Insecta Overview

Vincent H. Resh

University of California, Berkeley

University of California, Riverside

The species-rich superclass Hexapoda includes all insects and their near relatives that share the characteristic arrangement of having, as adults, three major body regions and six legs. The number of described insect species has increased greatly from the time of the early catalogers of life. Those 18th-century pioneers in biodiversity, such as Carl Linné, would not have conceived that there would be nearly a million species described by the 21st century. Most estimates today suggest that this number represents only 10 to 30% of the actual number of insect species thought to exist. The richness of living things is essentially the result of insect richness; animal biodiversity is therefore, in reality, mainly insect biodiversity.

Within the class Insecta, major forms of insects are grouped in orders. Ordinal-level groups represent divergent lineages that are nearly always recognizable by a set of distinctive characteristics. Almost always, an adult insect can be readily determined to order at a glance. The number of recognized orders has fluctuated slightly as entomologists' understanding of the included taxa and methods for classifying have developed. Classification schemes are both organizational systems and true scientific hypotheses. In this way they are dynamic, changing as new information becomes available. There are several important ways in which a classification may evolve. One is the subjective change in taxonomic rank. For example, in the 1950s all Ephemeroptera (the order containing mayflies) in North America were assigned to three families, and today they are in 21. Mostly, this is the result of raising subfamilies to family status.

If substantial evidence is found that a group previously recognized as an order is paraphyletic (i.e., does not contain all descendants of that group), then new monophyletic arrangements will be proposed. A good example is the order

Hemiptera. The taxa included in the order now were traditionally divided into two groups (often given ordinal status): Heteroptera (true bugs) and Homoptera. Recent analyses suggest a more complicated pattern of relationships. Three groups within the order Hemiptera are treated separately as suborders in this encyclopedia: Auchenorrhyncha (cicadas, spittlebugs, leafhoppers, treehoppers); Prosorrhyncha (Heteroptera and Coleorrhyncha); and Sternorrhyncha (aphids, psyllids, scale insects, whiteflies).

Perhaps the most exciting way that classifications may change is by the discovery of something genuinely novel. However, in insects, finding a truly new order (i.e., a group of taxa that have a combination of characteristics unique at that level) is an astounding event. At the time that this encyclopedia was ready for printing, a new order (Mantophasmatodea) was discovered in southern Africa. It is the first order of living insects to be described in over 80 years! This discovery reinforces the point that there is much left to learn about our earth's biodiversity.

In many classification systems, orders are grouped into superorders, but what comprises a superorder is far from fixed. For example, zorapterans are viewed as being in the superorder Orthopteroidea by some and in the superorder Hemipteroidea by others. Although a close link between Trichoptera (caddisflies) and Lepidoptera (moths and butterflies) is supported by a wealth of concordant evidence

TABLE I The Orders of Insects and Other Members of the Arthropod Superclass Hexapoda

Hierarchical category

Taxon

Families

Species

Class

Parainsecta

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