Predator Avoidance by Cricotopus: Cyclomorphosis and the Importance of Being Big and Hairy

In laboratory experiments, we tested in importance of body size and long abdominal hairs in determining susceptibility of two aquatic midges, Cricotopus sylvestris and C. bicinctus, (Diptera: Chironomidae), to predation by Hydra and the damselfly Ischnura verticalis. The chironomids co—occur on dense macrophytes in the Yahara River, Dane County, Wisconsin, USA, and differ structurally in that C. sylvestris has long hairs and C. bicinctus has short hairs. We also monitored Hydra density and C. sylvestris hair length for 2 yr. Large C. bicinctus were less susceptible to predation by both Hydra and small Ischnura, but prey size had no effect on susceptibility to large Ischnura. Long hairs offered no protection from Ischnura, but did protect C. sylvestris from Hydra, relative to short—haired C. bicinctus or C. sylvestris with experimentally shortened hairs. Hair length is negatively allometric relative to body size in ontogeny; hair absolute length remains constant as the larvae increase in size. Hair length was also cyclomorphic; successive generations of C. sylvestris showed a seasonal pattern of hair length. Hydra density also varied seasonally. When Hydra density was high, hairs on C. sylvestris were longest; when Hydra density was low, hairs were shortest. The observed cyclomorphic and allometric patterns of hair length were both beneficial characteristics in the presence of Hydra, and the benefits were additive.