It has been considered for a very long period already that insects originate from one ancestor form, but it is not so. Physical resemblance of insects isn't due to their similar descent, but is a result of adaptation to the environment and acquiring of convergence features. These conclusions were reached by genetics in a recent research. At least one group of primitive wingless insects known as springtails got out of water to land much earlier than other insects.
A group of Italian and American researchers with Doctor Francesco Nardi from the University of Siena at head represented a new view upon the genealogical tree of insect origin. At that the researcher focused not on the shape of their bodies, but on the similarity of their genes.
The scientists compared the key genome sequences of different insects with the ones of insects belonging to the variety of silverfish and to the two varieties of springtail. Silverfish belongs to a group which varieties are considered to be the closest relative to flying insects. And the analysis revealed substantial arguments in favor of the fact that although all of them originated from sea shellfish, that wasn't one and the same shellfish for all of them.
The results of the research break down the traditional stereotype thinking believing that insects belong to the monophyletic group; in other words, it's considered that they once originated from one six-legged ancestor that was common for all of them. Now it's clear that the physical resemblance of insects isn't due to their similar genealogy, but is a result of adaptation to the environment that resulted in resemblance of their forms (the phenomenon is known as convergence).
The difference of springtail from silverfish was distinct, although it had been considered earlier that the former originated from the latter. The crustaceous ancestor of springtail is separated from Crustacea with several evolution stages; Crustacea gave birth to other insects.
The mitochondrial data of the genome sequence reveal that the earthly hexapod arthropoda developed several times and emerged after at least two independent intrusions of their ancestor forms on land. Their similar features were smoothed away within millions of years of the evolution.
Insects usually have three pairs of legs, one pair of feelers and three parts of the body. They are the only members of anthropoda species having wings in their developed age, although direct presence of the wings is not always obvious.
Springtail is an insect living in ground, it is known all over the world. Thousands of springtail varieties exist on the Earth, including one variety in Antarctica and over 1,600 in Australia. The name of the insect comes from a unique forked pelvic backbone spine that can flex and spring; with the help of the backbone spine the insects can jump very high.
Crustacea that include crabs, shrimps, lobsters and shellfish are also anthropoda. They have four or more legs, two pairs of feelers, varying forms of the body that never has three basic parts like the body of insects, they also have no wings.
The scientific world still continues debating on the evolutionary connection between insects, Crustacea, Arachnida and multipeds.
Translated by Maria Gousseva
Read the original in Russian: http://science.pravda.ru/science/2003/6/20/55/9378_nasekomye.html
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