1920] Richmond, Studies on the Biology of the Aquatic Hydrophilidce 87 



Head quadrangular; antennal appendage about the length of the third segment; 

 mandibles symmetrical, each with two inner teeth; stipes swollen, not broad- 

 ened or depressed externally; ligula about as long as the first palpal segment 

 and not pointed; median line not impressed and pit absent; legs reduced but 

 complete; each abdominal segment with a ventral sucker 21. 



20. Head subspherical; ocular ares aggregated but more distant from each other; 



mandibles without inner teeth, furrowed on the inner side; mentum cordi- 

 form; tarsi absent; procercus and appendage of seventh intersegmental 



membrane fleshy and long Sphoeridium. 



Head ovate; ocular areas closely aggregated and sometimes appearing as one; 

 right mandible with a single inner tooth; mentum short, transverse; legs 

 absent; procercus and appendage of the seventh intersegmental membrane 

 reduced and barely indicated Cercyon. 



21. Clypeus tridentate Phcenonotum. 



Clypeus unidentate Colostoma. 



Key for the Identification of the Pup^e 



1 . Metathoracic wing-cases visible from above 2. 



Metathoracic wing-cases not visible 9. 



2. Motary styli of abdominal tergites in transverse rows of six each; abdominal 



tergites with small lateral tubercles; spiracles not hidden; abdominal pleu- 



rites confused with the tergites 7. 



Motary styli of abdominal tergites in transverse rows of four each; abdominal 

 tergites with large lateral tubercles; spiracles not hidden but partly concealed 

 by the lateral tubercles of the tergites; abdominal pleurites well defined. . . 3. 



3. Supraorbital styli less than two in number; metasternal spine and inner spur of 



metatibia prominent; eighth abdominal tergite with two small, rounded 



tubercles, each bearing a terminal stylus at the posterior margin 6. 



Supraorbital styli two in number; no metasternal spine; inner spur of meta- 

 tibia not prominent; eighth abdominal tergite without tubercles as above. .4. 



4. Pronotal styli 16 in number, all situated near the margins; cerci short. 



Helophorus. 



Pronotal styli 24-26 in number, all of them not situated near the margins; 



cerci long 5. 



5. Pronotal styli 26 in number, long and slender; abdominal styli varying in size, 



the lateral tergal styli very long and slender, the pleural styli very short and 

 conical; eighth abdominal tergite without appendages resembling cerci; 

 externo-caudal angles of ninth segment not acute; cerci long, very crooked 

 and multiannulate distally Berosus. 



Pronotal styli 24 in number; abdominal styli subequal; eighth abdominal 

 tergite with a pair of appendages resembling cerci; externo-caudal angles of 

 ninth segment prominent and acute; cerci long, only slightly rcooked and 

 apparently not annulate Laccobius.. 



