Sunday, August 31, 2003		Nature

How butterflies bluff the enemy
Nutan Shukla

Colourful eye-spots are located on the non-vital parts of a butterfly so that an enemy is not attracted to its vital organs

A variety of animals have evolved colours, patterns, etc, to attract a predator's attention, which very often result in them escaping with only minor damage or discomfort. The method is known as 'deception by invitation', a defence strategy put into operation once a prey has been discovered by the predator or when discovery is a distinct possibility.

Eye-like markings or spots on the wings of butterflies and moths, on the bodies of turtles, fish and caterpillars are part of this strategy and there is evidence that predators respond to them in two different ways. If they are small and exposed to the possibility of attack, they act as targets which misdirect the predators. Such eye-spots are situated on the non-vital parts of the body, like the edge of wings in the case of butterflies. If the spots are large, bright and conspicuous because of their colouring, fewer in number and if they are displayed by suddenly exposing them when the prey is disturbed, a predator is startled, frightened and is discouraged from investigating further. The interesting part is that such spots are usually found on, or near, the vital parts of the body.

The European grayling butterfly, which is found in places where there is bare ground, has two eye-spots near the outer edge of the underside of each forewing. When it alights on the ground, it intentionally displays one or both of these eye-spots for a few seconds as a part of its defence strategy. In a way it is a chance given to a predator, a bird or lizard, to attack it on an unimportant part.

Predators are in the habit of attacking the prey on their vital parts, like the head or eyes, so that they can overpower them easily. Experiments have also shown that when caterpillars with artificially painted eye-spots on unimportant parts were offered to birds, the avian predators pecked more often at the painted 'spots' rather than the other parts of the body, indicating that the eye-spots can fool the predators and divert their attention to unimportant parts.

The grayling butterfly also attracts lurking predators by displaying the eye-spots, but at the same time diverts their attention to the unimportant parts of their body. When attacked, it escapes by flying away merely with slightly damaged wings.

If it is not attacked, then it adopts the second course of action. It covers its forewings, which have eye-spots on them, with its camouflaged hind-wings, turns around to orient its wings towards the sun and leans to one side slightly so that its own shadow can be made to disappear and it merges perfectly with the colour of the ground on which it is resting. Since graylings live in the northern hemisphere, where the sun is never directly overhead, leaning on one side gets rid of the shadow.

Grayling butterflies are not social creatures once the breeding season is over. Several of them may be seen together in one habitat but this grouping is purely a response to abundant food. These butterflies emerge from their cocoons in July and after some time they develop their reproductive behaviour. The male, while in search of a mate, stops feeding and takes up position on the ground, or any vantage point. If the wait is long, his patience may fail and in his excitement he may pursue anything that passes by, be it a beetle, flies, small birds, and even falling leaves. Sometimes he may even chase his own shadow!

However, if he is in luck, he may encounter a female of his species. Should it be a receptive female, she reacts positively to the male's approach by alighting on the ground or any suitable spot. This is the cue he is looking for. If the living or non-living object that the butterfly has chased does not land, the male does not pursue it further. Once he is sure of a female's intention, he also lands nearby and soon starts walking around her until he is in the front, facing her. If the female is sexually immature, she lets him know with the flutter of her wings, and he again sets off in search for the right partner. If she keeps motionless, he begins his elegant courtship.

First, he flutters his wings in front of his prospective mate. Then in an effort to show the female the beautiful white-centred spots on his forewings, the male slightly raises his wings, opens and closes the front part of the wings rhythmically, his antennae quivering. This lasts from several seconds to one minute.

Now comes the most elegant part of the ritual. The male raises his forewings, opens them wide, and bows deeply. While still bowing, he folds the two forewings together and clasps the female's antennae between them. This is a butterfly kiss! Now the actual mating takes place. The male walks quickly behind the female and places his abdomen with its copulatory organ against the female's. He keeps facing away from her for some 30 to 40 minutes while he passes the sperm. The two animals then break contact and never see each other again.

Bowing is not just a posture: on the male's wings, exactly where he is pressing the female's antennae, are scent glands - a sign of his masculine maturity. When he clasps the female's antennae within his wings, he is essentially bringing her smell receptors into contact with his scent. Once she has sensed his particular scent, she is ready to copulate.