Insect coloration is diverse in appearance and function, and the cosmopolitan order Odonata containing both dragonflies and damselflies displays a wide range of wing colors in addition to a sophisticated vision apparatus (3, 28, 31). For odonates and many other insect groups, colors serve multiple functions including visual signaling such as intra- and inter-species signals, warnings to predators and camouflage, or in some cases as a means to support thermoregulation or desiccation tolerance in more adverse climatic conditions (2, 1, 5, 29). Previous work focusing on odonate wing color has documented complex ecological and evolutionary forces and interacting drivers that may be governing wing coloration (7, 8, 9).
Of particular note are key color differences between sexes (7, 9) related to their differing life history strategies. Male odonates often fly to secure and defend suitable territory for foraging and mating while females can be more cryptic, focusing on avoiding predators and seeking to oviposit eggs. Males with darker (7) or more vibrant wings (8, 19, 27) have been linked to more successful territoriality displays. These males have also been observed to be more successful in attracting females, demonstrating that wing color can also be a sexual ornament for males that either distinguishes the sexes or signals quality or age (9,19,27,42,43). In summary, the diverging wing color patterns of males and females appear to fit their divergent needs related to their life histories. Beyond key differences across sexes, odonates species also differ in other aspects of flight behavior(10). “Percher” dragonfly species spend most of their time stationary, making short but energetic flights, while “flier” species move continuously during active times. Percher flight style is associated with territoriality – male odonates choose perches in high-quality territories and aggressively defend this territory against intruding males (11). Thus, percher males may show increased wing coloration as a way to signal broadly to conspecific or heterospecifics. While there are likely some intermediates between percher and flier (10), a key point is that different species-specific flight behaviors may have a particularly strong impact on wing color, with the strongest forces operating on male wings.
So far we have focused on behavior and life-history characteristics, but structural constraints may also determine coloration. Organisms that are larger have a smaller surface area to volume ratio, providing an insulating effect that slows the rate at which they gain and lose heat (12). However, odonate wings are relatively thin, and assuming wing area and thickness is isometric with body size, larger wings may actually lose heat-per-unit-area at a similar or faster rate to smaller wings. The veins and muscles that large wings rely on for motion and rigidity may also take longer to heat than in smaller wings (13). For these reasons, we might expect differences in proportion of coloration in wings depending on size if pigments have a role in mitigating heat loss or supporting heat gain. However, it remains uncertain whether using wing color to warm the small volume of hemolymph circulating through the wings has thermoregulatory value either for capturing or retaining heat (33, 34). The only clear evidence of heat transfer between body and wings in Odonata comes from the libellulid Zenithoptera lanei, which has a complex multifunctional structure of the wing membrane not yet found in any other odonates. In this group, the wings are filled with tracheae and tracheoles that turn the wings into thermal windows, besides melanized cuticle, structural coloration, wax coverage and nanospheres (35). However, even if there is no direct thermoregulatory function for wing color, dark coloration may still relate to climate, for example via resistance to other stressful factors such as increased ultraviolet radiation. As well, it may be that larger odonates have both competitive advantages in cooler climate and proportionally more wing coloration for success against competitors. In this scenario, the thermoregulatory effect is indirect via body size but leads to relationships among the three. For all these reasons, there may still be strong interactions between darker coloration, body size and climate.
Despite significant efforts to untangle the complex factors that are driving coloration of odonate wings, no studies have quantified the ecological and evolutionary forces driving wing coloration in a broad, comprehensive framework. Previous work has focused on the darkest color on wings, although more recent work has called into question making simplistic assessments of function using one color alone (37). It seems probable that other colors common on wings (e.g. orange, yellow, brown, or white color) might perform different functions or be evolving in a different way compared to more melanistic colorations. We therefore developed and utilized a novel color clustering approach that takes a data-driven approach to infer color groupings and identify what kinds of colors are present across odonates. We used this approach to test the predictions that percher species and males in general (across flight categories) have more dark wing pigment, with percher males having the most pigment. We also tested whether there is a relationship between body size, wing color and climate. As a means for comparison, we performed these same tests on lighter wing colors discovered during color clustering. Finally, we tested if our main delimited color groupings–one darker and one lighter–are phylogenetically correlated, e.g., appearing together across the Nearctic odonate phylogenetic tree. Our approach explicitly allows capturing presence and absence of coloration, as well as measures of proportion of wing containing a particular color, and provides a means to test key predictions while also accounting for spatial and phylogenetic autocorrelation. This work provides a framework that extends our ability to understand the forces shaping insect color and can be extended broadly to other taxa and body regions.