READING IN CLASS


Read the text and say what information it carries.

Insects have the traits already discussed for the arthropods and the mandibulates - exoskeleton (of chitin, a light material but very supportive if a structure is small enough - in some ways like fiberglass), open circulatory system, segmented worm-type nervous system, mandibles, antennae - and a few specific traits of their own. Insects typically are divided into three sections: a head, thorax, and abdomen. The thorax is where all of the structures used for movement are located: six walking legs (several types of mouthparts and perhaps other structures are built from what used to be legs, but only six real legs remain), and typically four wings. Insects, unlike other winged animals, have not evolved their wings from pre-existing appendages, but rather from outgrowths of the thorax exoskeleton, possibly flaps that had been used to cover the gills of fresh water insects, or from the gills themselves. The progression probably went from blunt gliding structures, through steerable glider wings, to flappable wings, to systems that can beat wings up to at least a thousand times a second. Many types of insects use only the back pair of wings for flying; the front pair in those insects have generally evolved into protective covers for the back wings.

Insects typically go through a limited number of molts (6 is the typical number) between hatching and adulthood, and with very few exceptions only develop fully-functional wings during the last molt. If you are looking at an insect with full wings, it's almost certainly an adult and never going to get any bigger. Insects also go through two different pathways of development. In direct development (sometimes called simple metamorphosis), young insects look like miniature, wingless versions of the adults (these young are typically callednymphs). Crickets and grasshoppers develop this way. In indirect development (sometimes called complete metamorphosis), the larva is physically quite different from the adult. They often occupy different nichesthan adults as well, allowing more individuals to exist together in an ecosystem. Larvae grow, molt, grow, molt, et cetera, until they encase themselves in apupa form and completely reorganize themselves, a process calledmetamorphosis (sometimes breaking down to a "soup" of cells that then completely build a new individual) and emerge as adults. The terms "simple metamorphosis" and "complete metamorphosis" mentioned above can be confusing, and "metamorphosis" is usually just applied to animals that go through a major change. This is what butterflies do, as you probably know, but also beetles, bees, ants, and flies develop this way. In some cases, the adult acts almost exclusively to reproduce and spread offspring, not even being able to feed during their short time in that form.