3 External Structure 1. Introduction The extreme variety of external form seen in the Insecta is the most obvious manifestation of this group’s adaptability. To the taxonomist who thrives on morphological differences, this variety is manna from Heaven; to the morphologist who likes to refer everything back to a basic type or ground plan. | 3 External Structure 1. Introduction The extreme variety of external form seen in the Insecta is the most obvious manifestation of this group s adaptability. To the taxonomist who thrives on morphological differences this variety is manna from Heaven to the morphologist who likes to refer everything back to a basic type or ground plan it can be a nightmare Paralleling this variety is unfortunately a massive terminology even the basics of which an elementary student may find difficult to absorb. Some consolation may be derived from the fact that form reflects function. In other words seemingly minor differences in structure may reflect important differences in functional capabilities. It is impossible to deal in a text of this kind with all of the variation in form that exists and only the basic structure of an insect and its most important modifications will be described. 2. General Body Plan Like other arthropods insects are segmented animals whose bodies are covered with cuticle. Over most regions of the body the outer layer of the cuticle becomes hardened tanned and forms the exocuticle see Chapter 11 Section . These regions are separated by areas joints in which the exocuticular layer is missing and the cuticle therefore remains membranous flexible and often folded. The presence of these cuticular membranes facilitates movement between adjacent hard parts sclerites . The degree of movement at a joint depends on the extent of the cuticular membrane. In the case of intersegmental membranes there is complete separation of adjacent sclerites and therefore movement is unrestricted. Usually however especially at appendage joints movement is restricted by the development of one or two contiguous points between adjacent sclerites that is specific articulations are produced. A monocondylic joint has only one articulatory surface and at this joint movement may be partially rotary . the articulation of the antennae with the head . In dicondylic joints . most leg
