j. Beak short, robust, conical; the hairy fleck on the corium elongate, large, lying in the middle between the inner angle of the membrane and the outer vein parallel to the membrane margin; membrane margin S-shaped. k. The thick fore femur with a relatively deep longitudinal furrow to receive the tibia. Several American species (fig. 19f.). BELOSTOMA (= Lethocerus Mayer) kk. The less thickened fore femur without such a furrow. B. GRISEUS. BENACUS Stäl. jj. Beak slender, cylindrical; the hairy spot on the corium rounded lying next to the inner angle of the membrane. 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 gg. Eyes present. h. First two antennal segments very short, last two long, pilose, third thickened at the base; ocelli present, veins of the hemelytra forming cells. DIPSOCORIDÆ (= CERATOCOMBIDÆ) including SCHIZOPTERIDÆ.