Mulsant 1844 



32. Front tibiae abruptly notched and narrowed at apical 

 two-thirds (fig. 13:32e); elytra finely striate and punc- 

 tate, or only punctate Megastemum Mulsent 1844 



— Front tibiae entire; elytra strongly costate (fig. 13:316) 

 Pemelus Horn 1890 



Key to the Known Genera of Aquatic Nearctlc Hydrophilidoe 

 Larvae 



1. Nine complete abdominal segments, 10th reduced but 

 distinct; integument noticeably chitinized, spiracles 

 lateral, well developed (fig. 13:376) 



Helophorus Fabricius 1775 



— Eight complete abdominal segments, 9-10 reduced and 

 forming a stigmatic atrium (except in Berosus in which 

 atrium has not developed) 2 



2. Antennae with their points of insertion nearer the 

 externofrontal angles than are those of the mandibles 

 (mandible with a terminal seta, inner tooth, and lacinia 

 mobilis); labium and maxillae inserted in a furrow 

 on under side of head; gula well developed and attain- 

 ing occipital opening, (abdominal segments with well- 

 developed chitinous plates) (fig. 13:37c) 



Hydrochus Leach 1817 



— Antennae with their points of insertion farther from 

 externofrontal angles than are those of mandibles; 

 labium and maxillae inserted at anterior margin of 

 under side of head; gula reduced and not attaining 

 occipital opening 3 



3. First 7 abdominal segments with long lateral tracheal 

 gills; segments 9-10 reduced, no stigmatic atrium 

 present (fig. 13:37a) Berosus Leach 1817 



— Tracheal gills not nearly so prominent, or else absent; 

 abdominal segments 9-10 reduced, forming a stigmatic 

 atrium 4 



4. Ocular areas oval, larger, aggregated but more dis- 

 tant; legs well developed, visible from above except 



