Couplet 4

4a:  Hind coxa greatly enlarged, a ventral plate concealing trochanter and the basal half of femur, covering most of 3 basal ventrites (Figure 26-19B). 

Haliplidae   p. 405

4b:  Hind coxa greatly enlarged or not, if hind coxa greatly enlarged, then all ventrites visible laterally, coxa not concealing trochanter, basal half of femur or first 3 ventrites (Figure 26-4, 26-17)