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The Naucoridae and Belostomatidae are flattened insects, with four-segmented feelers and fore-legs inserted at the front of the prosternum.

By means of the oar-like hind-legs they swim actively through the water with the ventral surface upwards; the fore-legs are inserted at the hinder edge of the prosternum.

The prosternum is very large, subtriangular, concealing the insertion of the cox�, and extending over the front part of the mesosternum, as does this over the front of the metasternum.

Dorsal view of a male with the appendages cut short. a, Prosternum of prosoma. b, Metasternum of prosoma. prae-gen, The prae-genital somite.

Sternal elements of prosoma almost entirely absent, traces of a prosternum and metasternum alone remaining.