THE REGIONS OF THE BODY 27 



functional abdominal legs except in the Thysanura, and in the larvce of 

 Lepidoptera. A pair of 1- or many-jointed cercopods on the tenth seg- 

 ment; and in certain forms a pair of styles on the ninth segment. In 

 certain orders an ovipositor or sting formed of three, pairs of styliform 

 processes ; in Collembola a single pair of processes form ing the elater. 



The genital oj>enings opisthogoneate, usually single, but paired in 

 Thysannra (Lepisma), Dermaptera, and Plectoptera (Ephemeridce). 



The digestive canal in the winged orders is highly differentiated, the 

 fore-intestine being divided into an oesophagus and jiroroitricttlus, the 

 hind-intestine into an ileum, colon, and rectum, with rectal glands. 



The neri'ous system consists of a well-developed brain, in the more 

 specialized orders highly complicated; no more than thirteen pairs of 

 ganglia, which may be more or less fused in the more specialized orders. 

 Three frontal ganglia, and a well-developed, sympathetic system present. 



Stigmata confined (except possibly in Sminthurus) to the thorax and 

 abdomen, not more than ten pairs in all, and usually but nine pairs. 

 Tracheal system as a rule highly differentiated; invariably with tcenidia. 



Dorsal vessel with ostia and valvules; no arteries except the cephalic 

 aorta; no veins. After birth there is in the more specialized pterygote 

 orders a, reduction in the number of terminal segments of the abdomen. 



Development either direct (Synaptera), or with an incomplete (with 

 nymph and /ringed or imaginal stages), or complete metamorphosis; in 

 the latter case ivith a larval, pupal, und imago stage. 



The insects may be divided into two subclasses, --the Synaptera, 

 and the winged orders, Pterygota, of Gegenbaur (1877), since the 

 differences between the two groups appear on the whole to be of 

 more than ordinal rank. 



1. EXTERNAL ANATOMY 



a. The regions of the body 



The insects differ from other arthropods in that the body is divided 

 into three distinct regions, the head, thorax, and abdomen, the 

 latter regions in certain generalized forms not always very distinctly 

 differentiated. The body behind the head may also conveniently 

 be called the trunk, and the segments composing it the trunk- 

 segments. 



In insects the head is larger in proportion to the trunk than in 

 other classes, notably the Crustacea; the thorax is usually slightly 

 or somewhat larger than the head, while the hind-body or abdomen 



