Damselflies show abrupt, darting flight, which is the envy of aero-engineers. This amazing ability is used both to capture prey and, by males, to establish territories that can attract females. Insects are ectothermic, so maintaining this flying ability in the face of fluctuating environmental changes is a major challenge. Furthermore, body size has both a direct effect on manoeuvrability and an indirect effect, through its impact on heat retention. Two Japanese researchers from Kyoto University, Yuka Samejima and Yoshitaka Tsubaki, have studied how body size and temperature affect flight ability in this stunning insect.
The damselfly they chose to study – Mnais costalis – lives by fast-flowing mountain streams and shows male polymorphism: orange-winged larger males tend to have territories while clear-winged smaller males do not. These morphs reflect different mating strategies, with smaller males ‘sneaking’ mating opportunities.
The authors used an infrared thermographic camera to measure the surface temperature of males, which they manipulated in the laboratory by using a halogen lamp, and studied the flight performance of each male. They estimated maximum lifting force and size-corrected lifting force, which they measured by attaching weights to the insects' wings with fishing line. Size-corrected lifting force is an index of acceleration that is linked to the damselfly's superb aerial acrobatics.
The authors found that both measures of flight performance were positively correlated with body temperature. This is not particularly surprising, as it is well known that insect flight muscle activity increases with temperature. However, although body size led to higher maximum lifting force, it was negatively correlated with size-corrected lifting force. Simply put, larger males were less agile.
When the authors took their thermographic camera into the field, they discovered that the story was even more complex: larger, territorial, males showed substantial variation in body temperature, as their territory showed varying patches of light and shade. Smaller, non-territorial males, however, generally had higher body temperatures, as they tended to bask in sunlit areas, as part of their ‘sneaky’ mating strategy.
This combination of behavioural ecology and physiology enriches our understanding of the maintenance of polymorphic mating strategies in this species. Due to their smaller body size and their more constant, higher body temperature, smaller males are apparently more agile, and therefore gain an advantage in terms of ‘sneaky’ mating and avoiding predation. However, their smaller size means that they are less able to lift females – essential during mating – or to combat larger males.
The best strategy, it would appear, would be to be a large male with a perpetually sunlit territory. Indeed, the authors' unpublished data suggest that such males have higher reproductive success. However, such territories are rare and may be temporally or physically fragile; natural selection has led to the current polymorphism of alternative male strategies, with underlying alternative physiologies. Who would have thought that the beautiful flight of the damselfly concealed such complexity?
