Damselflies in the genus Mnais are unusual in having two types of males: i) orange-winged morphs and ii) clear or glassy- winged (hyaline) forms which look like the females. The orange-winged morphs are highly territorial and fight with each other over desirable territories which are attractive to the females for egg laying. The clear-winged morphs are not territorial and usual lurk in concealed spots around the margins of an orange-winged male’s territory, trying to copulate with any female that they can catch! The two morphs are essentially adopting different strategies for securing mates, and have been called ‘fighters’ and ‘sneaks’.
I did not know all this when I first started photographing these damselflies in Thailand, where one species, Anderson’s Greenwing (Mnais andersoni), is quite common in Chiang Mai province in the north of the country. The males are very eye-catching with their bright orange wings and yellow/green thoracic markings; mature males also have a chalky blue pruinosity – or dusty-like coating – covering the upper thorax and the eighth and ninth segments of the abdomen (below).
This chalky blue colour on the males almost looks like make-up to me! In some Mnais species , the thoracic colour changes from metallic green to copper, with maturity. Seen in detail, the thoracic strips of the mature males are very attractive (below).
The extent of the pruinosity on the thorax is apparent in the following photograph, a close up from above (below).
The orange wing is a form of ornamentation. The pigment levels increase during the breeding season but differ between individuals. It is thought that these secondary sexual characteristics are physiologically costly to produce, and are used by the females as a way of assessing the quality or condition of the males before deciding which individual to mate with.
Clear-winged males look very different from the orange-winged forms (below); they clearly lack the pruinosity on the thorax, and any pigmentation in the wing, apart from the tiny red pterostigma near the wing tip.
The transparent wings of the hyaline-winged males (and females) become light amber, coloured with age. Photographs of immature and mature stages of the two male morphs, and the females, are shown on Dennis Farrell’s terrific website on the dragonflies and damselflies of Thailand. It is thought that the colour changes and polymorphisms of the male morphs of M. andersoni are analogous to other species of the genus. Clear-winged males of Mnais pruinosa – and probably other Mnais species – are smaller, and are less successful in terms of reproduction per day than the more aggressive orange-winged forms, but they live longer! So over the course of their lifetimes, their relative reproductive successes are similar. The orange-winged forms seem to burn themselves out faster; through mating and fighting!
All females have hyaline wings (below) and can be readily distinguished from the males by the bulbous tip of their abdomen, which contains the ovipositor (below). Females have a white pterostigma (tiny cell on the leading edge of the wing near the wing tip) which is just visible in the following photo.
These different strategies of the two male morphs come with their own sets of costs and benefits. The timid, clear-wing males avoid confrontation, whereas the orange-winged forms always respond aggressively to the sight of another pair of orange wings! If successful in their contests, the orange-winged morphs gain a prized territory and benefit from the mating opportunities it affords them. The clear-winged forms on the other hand, benefit from not using up resources in fighting and resort to subterfuge, trying to mate with females attracted to the orange-wing territories. Their clear wings make it easier for them to lurk undetected (crypsis) and they also benefit from not having to invest resources in producing wing pigments. The downside (cost) of their ‘sneaky’ strategy, is that they get to mate less (on a day-to-day basis); but remarkably, they make up for this is the long run by living longer.
What is remarkable, is that the two strategies appear to be equally successful, although this may depend to some extent on the habitat and other contingent circumstances. The clear-winged males can be thought of as mimicking females, and their hyaline wings induce less aggression from orange-wing males than other orange-wing forms. They rely on subterfuge rather than aggression, but the females are not passive partners in all this, and will be making their own choices of whom to mate with. Females frequently mate with the non-territorial hyaline-winged males and it is possible that they have some hidden charms that the more bright orange, ‘in your face’ males lack?!
The damselflies shown here were photographed on Doi Inthanon – the highest mountain in Thailand.
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