90 Psyche [June 



3. Antennse exposed; front legs similar to intermediate; rostrum 4-segmented, 



first segment short; front and intermediate tarsi 1-segmented (?), hind 3-seg- 



mented; predatory OCHTERK)^ 



Antennse concealed; front legs raptorial; rostrum short, 4-segmented; front 

 and intermediate tarsi 1-segmented, hind 2-segmented; eyes very protu- 

 berant; short, stout, roughened forms; predatory 



*MONONYCHID^ or NERTHRID^ 



4. Hind tarsi without two distinct claws (except Plea which is less than 3 mm. 



in length); front legs not raptorial in structure; moderate or small in size. .5 

 Hind tarsi with claws; front legs raptorial 6 



5. Head overlapping thorax dorsally; body flattened above; rostrum short, hidden 



under a broad flat transversely striated plate (epistoma); front tarsi- 1-seg- 

 mented, palaeform (fig. 28); male genitalia asymmetrical; predatory; swim 



with dorsal side uppermost CORIXID.ffi 



Head inserted in thorax; body convex above; rostrum free, 3- or 4-segmented; 

 tarsi 2-segmented (front tarsi of the male sometimes 1-segmented) ; pre- 

 datory; swim with ventral side uppermost NOTONECTID.^ 



6. Membrane reticulately veined; large in size 7 



Membrane without veins; rostrum 3-segmented; front tarsi 1-segmented, 



intermediate and hind 2-segmented; hind legs not natatorial; predatory; 

 moderate in size NAUCORTD.^ 



7. Apical appendages of abdomen long and slender, not retractile; hind legs not 



much flattened; rostrum 3-segmented; tarsi 1-segmented; hind coxae rota- 

 tory; predatory NEPID.SI 



Apical appendages of abdomen short and flat, retractile; hind legs distinctly 

 flattened, natatorial; rostnmi 3-segmented; tarsi 2-segmented; hind coxae 

 hinged; predatory BELOSTOMATID^ 



8. Head shorter than thorax including scutellum 9 



Head as long as entire thorax, both elongated; ocelli absent; eyes distant from 



anterior margin of thorax; rostrum 3-segmented; tarsi 3-segmented; body 

 and appendages slender; generally wingless; predatory; aquatic, living on 

 the surface near shore; length about 10 mm HYDROMETRIDiE 



9. Claws of at least the front tarsi distinctly anteapical with terminal tarsal seg- 



ment more or less cleft (fig. 12); aquatic, living on the surface 10 



Claws all apical, last tarsal segment entire; aquatic and terrestrial forms. . .11 

 10. Hind femora extending much beyond apex of abdomen; intermediate and 

 hind pairs of legs approximated, very distant from front pair; ocelli present, 

 but sometimes very obscure; eyes close to anterior margin of prothorax 

 (fig. 29); rostrum 4-segmented, first and second segments short (fig. 29); 

 tarsi 2-segmented; parts of hemielytra more or less confluent (fig. 16); 



often wingless; predatory; moderate in size GERRID.ffi 



Hind femora not extending much beyond apex of abdomen; intermediate pair 

 of legs about equidistant from front and hind pairs (except in Rhagovelia); 

 oceUi obsolete or absent; rostrum 3-segmented; front tarsi 1- or 2-segmented, 

 intermediate and hind 2- or 3-segmented; predatory; usually live on surface, 

 but some species can dive; moderate or minute in size VELUD.^ 



