e. Hemelytra usually with a distinct clavus ([fig. 159]), clavus always ends behind the apex of the scutellum, forming the commissure. (Species having the wings much reduced or wanting should be sought for in both sections.)
f. Antennæ very short; meso- and metasternum composite; eyes always present.
g. Ocelli present; beak four-segmented. Ochteridæ and Nerthridæ.
gg. Ocelli wanting; antennæ more or less hidden in a groove.
h. Anterior coxæ inserted at or near anterior margin of the prosternum; front legs raptorial; beak three-segmented. Belostomidæ (with swimming legs), Nepidæ, Naucoridæ.
i. Metasternum without a median longitudinal keel; antennæ always four-segmented.
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.