CHAPTER 1 



Structure and Classification 



By W. C. Bentinck 



University of California, Berkeley 



Arthropods differ from other animals by a combination 

 of characters. They have a hardened or sclerotized 

 exoskeleton secreted by the epidermis and molted at 

 intervals. The body is segmented externally in varying 

 degree, and typically each somite bears a pair of 

 jointed appendages though these have been lost from 

 some somites of various forms. The circulatory system 

 is open with a dorsal vessel or heart, distributing 

 blood to organs and tissues, whence it returns through 

 body spaces to the heart. Respiration is accomplished 

 by means of tracheae, gills, lung books, or by diffu- 

 sion directly through the integument. The nervous 

 system is characterized by the paired ventral nerve 

 cord and dorsal brain. Reproduction is usually bi- 

 sexual, and most arthropods are oviparous. 



Insects may be distinguished from the other classes 

 of Arthropoda by the following characteristics of 

 the adults. The body is divided into three distinct 

 regions: the head, thorax, and abdomen. The head 

 bears one pair of antennae and mouth parts for chew- 

 ing, sucking, or lapping. The thorax typically consists 

 of three somites, each bearing a pair of jointed legs, 

 and the posterior two somites usually each bearing 

 also a pair of wings. The abdomen is comprised of 

 eleven somites or less with the terminal parts modified 

 as genitalia. 



The basic insect pattern has been modified in an 

 almost endless variety of ways throughout the course 

 of evolution. Hence there are numerous exceptions 

 to some of the generalizations mentioned above, 

 especially among immature stages. 



For the benefit of the beginning student, there 

 follows a brief discussion of the basic features of 

 insect anatomy. Most of the structures and regions 

 to be mentioned are illustrated in figures 1:1, 1:2, 

 1:3, and 1:4. 



EXTERNAL ANATOMY 



The exoskeleton is made up of numerous hardened 

 or sclerotized plates or regions called sclerites. 

 These are joined together by pliable membranes 



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which lend flexibility to the total structure. The 

 seamlike furrows or narrow membranous areas separat- 

 ing sclerites are called sutures. A typical unmodified 

 segment consists of four main regions; a dorsal region 

 or tergum, a ventral region or sternum, and on each 

 side a lateral region or pleuron. In each of these 

 regions the cuticle may be differentiated into separate 

 sclerites called tergites, sternites, and pleurites, 

 respectively. 



The head bears compound eyes on each side with 

 the antennae situated anterior to them and the three 

 simple eyes or ocelli arranged in a triangle between 

 them. Anterior to the median ocellus is a transverse 

 suture, the epistomal suture, which separates the 

 clypeus from the frons or region between the compound 

 eyes. 



The mouth parts are shown in detail in figure 1:2. 

 The labrum or upper lip is hinged to the front margin 

 of the clypeus and behind it is a pair of powerful 

 mandibles. Back of the mandibles are the maxillae, 

 each with three appendages; a lateral five-segmented 

 maxillary palpus; a somewhat fleshy appendage, the 

 galea; and an inner appendage, the lacinia, which 

 somewhat resembles the mandibles. The labium forms 

 the posterior wall of the mouth and bears laterally 

 the three-segmented labial palpi. The hypopharynx is 

 a soft tonguelike structure situated centrally between 

 the mouth parts. 



The thorax is comprised of three segments: the 

 prothorax, mesothorax, and metathorax, each bearing 

 a pair of legs and the latter two segments usually 

 each bearing a pair of wings. Generally there are two 

 pairs of thoracic tracheal openings or spiracles. 



The legs typically consist of five parts: the coxa, 

 a short basal segment which attaches the leg to the 

 body; the trochanter; the femur; the tibia; and the 

 tarsus which bears a pair of claws apically. The 

 tarsus may have one to five segments or, rarely, be 

 entirely absent. 



The adult plecopteron of figure 1:1 represents a 

 fully winged or alate insect of generalized type. 

 Some insects have shortened, stublike wings and 

 are termed brachypterous whereas others have none 



