k. Membrane large, furrow of the embolium broadened. Z. aurantiacum, fluminea, etc. Zaitha
kk. Membrane very short; furrow of embolium not broadened. Western genus. Pedinocoris
ii. Metasternum with a long median longitudinal keel. Southwestern forms. Abedus ovatus and Deniostoma dilatato
hh. Anterior coxæ inserted at the posterior margin of the prosternum; legs natatorial. Back swimmers ([fig. 19 b.]). Notonectidæ
i. Apices of the hemelytra entire; the three pairs of legs similar in shape; beak three-segmented; abdomen not keeled or hairy. Plea Leach
ii. Apices of hemelytra notched; legs dissimilar; beak four-segmented; abdomen keeled and hairy.
j. Hemelytra usually much longer than the abdomen; fourth segment of the antenna longer than the third segment; hind tarsi with claws. Bueno Kirk.
jj. Hemelytra but little longer than the abdomen; fourth segment of the antenna shorter than the third segment; hind tarsi without claws ([fig. 19b]). Notonecta L.
ff. Antennæ longer than the head; or if shorter, then the eyes and ocelli absent.
g. Eyes, ocelli, and scutellum wanting; beak three-segmented; head short; hemelytra always short; membrane wanting. Insects parasitic on bats. Polyctenidæ