The life-cycle of one of Britain’s rarest insects, the caddisfly Hagenella clathrata, is inextricably linked to Purple Moor-grass Molinia caerulea, a plant that is not always welcomed in habitat management circles.
Over the past few years I have developed an interest in caddisflies – their life-cycles and ecology in particular – thanks in part to the enthusiasm of friend, neighbour and seasoned entomologist Graham Vick. All caddis are tied to water in one way or another by the aquatic or semi-aquatic requirements of their larvae and in Britain the group comprises some 200 species. Larval cases arouse a passing curiosity in most naturalists but the adults must be one of the most overlooked and neglected insect groups going.
In a way I can understand this given that with many species adults are enigmatically hard to observe and generally even harder to identify: the majority look rather uniformly brown and examination of wing venation and genitalia structure are needed to be certain what you are looking at. There are a few exceptions of course and my interest was piqued by Graham’s tales of one in particular – Hagenella clathrata. It sometimes goes by the English name Window-winged Sedge, the term ‘sedge’ being a fly-fishing soubriquet applied to the group as a whole. Hagenella must be one of the most attractive caddisflies going. It is also one of the rarest (or perhaps least-recorded), being known from just a handful of sites on raised bogs, wet heaths and the margins of enchantingly-name schwingmoors (quaking bogs to you and me). Northern Britain was thought to be its stronghold until relatively recently when its presence was confirmed on a few Surrey heaths by a band of seasoned local entomologists, along with Graham and legendary caddis expert Ian Wallace.
Above: Hagenella clathrata, one of Britain’s rarest and most striking caddisflies. Paul Sterry/Nature Photographers Ltd
Given Hagenella’s distinctive appearance and the fact that adults are day-flying you might imagine it would be easy to observe. Not a bit of it. It bears a superficial resemblance to a Chequered Skipper Carterocephalus palaemon and has all of that species’ frustratingly enigmatic field characters. The following summary of its habits is based on personal observations and more extensive field work by Graham and Ian. Hagenella is on the wing in late May and early June, only flies on hot, sunny days, and seemingly only between the hours of about 16.00 and 17.30. On dull days, and when the sun is obscured by a passing cloud, it disappears without a trace. During the appointed time and only if the sun is shining, males perch on birch and willow bushes and take to the wing (flying as fast as a dragonfly) if they get a pheromonal whiff of a female (at least that’s what I assume stirs them into action). Females seem to lurk around clumps of vegetation and dive for cover at the first sign of danger.
Above: Egg mass and developing larvae of Hagenella clathrata. Paul Sterry/Nature Photographers Ltd
Above: Hagenella larva with its case made from fragments of birch leaves and strips of Molinia stem. Paul Sterry/Nature Photographers Ltd
My ecological curiosity was aroused by Hagenella’s chosen habitat. As with other locations in the UK, on Surrey heaths it is entirely restricted to areas where Molinia caerulea (Purple Moor-grass) is dominant. Previously, I had only really come across Molinia in conservation literature alongside the words ‘control’ and ‘management’. Apparently it can interfere with the perceived ‘favourable condition’ habitat quality so beloved of statutory nature conservation bodies with regards to Sites of Special Scientific Interest in particular. In places it is certainly a dominant plant but perhaps that might be for good, biological reasons. Anyway, regardless of its appearance and status it is vital to the life-cycle and survival of Hagenella clathrata. In prime Molinia habitat, dank-looking pools are present between mature clumps and it is amongst the weedy-looking Sphagnum and decaying plant debris that the remaining stages of the life-cycle (egg-laying, larval growth and pupation) appear to take place.
Above: A typical clump of Molinia, otherwise known as Purple Moor-grass. Paul Sterry/Nature Photographers Ltd
So I would urge anyone with responsibility for habitat boasting swathes of Molinia clumps to be more circumspect in their approach to ‘managing’ the plant. Who knows, you might have one of Britain’s rarest insects hiding in plain sight on your land. With such a tricky species to observe, absence of evidence is not evidence of absence. For more information about Hagenella, look at Ian Wallace’s excellent Buglife dossier on the species: https://www.buglife.org.uk/sites/default/files/Hagenella%20clathrata%20species%20dossier.pdf
As a further indication of the challenges of observing caddisfly adults and how easy it is to overlook them, it is worth considering one of the commonest species in the part of north Hampshire where I live. Here, Glyphotaelius pellucidus is associated with seasonally-drying woodland streams. The larvae make distinctive, flattened cases from leaf fragments and are fairly easy to find as they trundle along stream beds in late winter. But the egg-masses are the most conspicuous stage in the life-cycle: protected in jelly-like blobs, they are laid on leaves overhanging streams in early autumn. That the streams are typically dry at that time of year makes what happens next all the more remarkable: come the rains and flowing streams, the egg-masses liquefy and drip tiny wriggling larvae into the water below. How the adults recognise sites that will subsequently become streams, and hence where to lay their eggs, remains a mystery.
Above: Jelly-like egg masses of Glyphotaelius pellucidus. Paul Sterry/Nature Photographers Ltd
Above: Adult Glyphotaelius pellucidus. Paul Sterry/Nature Photographers Ltd
Despite the relative abundance of egg-masses (perhaps 100+ suspended over a 20 metre stretch of stream) I can probably count the number of times I have ever seen an adult (away from a light trap) on one hand. And that’s with a species that is undeniably common and an observer who is actively on the look-out for them.