2. External anatomy
Insects are segmented invertebrates that possess the articulated external skeleton (exoskeleton) characteristic of all arthropods. Groups are differentiated by various modifications of the exoskeleton and the appendages — for example, the Hexapoda to which the Insecta belong (section 7.2) is characterized by having six-legged adults. Many anatomical features of the appendages, especially of the mouthparts, legs, wings, and abdominal apex, are important in recognizing the higher groups within the hexapods, including insect orders, families, and genera. Differences between species frequently are indicated by less obvious anatomical differences. Furthermore, the biomechanical analysis of morphology (e.g. studying how insects fly or feed) depends on a thorough knowledge of structural features. Clearly, an understanding of external anatomy is necessary to interpret and appreciate the functions of the various insect designs and to allow identification of insects and their hexapod relatives. In this chapter we describe and discuss the cuticle, body segmentation, and the structure of the head, thorax, and abdomen and their appendages.
First some basic classification and terminology needs to be explained. Adult insects normally have wings (most of the Pterygota), the structure of which may diagnose orders, but there is a group of primitively wingless insects (the “apterygotes”) (see section 7.4.1 and Box 9.3 for defining features). Within the Insecta, three major patterns of development can be recognized (section 6.2). Apterygotes (and non-insect hexapods) develop to adulthood with little change in body form (ametaboly), except for sexual maturation through development of gonads and genitalia. All other insects either have a gradual change in body form (hemimetaboly) with external wing buds getting larger at each molt, or an abrupt change from a wingless immature insect to winged adult stage via a pupal stage (holometaboly). Immature stages of hemimetabolous insects are generally called nymphs, whereas those of holometabolous insects are referred to as larvae.
Anatomical structures of different taxa are homologous if they share an evolutionary origin, i.e. if the genetic basis is inherited from an ancestor common to them both. For instance, the wings of all insects are believed to be homologous; this means that wings (but not necessarily flight; see section 8.4) originated once. Homology of structures generally is inferred by comparison of similarity in ontogeny (development from egg to adult), composition (size and detailed appearance), and position (on the same segment and same relative location on that segment). The homology of insect wings is demonstrated by similarities in venation and articulation — the wings of all insects can be derived from the same basic pattern or groundplan (as explained in section 2.4.2). Sometimes association with other structures of known homologies is helpful in establishing the homology of a structure of uncertain origin. Another sort of homology, called serial homology, refers to corresponding structures on different segments of an individual insect. Thus, the appendages of each body segment are serially homologous, although in living insects those on the head (antennae and mouthparts) are very different in appearance from those on the thorax (walking legs) and abdomen (genitalia and cerci). The way in which molecular developmental studies are confirming these serial homologies is described in Box 6.1.