On sunny days, the glint of a banded or beautiful demoiselle is quite a sight to behold. At just 4.5cm long, these species are Britain’s biggest damselflies – the smaller, slenderer cousins of dragonflies. Their two pairs of wings are also the same shape, whereas dragonflies tend to have bottom wings that are broader and shorter than the top ones. In comparison to that of their larger relatives, the flight of damselflies is slow, gentle, and fluttery.
Like many wildlife species, demoiselles are sexually dimorphic, which means that the sexes are visually distinguishable from each other. The males are inky blue with a turquoise sheen (both species), while the females are metallic green (banded) or greenish-bronze (beautiful). Due to these similarities, the two species can look very similar at first glance, but they can quickly be identified by their unique wing colours.
Unlike dragonflies, which hold their wings out to the side, demoiselles fold theirs vertically along their bodies. Those of the males are dark all over (beautiful) or have a dark patch in the middle (banded). In females, the difference is more subtle: their wings are brown (beautiful) or green (banded), with a faint white spot at the tip. In fact, demoiselles are the only British damselflies whose wings have colour!
Demoiselles start their lives as eggs, which the females inject into the stems of plants like water crowfoot – they may even go below the surface to find a good spot. Two weeks later, the young hatch as larvae; a life stage that lasts for two years. They spend this time among submerged plants, eating smaller invertebrates and hiding from predators. In the higher flows of winter, they may hunker down in mud, silt, and gravel.
Upon reaching adulthood, the larvae climb up emergent plants that root in the water but extend above the surface. Once in the fresh air, they shed their exo-skeletons and take to the sky on their newly unfurled wings. In flight, both species are fierce predators that catch other insects with their six bristly legs. You might also see them basking in the morning sun and sheltering in vegetation as night falls or wet weather arrives.
For the most part, though, demoiselle adulthood is about reproduction. The males take and defend small territories, performing dances to attract passing females. If the wooing goes well, each pair forms a “wheel” or “heart” shape for mating, and he leads her to a good egg-laying site. Look out for this amazing spectacle between April and October, but particularly on bright, dry, windless days between May and August.
The lush plant life of our chalk streams makes them great habitats for both demoiselle species. Being more sensitive, beautiful demoiselles are especially fond of the gravelly beds, consistent temperature, and clean, oxygen-rich water. If we want to give these incredible insects a home, we need to keep our streams healthy by tackling pollution and agricultural runoff, reducing water abstraction, and letting vegetation flourish along the riverbanks.
If you’d like to learn more about chalk stream wildlife, check out the Watercress and Winterbournes scheme by visiting hiwwt.org.uk/winterbournes. Our work is supported by the National Lottery Heritage Fund.
Photo: Male banded demoiselle Copyright © Mark Hamblin – 2020VISION
