High altitudes pose a number of problems for organisms. As you ascend a mountain, temperature falls by ~6 ° C per kilometer, and oxygen partial pressure (PO2) falls precipitously. These changes are likely to be particularly challenging for ectothermic insects, which can't regulate body temperature endogenously and which have the highest metabolic rates (and therefore the highest oxygen demands) known. Flying insects face a third problem--reduced air density. Forces produced by flapping wings depend on air density, so the altitudinal reduction in air density should challenge their ability to remain airborne.
One way that insects may overcome reduced air density at altitude is through changes in flight morphology. Increased wing surface area relative to body mass (i.e. reduced wing loading) could ameliorate the challenge of flight in reduced air densities. Museum data suggest this pattern for bumblebees (Dillon et. al, 2006)
Insects may also compensate for reduced air density at altitude by changing wingbeat kinematics. Ongoing work with bumblebees in the laboratory and field suggest that bumblebees increase stroke amplitude to fly in reduced air densities.
Melanie Frazier and I used a novel flight assay and a factorial experimental design to ask whether temperature, pressure, and their interaction affected flight ability and motivation in Drosophila melanogaster. Surprisingly, we found that flies were able to fly at nearly all temperatures and pressures, including 33% sea level air pressure. However, moderate decreases in temperature and pressure strongly decreased their motivation to fly. This led us to wonder to what extent flight motivation rather than physiological limitations could lead to the evolution of flightlessness at high altitude (Dillon & Frazier, 2006).
In collaboration with Ray Huey, I have been looking at the functional and molecular basis of thermal sensation in Drosophila (Dillon et. al, 2009). We are examining thermoregulatory behavior of fly mutants, flies with temperature sensing organs removed, and of flies lacking a single thermoTRP channel--TRPA1. Paul Garrity's group has shown that this single channel plays an important role in avoidance of high temperatures in Drosophila larvae. We have found similarly aberrant thermoregulatory behavior in adults lacking the channel. We have also done some artificial selection on thermal preference to look at fitness consequences of behavioral thermoregulation.
In collaboration with Robert Dudley, I have been looking at the scaling of insect flight performance. Load-lifting performance declines with body size among 11 euglossine bee species (Dillon & Dudley, 2004). We are interested in whether this is a general pattern across other bees and insects, particularly when phylogenetic relationships are accounted for.
We have a host of other ongoing projects/interests, including Bergmann size cline in insects, global temperature fluctuations, integrating physiology with global climate change, and thermoregulatory physiology across elevation.