Faculty Mentor Department

Keywords

Abstract

Extremes of body size captivate biologists. In insects, the lack of extant giants has prompted the question, what is constraining insect size? While multiple physiological and ecological hypotheses have been presented, there is no widely accepted explanation. One unexplored physiological hypothesis is that large insects are unable to shed metabolic heat rapidly enough and are at increased risk of overheating. My project examines this idea using larvae of the Japanese rhinoceros beetle (Trypoxylus dichotomus), chosen for their huge size, simple body plan, and underground lifestyle. Using CO2 respirometry, I measured larval metabolic rates at room temperature. Although these beetle larvae are among the largest insects ever measured, their metabolic rates fell squarely on the expected values extrapolated from other, smaller insects. This permitted me to build a simple mathematical model of heat balance for insects across a wide range of body sizes. Specifically, I converted my measured rates of gas exchange into rates of metabolic heat production, and used the model to predict how much equilibrium body temperature would increase in insects larger than those that naturally occur. I then used CO2 respirometry during temperature ramping experiments to determine larval critical thermal maxima (CTmax). This showed that larvae could survive temperatures of 43.5-47˚C. Together, these show that for every 100-g increase in body size, there is a 0.5˚C increase in equilibrium body temperature, and that body temperatures are predicted to be well below the thermal maximum for animals ten times larger than any extant insect. In addition, larvae placed on runways extending across a thermal gradient were surprisingly active, and clearly capable of behavioral thermoregulation through movement to cooler locations. Collectively, my results suggest that insect size is not limited by metabolic heat production. This study provides a greater understanding of insect size constraints and behaviors associated with thermal regulation.