Insect Diversity

Abstract

From their beginning as primitivelywingless thysanuranlike creatures, the insects have undergone a vast adaptive radiation to become the world’s most successful group of living organisms. Undoubtedly, a large measure of this success can be attributed to the evolution of wings. The Paranotal Theory suggests thatwings evolvedfrom rigid outgrowthsof the terga inadult insects, their initialfunction being the protection of the legs. Increase inthesize of the paranotallobes was associated withimprovement in attitudinal control as the insects dropped from vegetation in order to escape predation. In the Paranotal Theory articulation of these winglets was a later development associated with improving attitudinal control. An idea that is becoming more popular is that wings developed from already articulated structures. These were present on all thoracicand abdominal segments in aquatic juvenile insects andfrom the outset were hingedin associationwith their function as gills, spiracular flaps, or swimming organs. Before they became large and powerful enough for true flight, the wingsmay have been used to propel insects over the water surface (surface skimming), as seen in some extant stoneflies and mayflies. The Pterygota had a monophyleticorigin intheDevonian and soon split into two major evolutionary lines, thePaleoptera and the Neoptera. By the late Carboniferous several distinct neopteran groups were established. These were the pliconeopterans (ple- copteroids, orthopteroids, and blattoids), which are perhaps polyphyletic, the paraneopter- ans (hemipteroids), and the oligoneopterans (endopterygotes), both of which are probably monophyletic. Of these, theendopterygotes have been by far the most successful.This is related, in large part, to theevolution of a pupal stage withinthe group. Various theories have been advancedfor theoriginof the pupal stage. The most likely theory proposes that it is equivalent to the final nymphal instar of the original exopterygote ancestor. Its initial function was to provide space for wing and wing muscle development. However, its evo- lution also facilitateddivergence of adultandlarvalhabits, so that the two stages no longer competedfor the same resources. It also became a stage inwhich species could survive adverse climatic conditions, especially freezing temperatures. Insect success (i.e., diversity) is related not only to the group’s adaptability but also to the environment in which they have evolved. Being arthropods, insects possess a body plan that is superior to that of other invertebrates. They are generally small and ableto fly. They usually have a high reproductive capacity, often coupled with a life history that isshort and contains a pupal stage. Because offeatures peculiar to them, four orders (Coleoptera, Lepidoptera,Hymenoptera, and Diptera) are especially successful, comprising almost 75% of the described species. Insect evolution was coincident with the evolution of land plants. The insects were among the first invertebrates to establish themselves on land.Byvirtue of theiradaptability they were abletocolonize rapidlythenew habitats formed as a resultof climaticchanges over the earth’s surface.