84 Bulletin American Museum of Natural History [Vol. XLII 



Head slightly elevated, nearly horizontal; epicranial suture absent; ocular 

 areas in groups of six; mandibles sharply pointed, with inner teeth; maxilla 

 palpiform; labium and maxilla inserted at the anterior margin of the under 

 side of the head; gula reduced and distant from the occipital opening. 

 Cerci well developed and three-segmented Helophorus. 



3. Antennae very short; ocular areas aggregated; clypeus lacking or fused with 



labrum; no well-defined spiracles; tracheal gills on the prothorax, and first 

 and eighth abdominal segments; cerci absent; each abdominal segment with 

 a continuous band of chitin ; tenth or anal segment with a ventral sucker and 



two terminal pencil-shaped appendices Hydroscapha. 



Antennae rather long; ocular areas distant; clypeus and labrum both distinct; 

 nine pairs of annuliform spiracles; no tracheal gills; cerci two-segmented 

 and well developed; each abdominal segment with a ventral and dorsal 

 plate of chitin; tenth or anal segment without a ventral sucker and with two 

 small terminal hooks : 4. 



4. Antennae usually shorter, with no prominent inner swelhngs; setae on the clypeus 



not placed at the anterior margin and the two median one's distant from each 

 other; lacinia mobilis narrower; inner lobe of maxillae not distinctly divided 



apically; cerci nearly contiguous proximally and divergent Ochthebius. 



Antennae usually longer, with prominent inner swelhngs; setae on the clypeus 

 placed at the anterior margin and equidistant; lacinia mobilis broader; inner 

 lobe of maxillae distinctly divided apically ; cerci widely separated proximally 

 and nearly parallel 5. 



5. Third segment of antennae without inner swellings, second segment with a 



single antennal appendage; a pair of pectinate setae at the anterior margin of 

 the labrum; inner lobe of maxillae slightly divided apically; labium broadened 



distally. Hyolrama. 



Third segment of antennae with an inner swelling; second segment with two 

 slender antennal appendages; no pectinate setae at the anterior margin of the 

 labrum; inner lobe of maxillae strongly divided; labium not noticeably 

 broadened distally, although sides of mentum are rounded Limnebius. 



6. Head slightly inclined or horizontal; antennae with their points of insertion 



nearer the externo-frontal angles than those of the mandibles; ocular areas 

 in groups of five or six; labium and maxillae inserted in a furrow on the 

 under side of the head; gula well developed and attaining the occipital 



opening 7. 



Head elevated; antennae with their points of insertion farther from the externo- 

 frontal angles than those of the mandibles; ocular areas in groups of six; 

 labium and maxillae inserted at the anterior margin of the under side of the 

 head; gula reduced and not attaining the occipital opening 8. 



7. Ocular areas in groups of five, not aggregated; epicranial suture very short, 



nearly absent; mandibles sharply pointed and with inner teeth; palpifer 

 with an inner claw-like lobe; abdominal segments without chitinous plates. 



Spercheus. 



