Testing life history growth patterns in the aquatic dipteran larvae of Corethrella appendiculata

Background/Question/Methods

Organisms exhibiting complex life cycles may face tradeoffs in nutrient storage and timing of life history transitions when encountering suboptimal conditions. Among holometabolous insect species, a variety of developmental strategies have been discovered in the transition from larval to adult life stages, including plasticity or canalization in the pupation threshold, post-threshold period and final adult size. The developmental strategy of an aquatic dipteran was investigated using final instar larvae of the midge Corethrella appendiculata, a predator of container-dwelling mosquitoes. Previously developed models comparing daily food intake to total development time in a variety of nutrient environments were used to differentiate among competing hypotheses of plasticity and canalization in the nutrient threshold required for pupation and in the length of the post-threshold period.

Results/Conclusions

Final instars of both male and female C. appendiculata larvae exhibited a canalized pupation threshold followed by a canalized post-threshold period. The best-fit developmental model predicts that males must reach a nutritive threshold of 6.73 mosquito prey after which 6.33 days of post-threshold development occur before pupation. Females show a higher threshold of 12.09 prey followed by a 7.23 day post-threshold period. Corethrella appendiculata also displayed protandry with males entering the final instar earlier than females, while final instar females consumed more prey per day suggesting a possible tradeoff in optimal developmental rate between sexes. The canalized developmental pattern displayed in C. appendiculata larvae is similar to the strategy elucidated in Manduca sexta caterpillars and is likely tied to the preferred habitat of the midge. Midge larvae are found in relatively established, permanent container habitats which may contribute to the canalized development and lengthy post-threshold periods when compared with aquatic dipterans from more temporary container environments.