Fig. 1. (A) Anterior aspects of Procambarus clarkii, showing the long paired second antennae (asterisks) and the biramous antennules, the lateral flagella of which are indicated by white arrows. There is an upward curve to each lateral flagellum, which in this large animal is 2.5 cm long. (B) Several annuli of a lateral antennular flagellum of P. clarkii. Aesthetasc sensilla (asterisks) are arrayed ventrally on each annulus of the distal half of the flagellum, usually in groups of 2–4. At least three other types of sensillum occur on the medial and lateral flagella, including the most numerous class, the beaked hairs, indicated here with white arrows, and a standing feathered hair (white caret).