69 

 Bentmck: Structure and Classification 



Fig. 1:1. An adult Plecoptera, Pteronorcys californica Newport, illustrating general 

 insect anatomy (Essig, 1942). 



at all or only tiny vestiges thereof and are referred 

 to as apterous. The main wing veins develop along 

 the course of tracheae present in the developing wing 

 of the immature insect. The wing venation varies 

 greatly from order to order but certain similarities in 

 basic pattern lend support to the theory that all the 

 diverse patterns have been derived from a single 

 type of primitive venation. In figure 1:4 is shown the 

 venation pattern which students generally regard as 

 the archetype pattern from which the venation of 

 modern insects has been derived. The names of 

 the veins are indicated with the standard abreviations 

 used for the Comstock-Needham system of wing-vein 

 nomenclature. 



In wings of modern insects the precosta (Pc) is 

 lacking, and the anterior fork of the media (MA) is 

 usually absent. The remaining veins may undergo 

 modification by fusion of adjacent veins or by partial 

 or complete suppression of one or more veins. In some 

 orders the pattern is greatly complicated by the 

 addition of secondary veins, and in other orders there 

 are forms in which practically all venation has been 

 lost. 



The abdomen typically consists of eleven segments 

 the last of which bears the cerci. The Collembolla 

 are a notable exception in possessing never more than 

 six abdominal segments, either in the embryo or the 

 adult. Various numbers of the terminal segments are 

 involved in the formation of the external genital 

 apparatus and ovipositor or egg-laying structure. 

 Typically there is a pair of spiracles on each of the 

 first eight abdominal segments, but some or all of 

 these may be absent in some insect. Likewise the 

 number of abdominal segments may be actually or 

 apparently reduced or the first segment may be 

 incorporated into the thoracic region. 



INTERNAL ANATOMY 



Just within the body wall and attached to it are the 

 muscles. The alimentary canal, divisible into fore-, 

 mid-, and hind-gut, occupies the center of the body 

 and extends from one end to the other. The fore-gut 

 is further divided into the pharynx, esophagus, crop, 

 and proventriculus. The mid-gut represents the stom- 



