Effect of Temperature and Hostplants on Survival, Development and Body-Size in 3 Tropical Satyrine Butterflies From North-Eastern Australia

Abstract

The influence of temperature on egg survival and development rate, and the effect of two hostplant species, native kangaroo grass, Themeda triandra, and introduced Guinea grass, Panicum maximum, on larval survival, development and pupal weight, were examined in three species of tropical satyrine butterflies of the genus Mycalesis (the bushbrowns) under laboratory conditions. In all three species maximum egg survival occurred at 26 degrees C. Eggs of M. perseus survived well between 17 and 35 degrees C, whereas those of M. terminus and M. sirius survived poorly above 30 degrees C and, in the case of the latter species, below 23 degrees C. Maximum egg development rate occurred at 35 degrees C in M. perseus and about 30 degrees C in the two other species. Thus, optimal temperatures for development and survival were in the range of 30-35 degrees C for M. perseus and 25-30 degrees C for both M. terminus and M. sirius. Temperature thresholds were similar in all three, about 11 degrees C. Larvae of the three species survived significantly better on Themeda but took substantially longer to develop and attained lower pupal weights than when reared on Panicum, especially those of M. sirius. On the latter host at 25 degrees C, development from egg to adult varied from 40.4 days (male M. perseus) to 50.4 days (female M. terminus). Development in M. sirius was more variable, with larvae completing either five or six larval instars. Overall, males developed significantly faster than females, and males were smaller in size than females in all three species. Under field conditions populations are thus seasonally multivoltine, protandrous and sexually dimorphic with respect to body size. When reared on Panicum at 25 degrees C, a positive correlation between mean development time (from egg to adult) and mean body size (pupal weight), between species within the genus, revealed that smaller adult size in M. perseus was associated with faster maturation, while larger adult size in M. terminus was associated with slower development. Despite body-size effects, differences in rate of development and embryonic survival in these satyrines are closely tied to ecological differences in geographic distribution, habitat preference and resource durational stability. Potential selective pressures likely to influence size and development amongst these species, and between males and females, are discussed.