2 Insect Diversity 1. Introduction In this chapter, we shall examine the evolutionary development of the tremendous variety of insects that we see today. From the limited fossil record it would appear that the earliest insects were wingless, thysanuranlike forms that abounded in the Silurian and Devonian periods. | 2 Insect Diversity 1. Introduction In this chapter we shall examine the evolutionary development of the tremendous variety of insects that we see today. From the limited fossil record it would appear that the earliest insects were wingless thysanuranlike forms that abounded in the Silurian and Devonian periods. The major advance made by their descendants was the evolution of wings facilitating dispersal and therefore colonization of new habitats. During the Carboniferous and Permian periods there was a massive adaptive radiation of winged forms and it was at this time that most of the modern orders had their beginnings. Although members of many of these orders retained a life history similar to that of their wingless ancestors in which the change from juvenile to adult form was gradual the hemimetabolous or exopterygote orders in other orders a life history evolved in which the juvenile and adult phases are separated by a pupal stage the holometabolous or endopterygote orders . The great advantage of having a pupal stage although this is neither its original nor its only significance is that the juvenile and adult stages can become very different from each other in their habits thereby avoiding competition for the same resources. The evolution of wings and development of a pupal stage have had such a profound effect on the success of insects that they will be discussed as separate topics in some detail below. 2. Primitive Wingless Insects The earliest wingless insects to appear in the fossil record are Microcoryphia Archeognatha bristletails from the Lower Devonian of Quebec Labandeira et al. 1988 and Middle Devonian of New York Shear et al. 1984 . These together with fossil Monura Figure and Zygentoma silverfish Figure from the Upper Carboniferous and Permian periods constitute a few remnants of an originally extensive apterygote fauna that existed in the Silurian and Devonian periods. Primitive features of the microcoryphians include the monocondylous .
