Transformation

I have been fascinated by the transformations of butterflies, moths and dragonflies since childhood; that one body form can be turned into something completely different is amazing. While the changes in butterflies are striking enough, it is those of the dragonflies that I have always found the most interesting. The transition from an aquatic larva to a winged, non-aquatic adult is truly remarkable. In some ways the metamorphosis of a butterfly appears to be a passive process, the larva settling down in a sheltered location to become a pupa from which an adult will ultimately emerge, often many months later. In dragonflies it feels so much more active and immediate, the adult emerging from a larval form that has itself only recently emerged from the water. Of course, this is an illusion in the sense that it is the adult itself, held within its old larval skin, which drags itself from the water to clamber up a suitable stem.

Many of the British dragonfly species (and I include damselflies under this broader title) emerge from mid- to late-morning, which offers the interested observer with the opportunity to witness the breathtaking series of behaviours which deliver the winged adult into the world. A few of the larger species, however, leave the water at dusk, completing their emergence by the early hours. Most individuals select an emergent plant as their place of transformation, clambering up to a position well clear of the water. Once a suitable position has been found, the larva attaches itself head up and then proceeds to swing its body about in a characteristic and jerky manner. It is thought that this behaviour, which also includes various circling leg movements, allows the dragonfly to determine that it is securely attached to its support and also that there are no obstructions close by that might hinder the expansion of its wings. If the wings push up against an obstruction during their expansion phase then they can be severely damaged.

The first stage of the transformation involves a switch from aquatic respiration to aerial respiration. The cuticle then splits and the head, legs and thorax are pulled clear of the larval skin. This appears to be a demanding process because it is invariably followed by a long period during which the insect remains immobile and resting. The abdomen is then withdrawn from the skin and the process of expansion begins, with both the wings and abdomen very slowly increasing in size. Once the process of expansion is complete, the now fully formed adult remains on its support until it is ready to take to the air for its maiden flight, leaving behind the larval skin and all evidence of its once aquatic lifestyle.

A white-letter day

It has not been much of a summer for our resident butterflies and the cool, often wet, conditions will have undoubtedly caused problems for many species. While a single poor summer, or even a short run of poor summers, may not prove catastrophic, it can exacerbate the impact of long-term declines brought about by other factors. The poor conditions tend to have an impact on butterfly survey work as well, limiting opportunities to get out into the countryside to locate populations of some of our more under-recorded or localised species. Three such species are on the wing now; the White Admiral, the Purple Hairstreak and the White-letter Hairstreak. Of these, it is the White-letter Hairstreak that we should, perhaps, be most concerned about.

This is a species that has undergone a significant decline since the 1970s, in part a reflection of its association with the various species of elm. The widespread outbreak of Dutch Elm Disease in the 1970s resulted in the loss of some 10 million elms, and with them, the removal of the favoured larval foodplants.  The spores of this fungal disease were carried by bark beetles and transferred from tree to tree as the beetles moved about to seek food or lay eggs. At the time, experts predicted that the loss of the elms would be extremely damaging, suggesting that the future of the White-letter Hairstreak hung in the balance. However, the predicted sudden and catastrophic loss of the butterfly did not happen, largely because the butterflies took to feeding on the regrowth brought about by the elm’s ability to sucker readily; they also exploited some of the more disease-resistant varieties of elm. As a result, colonies still cling on across a large part of central and eastern England albeit at a greatly reduced level.

Our understanding of how the White-letter Hairstreak population has changed comes from regular monitoring work, carried out annually across a series of sites through the UK Butterfly Monitoring Scheme. What the scheme doesn’t give us, however, is a clear picture of where the remaining colonies are located. This is where we, as amateur naturalists, come in. Over the next few weeks, the hairstreaks will be on the wing and can be spotted making their erratic spiralling flights around the tops of elm trees. Here they will feed on aphid honeydew. They will also descend to nectar on bramble or thistles, so it is worth staking out a group of elms with nectar sources below. You won’t see the dark uppersides of the wings because the butterfly perches wings closed, but you might just see the orange stripe and white ‘W’ on the underwing. If you do, make sure you let Butterfly Conservation know.