488 INVERTEBRATE MORPHOLOGY. 



they do upon vegetable food either in the adult or larval 

 stages, and occurring occasionally in enormous numbers, form 

 powerful enemies to the horticulturist and agriculturist, the 

 Kocky Mountain Locust, for example, devastating at times 

 the crops, while fruit and forest trees are injured by the at- 

 tacks of other forms. 



The Insecta differ from other Tracheata in having the 

 body divided into three well-marked regions. The most an- 

 terior of these is the unsegmented head, bearing the antennae 

 and the masticatory appendages, and immediately following 

 it is the thorax, composed of three segments, the prothorax, 

 mesothorax, and metathorax, each of the last two bearing 

 usually a pair of wings upon its dorsal surface, while pos- 

 teriorly is the segmented abdomen composed typically of ten 

 segments, sometimes as broad as the thorax at the junction 

 with that region, sometimes contracted to a narrow stalk. In 

 many cases, however, the apparent number of segments falls 

 below ten owing to the fusion of certain of the posterior seg- 

 ments or the union of the anterior segment with the thorax, 

 and in the Butterflies and two-winged Flies the thoracic seg- 

 ments seem to be reduced to two owing to the close associa- 

 tion of the metathorax with the first abdominal segment. 



four pairs of appendages are borne by the head. The 

 antennae, and indeed all the appendages, vary greatly in 

 shape in the various groups, but are usually long slender 

 niultiarticulate structures provided with sensory hairs. The 

 masticatory appendages are a pair of mandibles and two 

 pairs of maxilhe, which are variously specialized for biting, 

 piercing, or sucking. The most typical condition is that in 

 \vhich the entire apparatus is adapted for biting and that may 

 be described here, leaving special modifications to be con- 

 sidered in connection with the orders in which the}' occur. 

 The mandibles (Fig. 225, 67) are strong unjoiuted toothed 

 plates which meet together in the middle line and are pro- 

 vided with strong muscles. The first maxillae, or, as they are 

 usually termed, the maxillae (Fig. 225, B], on the other hand 

 are distinctly jointed, and consist of a basal joint, or cardo, 

 succeeded by a second joint, or stipes, which bears on its 

 outer side a multiarticulate palpus (p) and terminates in one 



