ABSTRACT
The egg and larval instars (L1-L4) of Culicoides sonorensis Wirth and Jones are described. Average (range) of body lengths of a laboratory colony (Van Ryn) were 0.45 (0.43-0.48) mm for eggs, 0.72 (0.39-0.92) mm for L1, 1.8 (1.3-2.2) mm for L2, 3.6 (2.2-5.0) mm for L3, and 5.9 (5.0-6.9) mm for L4. Head capsule lengths were 0.067 (0.064-0.070) mm for L1, 0.100 (0.094-0.110) for L2, 0.157 (0.144-0.168) for L3, and 0.316 (0.300-0.324) mm for L4. Wild and colony L4 larvae differed in many standard metrics such as head length or width, but head ratios and pharyngeal armature measurements were comparable. Standard external and internal features of the head capsule are described. Scanning electron microscope photos revealed pharyngeal armature structure was similar for L2-L4. The pharyngeal armature and piercing mandibles suggest a generalist feeder and opportunistic predator. Egg ansullae were randomly distributed (as opposed to rows in most other Culicoides spp.) and averaged 1.98 ansullae per 25 µm2 of egg surface. Ansullae were widened and porous apically, averaging 1.29 microns long, 0.65 µm at the base, and 1.06 µm at the apex. Only the L1 had a proleg, structurally complex, and featuring five rows of posteriorily directed, shorter spines (1-3 µm long) and 8-10 setae 18 µm long with hooked tips. On terrestrial mud, this proleg was used for surface crawling, and the L1 navigated dry conditions better than the more aquatic L2-L4 stages. Eggs are laid above waterline, and the L1 proleg is probably an advantage in terrestrial surface movement.
