1 Arthropod Evolution 1. Introduction Despite their remarkable diversity of form and habits, insects possess several common features by which the group as a whole can be distinguished. They are generally small arthropods whose bodies are divisible into cephalic, thoracic, and abdominal regions. | I Evolution and Diversity 1 Arthropod Evolution 1. Introduction Despite their remarkable diversity of form and habits insects possess several common features by which the group as a whole can be distinguished. They are generally small arthropods whose bodies are divisible into cephalic thoracic and abdominal regions. The head carries one pair of antennae one pair of mandibles and two pairs of maxillae the hind pair fused to form the labium . Each of three thoracic segments bears a pair of legs and in the adult the meso- and or metathoracic segments usually have a pair of wings. Abdominal appendages when present generally do not have a locomotory function. The genital aperture is located posteriorly on the abdomen. With few exceptions eggs are laid and the young form may be quite different from the adult most insects undergo some degree of metamorphosis. Although these may seem initially to be an inauspicious set of characters when they are examined in relation to the environment it can be seen quite readily why the Insecta have become the most successful group of living organisms. This aspect will be discussed in Chapter 2. In the present chapter we shall examine the possible origins of the Insecta that is the evolutionary relationships of this group with other arthropods. In order to do this meaningfully it is useful first to review the features of the other groups of arthropods. As will become apparent below the question of arthropod phylogeny is controversial and various theories have been proposed. 2. Arthropod Diversity Arthropods share certain features with which they can be defined. These features are segmented body covered with a chitinous exoskeleton that may be locally hardened and is periodically shed tagmosis the grouping of segments into functional units for example head thorax and abdomen in insects presence of preoral segments paired jointed appendages on a varied number of segments hemocoelic body cavity containing ostiate heart enclosed within a .
