Silkmoths are essentially tropical insects, occurring in their largest numbers and variety on the continents of South America, Africa and Asia. It is, therefore, a surprise to many, to discover that one species actually occurs here in Hampshire
There is currently in the region of 2300 different species of silkmoths in the world, together forming a family known as the Saturniidae. Most are medium to large insects, the Caesar moth, Attacus caesar, of the southern Philippines, having the second largest documented wingspan of any moth in the world at some 25.5cm.
Many make use of flash, or 'alarm', colours and markings. Whilst at rest, for example, the adult moths are normally inconspicuous, melding perfectly with the surrounding vegetation. When disturbed or threatened however, they suddenly flap or spread their wings, exposing striking markings or bright-coloured eye-shaped spots. Predators are startled and either momentarily thrown off-guard, giving the moths time to escape, or are deterred entirely from devouring them.
Their larvae, or caterpillars, are no less spectacular. Most are vividly coloured and many have thorny, wart-like growths on their bodies. Some larvae of certain Asian species grow quite large often reaching 10cm or more in length and the thickness of a man's thumb.
When pupating, the larvae spin silken cocoons amongst the leaves of their foodplant; on the ground beneath rotting vegetation or suspended pendulously from branches. The white or yellow cocoons of the true silkmoth, or 'Mulberry Worm' Bombyx mori of China, can be reeled off into a single thread ranging in length from several hundred metres to an incredible two miles.
Originally cultured by the Chinese, as early as 2600 BC, the silk industry then spread to Bukhara and later to Rome from where it entered the commerce of the western world. The insect has since been responsible for the financial development of a number of Asiatic, Oriental and Arabian peoples and its influence on certain world political developments has been almost unmatched by any other animal species.
Despite the numerous synthetic materials of our modern world this natural silk remains indispensable. Consequently, the silkmoth ranks as the most economically important of all insects. Today, the silk industry produces more than 130 million kilograms of raw silk each year, worth about 250 million U.S. dollars. Our very own, and sole native representative of the silkmoth family, the Emperor moth Saturnia pavonia, is distributed fairly widely over much of the British Isles although, in certain regions, is rare or absent altogether. Here in Hampshire, it can be found across the county, from the chalk grasslands of the Downs to the lowland heath and ancient woodlands of the New Forest. Although smaller than many of its tropical relatives, it is nevertheless one of our largest and most spectacular moths. The female, which is larger than the male by about seven millimetres, has a wingspan of up to 8.5cm.
The colouring differs between the sexes, the female being an overall bluish-grey and the male a darker purplish-grey suffused with rosy or rusty shades. The patterns on the wings of both sexes are similar, and the characteristic 'eyes', one on each fore-and hind-wing, makes identification easy. As with other members of this family, the Emperor does not feed when an adult, its large, fluffy body containing enough fat from its larval stage to sustain it during its comparatively short life.
The males fly strongly during the day and can often be seen flying rapidly across moorlands and heaths, and around the edges of woodland on warm summer afternoons, their bright colours often resulting in them being mistaken for butterflies. Although the female can sometimes be found in the daytime, resting on fence posts, tree trunks, or amongst vegetation, she is strictly a night-flyer.
A newly emerged female Emperor moth will generally attract a great many males, drawn to her by strong scents known as pheromones which she expels from a special organ at the tip of her abdomen. Males will often fly several kilometres to mate with a female, zigzagging transversely into the wind as they home in on her, detecting her inviting scent by means of chemoreceptors located on their large feathery antennae. The male that is the first to arrive obviously has the best chance to mate with her.
Once mated, the female lays her eggs in neatly arranged batches on or around the stems of various plants such as heather, bramble, and sallow. Depending on the temperature, the eggs hatch some two to three weeks later. The hairy black, newly-hatched larvae first devour their egg shells – a source of important protein – and then group together for communal protection before they start to feed upon plant material. As they get bigger, they leave the group to go their own ways. During the four to six weeks that they spend as larvae they undergo a number of moults, or skin changes known as instars, after some of which they take on a new colour. In the early stages they are black with an orange stripe along each side and later they are black but ringed with orange. Fully grown, the larvae are amongst Britain's most attractive caterpillars and, after their penultimate moult, are bright green with velvety jet-black markings and pink, yellow, blue, or black warts, from which arise short bristles.
Before their final moult, when they change into pupae, the larvae each make an interesting and cunningly constructed ginger-coloured fibrous cocoon in between two or three leaves. The cocoon is bottle-shaped and made in such a way that the emerging adult moth can pass out through it quite easily, but something trying to gain entrance is prevented from doing so by a ring of coarse converging fibres forming a door.
It is not unusual to find two pupae inside the same cocoon particularly when the larvae are living in overcrowded conditions. When the leaves eventually fall from the trees and bushes in the autumn, the cocoons fall with them to the ground where they remain until the following spring. Unlike the tropical silkmoths which can be continuously brooded, the Emperor moth has just one annual brood, very occasionally two, and it is mainly for this reason that this species is not used commercially.
Because the adult moths do not feed, they do not seek out nectar-bearing flowers like other moths and butterflies. Nevertheless, they can be attracted to gardens planted with one or more of the trees and shrubs that their larvae feed on, such as hazel, meadowsweet, hawthorn, apple, plum and heathers.
