HETEROCERIDAE. 117 



Body rounded ; antennae distant, club slender. Lutrochus. 



Body oblong, elongate ; 



Antennae approximate, second joint moderate, club pectinate. 



Pelonomus. 



Antenna; distant, second joint much dilated, club lamellate. Helicuus. 



Lutrochus luteus is found in Texas; Pelonomus obscurus in 

 the Southern and Western States ; Helichus is widely distributed, 

 and is fouud clinging to stones under water, in rapid streams ; 

 the other two genera are found at the margin of streams, under 

 stones, &c. 



Sub-Family III.— ELMIDAE. 



The anterior coxae are rounded, without trochantin ; the abdo- 

 men has five ventral segments, the fifth rounded at tip ; the front 

 is not prominent ; the palpi are short ; the antenna? inserted upon 

 the front, near the eyes, slender, slightly thickened externally ; 

 middle eoxie widely distant ; posterior coxae separated, transverse, 

 not dilated into a plate protecting the thighs ; legs exceedingly 

 long ; side pieces of the metathorax narrow, epimera not visible ; 

 epipleurae narrow, extending to the apex. 



These insects are only found adhering 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 ; 

 Antennae 11-jointed ; 

 Anterior tibiae pubescent internally. Limjuus. 



Anterior tibiae glabrous internally. Stexelmis. 



Antennae 6-jointed. Mackonychus. 



Head free ; prosternum not lobed beneath ; antennae 11-jointed. 



Ancyeoxyx. 



Xo species of this sub-family has occurred in our fauna, except 

 in the Atlantic district. 



Fam. xxx.— heteroceejdae. 



Mentum large, oblong, deeply emarginate in front; ligula 

 coriaceous, prominent, bilobed, without paraglossee ; palpi 

 3 jointed, moderately long. 



