The Tribune - Spectrum


Sunday, May 27, 2001

They use projectiles to catch prey
Nutan Shukla

ANT-lions have no relationship with the members of the cat family. They are larvae of ‘doodle-bugs’, insects related to lacewings and alder flies. These larvae are the members of the group of animals who throw projectiles to catch prey.

Living in sandy places in the tropics or sub-tropics, they dig pits as large as 4 inches in diameter and 20 inches deep. These conical depressions act as death-traps for insects on which the larvae feed. To construct the trap, the ant-lion walks backwards in a circle, gradually spiralling inwards. It takes sand on its head and throws it either to one side or to the other, creating a depression of the appropriate size.

After the work is done, this predatory insect hides by burying itself at the bottom of the pit, with just its enormous, pincer-like jaws protruding and waiting for an unfortunate victim to blunder in. Once the victim is at the bottom of the pit, it is almost impossible to climb up because the walls of the pit are very steep and are lined with fine sand, which is far less stable than coarse sand. If, however, the prey seems to be making good its escape, the ant-lion throws sand at it, knocking it back into the centre of the pit. There, it can grasp the victim, pierce it and suck out the body fluids.

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There are very few animals who hurl projectiles at their prey. One such predator is the archer fish. Ranging from India to northern Australia, this fish is found in different kinds of waters, including fresh, saline and brackish, particularly in mangrove swamps. This fish usually catches insects sitting outside water. The prey sitting on an overhanging twig or leaf is brought down in the water by shooting a spit on it.

To be able to spit droplets of water to any distance, the fish has developed specialised features in its mouth. It has a groove in the roof of the mouth and the tongue is modified to press against the groove to form a ‘tube’, in effect the barrel of a water pistol. At the moment of discharge, the tongue is pressed to the roof of the mouth, the gills closed, and the front of the tongue flicks out the water droplets. A mature fish can knock down an insect which is up to 5 ft above the water surface, and with a degree of accuracy.

The most famous of the stone-throwers, perhaps, is the Egyptian vulture. It throws rocks at ostrich eggs. Its aim is not very accurate and needs many attempts before breaking them open. The habit seems to come from the vulture’s innate tendency to throw eggs. The birds raid pelican and flamingo colonies for eggs which they can hurl down and break. Faced with an ostrich egg, they do the only thing that comes naturally —throw an egg-shaped stone at it. The behaviour is not learnt but acquired simply by trial and error during the bird’s lifetime.

Immature throwers have been seen to throw rocks more than 70 times and then give up in disgust. More mature birds may achieve the skill of one individual which scored 38 direct hits out of 64 tries. It is thought that the Egyptian vulture began its rock-throwing tradition by throwing the eggs of smaller birds on the ground. Only when it was confronted with the frustration of not being able to carry an enormous ostrich egg aloft did it resort to throwing rocks at it.

An Australian bird, the black-breasted buzzard, is known to do the same with the eggs of emu, an ostrich-like bird endemic to Australia. The buzzard chases away the parent emu from the nest and then drops rocks on the eggs.

The spitting spider spits a glob of sticky material to a distance of 2 cm, sticking the prey to any convenient surface. This arachnid prefers moths and also has a liking for other spiders, too, whom it sticks to its web with its spit.

Some members of the mongoose family also perform the throwing act, but it is different from that of the Egyptian vulture. Instead of throwing projectiles, it smashes eggs on rocks by throwing. The Egyptian mongoose, an animal considered sacred by the ancient Egyptians, picks up an egg with its forepaws and throws it between its back legs against a large stone being used as an anvil.