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In the key to American Chaoboridae:

First tarsomere shorter than second. Halter with numerous setae on stem and knob. Gonostylus with spiniform seta at apex. ... Mochlonyx

First tarsomere longer than second. Halter with two groups of two to five setae on knob. Gonostylus without apical spiniform seta at apex. ... Chaoborus