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Insects

The vast diversity of insects results in numerous variations in all but their most general features. The insect body is divided into three parts: the head, the thorax, and the abdomen. The head has mouthparts, simple and/or compound eyes, and one pair of antennae, sometimes absent in immature stages but usually present in the adult. The thorax has three pairs of jointed legs in all stages and two pairs of wings in the adult. In some insect groups, one pair of wings is modified for other purposes. For example, in beetles the front pair of wings form a hardened, protective cover (the elytra) over the single pair of functional wings. The abdomen contains the digestive and reproductive organs.

A key distinction in identifying insects and understanding their life cycles is the fact that they have two different modes of metamorphosis, in which they transform from immature stages to adulthood. Insects such as grasshoppers and aphids undergo incomplete metamorphosis (hemimetabolous development) in which the immature stages (nymphs) resemble small adults with reduced or no wings. In comparison, moths and beetles undergo complete metamorphosis (holometabolous development) in which the immature body (larvae) is very different from that of the adult. For example, a caterpillar is the immature stage of a moth or butterfly. So, what may be obvious in one life stage may be reduced or absent in another. In these insects, the mobile, feeding larval stage progresses to a quiescent pupal stage before becoming an adult.

Insects are a very diverse group of organisms. Globally, there are about 1 million known species in 43 orders (see the Catalogue of Life). The actual number of species is much higher because not all species have been identified and catalogued by insect taxonomists. In Canada, it is estimated that there are about 55,000 species (about 30,000 known species and 25,000 estimated yet to be found). For general information on the main orders of insects found feeding on trees in Canada, click on the following links: