SECTION IV 



KEY TO GENERA OF AQUATIC AND SEMI -AQUATIC DRYOPOID BEETLE LARVAE 



OF THE UNITED STATES 



1 Broadly ovoid in shape and very much flattened; lateral 



margins of each segment greatly expanded, the head com- 

 pletely concealed from a dorsal view by the expanded 

 anterior pronotal margin (water pennies) . . . PSEPHENIDAE ... 33 

 Less broad and flat, usually slender with a round or 



triangular cross section; head exposed from dorsal view .... 2 



2 (1) Ninth abdominal segment with a movable ventral operculum 



closing a caudal chamber (Fig . 13) 3 



Ninth abdominal segment without an operculum. .PTILODACTYLIDAE. . 7 



3 (2) Body cylindrical, with abdominal sternites and pleurites 



greatly reduced, the tergites almost forming complete 

 rings on first 5 segments and forming complete ones on 

 segments 6-9; without retractile gills; abdominal spir- 

 acles lateral on segments 1-7 and dorsal on segment 8 



(Unlikely to be found in our streams.) DRYOPIDAE 5 



Body usually not cylindrical; abdominal sternites not 

 greatly reduced on anterior segments; with retractile 

 filamentous caudal gills emerging from caudal chamber 4 



4 (3) Operculum with a pair of internally attached hooks (Fig. 13)... 9 



Operculum without hooks, but with a flat, movable, dorsal 

 sclerite attached to each lateral margin; thoracic seg- 

 ments and first 8 abdominal segments each with a 

 dorsolateral flattened projection bearing many hairy 

 filaments (Fig. 132): CHELONARIIDAE: ChelonaHion 



Figs 132-133 Left lateral aspect of larva: 132- Chelonariim sp. 

 (from Boving § Craighead); 133- Dryops sp. (from Bertrand) . 



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