166 COLEOPTERA OF NORTH AMERICA. 



These insects are only found adliering to stones or plants be- 

 neath the surface of the water; the larvae are similar in form to 

 those of the other sub-families, except that the segments are not 

 united to the margin, which thus appears incised. 



Head protected beneath by a lobe of the prosternum ; 

 Ant<!unEe 11-jointed ; 



Anterior tibia; pubescent internally. Elmis. 



Anterior tibiffi glabrous internally. Stenelmis. 



Antenna 6-jointed. Macronychus. 



Head free; prosternum not lobed ; antenna^ 11-jointed. Ancyronyx. 



Ancyronyx occurs in the Atlantic region only; the other genera 

 are represented on both sides of the continent. 



Fam. XXXVIII.— HETEROCERIBAE. 



Mentum large, oblong, deeply emarginate in front; ligula 

 coriaceous, prominent, bilobed, without paraglossas; palpi 

 3-jointed, moderately long. 



Maxillae exposed at the base, wliicli is elongated ; lobes 

 two, coriaceous, not armed, but sparsely ciliate; palpi 4- 

 jointed, short. 



Antennae inserted at the internal margin of the eyes, but 

 in front, short, 11-jointed, joints 5-11 forming an oblong 

 serrate club. 



Head large; eyes rounded, finely granulated; front promi- 

 nent; labruni large, rounded, ciliate over its whole surface; 

 mandible stout, prominent, fringed internally with a ciliate 

 membrane, and furnished externally wath a strong carina. 



Prothorax transverse, with rounded angles, side pieces not 

 separate; prosternum lobed in front, acute behind; anterior 

 coxal cavities widely open behind. 



Mesosternum very short, deeply emarginate; side pieces 

 small, diagonally divided. 



Metasternum moderate, meeting the first ventral segment; 

 side pieces wide. 



Elytra entirely covering the abdomen. 



Abdomen composed of five nearly equal ventral segments, 

 the fifth only being movable, the others connate; the first 

 marked each side with an elevated curved line reaching the 

 posterior margin.* 



* This elevated line is finely striate transversely, and is a stridulating 

 oro-an ; the hind legs, by friction against it, produce a quite distinct sound. 



