Emperors of the Moor.

 I love the big Saturniid moths. Ever since I was a kid and I got to watch an Indian Moon Moth emerge from its cocoon at home, a story of which my parents love to remind me, I have loved the big Saturniids, their big, ornamented, often bright green caterpillars, their silk-spinning and their cocoons, a palace for their pupae, and the big furry faces of the adult moths, with their dark compound eyes, their soft and luxuriant looking fur, and their surprisingly appealing faces.  Huge, patterned wings of eye spots, translucent windows and elegant pastel colours. The Saturniid moths are heavy duty night butterflies of immense beauty.   Although a group of them are called ‘Giant Silk Moths’ they are not closely related to Bombyx mori the domestic species so brutally harvested to make ties and party dresses. They are big, widely distributed, heavy moths, and the family includes the largest moth in the world, the Giant Atlas Moth (Attacus atlas).

Here in Europe, and Britain especially, we have something of a paucity of Saturniid moths. While the beautiful Spanish Moon Moth (Graellsia isabellae), a species I dream of seeing in the wild, lives in Mediterranean Europe, and the Tau Emperor (Aglia tau) sits just across the channel as close as Northern France and Germany, we only have one breeding Saturniid in these islands, and it is far from the biggest of them. However, the Emperor Moth, Saturnia pavonia, is a stunning beast, no less.  Males are finely patterened with rich grey forewings with eye spots, and purple fringing, and just a hint of a snakes’ head pattern on the wingtips, recalling some of its giant relatives. Their hindwings are orange, with dark veins like window lead, and another set of eye spots.  The females are grey, but no less spectacular, slightly larger, broad in the abdomen, and retaining the dark venation, with purple notes like the heather over which they fly. The eye spots are presumably aposematic colouring to threaten potential predators, and do look owl-like. The highly active males patrol the heather and grassland on sunny days in April and May. First impression is of an orange moth, the colour of its hindwings, often part concealed at rest. Having done all their eating as larvae, adult emperors do not feed, representing just a short-lived dispersal stage to mate and produce eggs.  Males spend their lives frantically looking for an opportunity to breed before their stored supply of energy runs out.  Their broad, feathered antennae serve to detect the chemical signals given off by females. Antennae provide a way of distinguishing the sex of Saturniids, under less pressure to detect chemical signals over long distances, the females have relatively thin antennae.  These moths can be attracted using a pheromone lure, designed to imitate the chemical signals of a female Emperor Moth. It works on similar principles to lures used to control pest species like Codling Moth or House Moths, but we were not using it to harm the moths, merely as an opportunity to observe and marvel.  When we, myself and a few colleagues from Butterfly Conservation, hung up the pheromone lure, from a fence in a little birch-and-heather patchwork not far from Aviemore in the Cairngorm Mountains, the male Emperors were with us in minutes.  Confused, they searched for the female, perching up on pines, posts and people, fluffy winged bundles of sexual frustration. The source of their confusion was soon taken away and repackaged, the moths’ energy reserves are valuable, and, the onlookers satisfied with excellent views, the moths melted out over the heather moorland once again.

Emperor Moth, Aviemore, May 2023

Females are relatively sedentary, seldom wandering far from their host plant, retaining their energy to distribute their eggs, and to escape from predators should the need arise.  They need to save their resources. As such, their lifespan tends to exceed that of the males, sometimes spending 10 days on the wing. A proximity to the host plant and a sedentary lifestyle saves them the active hunt for somewhere to lay that is a part of the lives of longer-lived lepidoptera, and like most Saturniidae, they don’t seem interested in finding the correct host plant. Fortunately, the larvae have fairly broad tastes, munching on heathers, birch, bramble and several other plant species, contributing to their surprisingly wide range in the UK, from the Highlands to open country in Essex and Kent, and down to Cornwall.

Upon hatching, the larvae are black and hairy at first, growing quickly, until by high summer the caterpillars are well grown and magnificent. A large Emperor Moth larva is bright green, studded with dayglow yellow, or sometimes other colours, points like gemstones, each surrounded by hairs, or perhaps bristles. It spends its summer eating, before spinning a cocoon, undergoing its final moult, and becoming a pupa. It overwinters in this state, as like all lepidoptera its body is essentially turned to soup and rebuilt into its final form. In Spring the moth emerges again, through a unique one-way valve in its cocoon, in contrast to other Saturniids, which instead produce chemicals to melt a hole through their silken palace. Then they face the world and fly, the cycle beginning anew.

Emperor Moth Larva on Orkney

Keep an eye out from April-early June for the Emperor Moth. They can be seen across the country, I have encountered it on the coastal marshes of Essex, the West Country heaths, up here in the Cairngorm Mountains in Scotland, and even, as a fat green, yellow-studded, heather munching larva, next to the Ring of Brodgar on the Isle of Mull.  Sometimes the larvae are easier to find than the moth, and tend to be active at the times of year we’re taking our summer holiday, so take a look for them too. In heather habitats their bright green colouration and markings make them surprisingly easy to spot.  And be impressed, by our only native Saturniid moth. A truly magnificent beast.