Insect size and shape responses to temperature : a case study of British Odonata and Chironomidae (Diptera)

This PhD project investigated the response of wing length (as a proxy of body size) of Odonata and Chironomidae, and wing shape of Odonata to temperature and latitude. Three complementary data sources were used: natural history collections, field data, and mesocosm experiments. While natural history collections are valuable resources of specimens collected over long historical time scales, field data provides a modern perspective and mesocosm experiments provide a window on the future, under a predicted climate warming scenario. This PhD project used 5,331 museum specimens of 14 British Odonata species representing different life cycle types to examine the potential drivers of body size and wing shape responses to latitude and temperature (Chapter II and V). To control for latitude a field survey was performed during the summer of 2018 at Edington, Somerset to compare modern wing length of three species with historical data based on museum specimens of the same species collected at the same locality (Chapter III).Furthermore, to investigate body size responses of chironomids under a future temperature scenario, 1,976 adult specimens of six chironomid species were collected from mesocosm experiments which comprised ponds at ambient temperature and ponds maintained at 4℃ higher than ambient (Chapter IV). The results of Chapters II, III and IV showed that species and suborder (within Odonata) were significant factors affecting the magnitude of the temperature-size responses in Odonata and Chironomidae. Wing lengths of Zygoptera (Odonata) and Chironomidae are more sensitive to temperature and collection date than Anisoptera (Odonata). Zygoptera and Chironomidae tend to get smaller with increasingt emperature, likely due to higher temperatures disproportionately increasing developmental rate, resulting in smaller adults. Anisoptera showed no significant correlation with temperature, possibly due to selection for larger individuals in Anisoptera which are strongly territorial species. Adults of Zygoptera and Chironomidae emerging towards the end of the summer tend to be smaller than those emerging earlier in the season, likely due to larval development being time-constrainedlater in the season and as a result, larvae accelerate their developmental rate which comes at the cost of a smaller adult body size. The results of Chapter V indicated non-significant correlations between environment and wing shape in Anisoptera, while there were significant influences of latitude and mean seasonal temperature on wing shape in Zygoptera species, with broader and shorter wings found at lower latitudes with warmer temperatures. This finding corresponds well with a result of Chapter II which found shorter wing length with increasing temperature in all zygopteran species in the study. Overall, the results of this PhD project show that there are different factors influencing the temperature-size responses of insects, including phylogenetic relationships, sex, behaviour and life cycle types. Although this study found no universal temperature size responses in the focal taxa, Zygoptera and Chironomidae tend to have stronger negative body size responses to warming temperature and emergence date than Anisoptera. In addition, the study shows that wing shape variation in Zygoptera is more sensitive and adaptive to latitude and temperature than in Anisoptera.