The Hexapoda, essentially the insects, can be divided into pterygotes (with wings) and apterygotes (without wings) and include the springtails, dragonflies, cockroaches and locusts. The group may prove to have as many as 10 million living species when the rich faunas of the tropics have been completely described. The subphylum also includes the onychophorans, with flexible segmented bodies and unjointed limbs propelled by changes in blood pressure analogous to the water vascular system of the sea urchins. The hexapods have unbranched or uniramous appendages, a simple gut, a single pair of antennae and a pair of mandibles, together with a toughened head capsule. Insects have six limbs.
The oldest insect is probably the springtail Rhyniella praecursor from the Lower Devonian Rhynie Chert of the Orcadian Basin of northeast Scotland (Fig. 14.19). Conrad Labandeira (Smithsonian Institution, Washington) and his colleagues have shown that insects diversified earlier than had been thought (Labandeira 2006), and the group probably originated in freshwater during the Late Silurian, which may account for the poor fossil record of the group before the Devonian (Glenner et al. 2006). Early and Mid Devonian faunas are now well known from Rhynie, Gaspé, Québec and Gilboa, New York State and these probably coincided with the diversification of land plants. And by the Late
Carboniferous a very diverse insect fauna had evolved, with forms such as the dragonflies and mayflies capable of powered flight (Box 14.7). By the end of the Permian, most of the familiar insect orders had appeared. During the later Mesozoic and Cenozoic, significant coevolutionary relationships were established between plants and insects, particularly between flowering plants and insect pollinators, and possibly even between spiders and flies (see Box 14.6). Moreover, by the Miocene, fossil hair trapped in amber together with the sand fly Lutzomyia suggests that these blood suckers were already feeding on mammals in arboreal nests during the Mid Tertiary (Penalver & Grimaldi 2006).
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