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Living fossils hold clues to the evolution of insects

Posted in Biology, Insects, Nature, Prehistory, Wildlife on Monday, 29 April 2013

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This edited article about insects originally appeared in Look and Learn issue number 234 published on 9 July 1966.

Primeval forest, picture, image, illustration

Huge dragonfly-like insects lived in prehistoric forests

Two questions which will probably occur to anybody who looks at insects in all their variety are: “Why are there so many of them?” and “Where did they come from?”

Study of the lives of insects gives us an idea as to why there should be so many, for every different sort does a particular job which is not done by anything else in the area in which it lives. The large numbers of different species can be accounted for by the limited abilities of each one, for there are few “Jack-of-all-trades” among insects.

To discover how insects came into being is much more difficult, for in the animal kingdom are many related creatures. Spiders and crabs, for example, both have jointed legs attached to a hard skeleton outside the body. But these are cousins, not ancestors, to the insects.

We know what the ancestors may have looked like, however, through the discovery of a “living fossil” type of animal, known from many tropical parts of the world. This is called Peripatus, or Velvet Worm, because its body is covered by a huge number of tiny bumps which look like plush.

At first sight Peripatus also looks like a worm, for its body is segmented, without grouping into head, thorax and abdomen. Until looked at closely, it appears to be entirely soft-bodied, but in fact each leg is tipped with a hard claw and in the mouth is a pair of tough jaws with which it slices up the insects and other small creatures on which it feeds.

This may seem a poor foundation for the insects’ family tree, but there are other similarities. For example, Peripatus breathes through a series of holes along the sides of the body, as do many insects.

It is thought that an animal something like this, which lived many millions of years ago, may have become more efficient by using parts of its body for certain functions only, and that this eventually led to the compartmentalized body structure of the insects.

The Firebrat has some relatives which give us a clue that this may be right, for they carry tiny flaps on the lower surface of the abdomen. These are probably all that survives of the legs which a Peripatus-like ancestor carried on every segment.

Another insect gives us an idea of how antennae may have developed. This creature, which is called a Proturan, has six legs, but the front pair are never used for walking. Instead, they are held up over the head and used for touching and exploring things.

Both the proturan and the firebrat are called “primitive” insects, for they are like early insects in several ways. For instance, although their bodies are compartmented, their lives are not, and a young firebrat looks very much like an adult, except for the difference in size. Neither has wings, for these were a later development of insect structures.

Primitive wingless insects still thrive in many places and, where conditions are right, more advanced forms have been unable to oust them. In Britain, the commonest insects are probably a primitive, wingless kind called Springtails.

These live in huge numbers in the top few inches of the soil. It has been estimated that about 51,000 of them may live in a square yard of soil of a beech forest, and about 250,000,000 in an acre of grassland.

Springtails are tiny creatures which, when disturbed, leap rather like a pole vaulter, using a large, stiff prong which is normally tucked underneath the abdomen. They are almost the only insects able to stand sea-water, and one kind is common in rock pools and on breakwaters.

Among the reasons for thinking that primitive, wingless insects are most like the first insects to evolve is the fact that they occur earlier in the fossil record than any other kind of insect. They are known from rocks in Scotland which date back about 370,000,000 years. Winged insects are first known from rocks which date back about 280,000,000 years.

These early winged insects had wings which had developed as flaps from the top of the thorax, each pair matching a pair of legs, so there were six wings in the earliest forms. The first pair of wings tended to be smaller than the others, and finally they disappeared altogether.

At the time when the great coal forests stretched across much of the world, huge six-winged dragon-fly-like insects clumsily flapped among the trees. Large cockroach-like creatures are also known from this distant age. In their way of life these creatures probably foreshadowed the small birds and mammals of a modern forest. The earliest of them were plant feeders, although carnivorous forms evolved later.

Some of the most beautiful fossils ever found are the remains of insects which were trapped in the sticky gum exuded by some trees. This became amber, and in it the remains of ancient insects may be seen. The fossil of the Ant Lion dates back millions of years. Because it is so well preserved, scientists can study the specimen in detail.

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