similar insects differentiated from other insect groups. Over time, a relatively stable classification system has developed but differences of opinion remain as to the boundaries around groups, with "splitters" recognizing a greater number of groups and "lumpers" favoring broader categories. For example, some North American taxonomists group ("lump") the alderflies, dobsonflies, snakeflies, and lacewings into one order, the Neurop-tera, whereas others, including ourselves, "split" the group and recognize three separate (but clearly closely related) orders, Megaloptera, Raphidioptera, and a more narrowly defined Neuroptera (Fig. 7.2). The order Hemiptera sometimes was divided into two orders, Homoptera and Heteroptera, but the homopteran grouping is invalid (non-monophyletic) and we advocate a different classification for these bugs shown stylized on our cover and in detail in Fig. 7.5 and Box 11.8.

In this book we recognize 30 orders for which the physical characteristics and biologies of their constituent taxa are described, and their relationships considered (Chapter 7). Amongst these orders, we distinguish "major" orders, based upon the numbers of species being much higher in Coleoptera, Diptera, Lepidoptera, Hymenoptera, and Hemiptera than in the remaining "minor" orders. Minor orders often have quite homogeneous ecologies which can be summarized conveniently in single descriptive/ecological boxes following the appropriate ecologically based chapter (Chapters 9-15). The major orders are summarized ecologically less readily and information may appear in two chapters. A summary of the diagnostic features of all 30 orders and cross references to fuller identificatory and ecological information appears in tabular form in the Appendix.

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