238 



SYSTEMATIC ZOOLOGY. 



used in flight, but in certain groups the front pair are 

 much thickened and hardened, so that they are converted 

 into wing-covers (elytra) which protect the hinder wings 

 when at rest. 



The abdomen is normally composed of ten segments, 

 but this number may be reduced. In some insects the 

 abdomen joins the thorax by its whole width, while in 

 others it is contracted in front to a slender stalk as in 

 the ants and wasps. The appendages of the abdomen 

 are never locomotor in function in the adult. In the 

 lowest insects rudimentary appendages may occur on all 

 segments of the abdomen, but in the higher groups only 

 three pairs, at most, occur, and two of these are modified 

 into an organ (ovipositor] for laying the eggs. In the 

 bees, wasps, etc., the ovipositor is at the same time an 

 offensive weapon, the sting, there being connected with it 

 a poison-gland in the abdomen. 



The alimentary canal has few convolutions. Into the 

 mouth-cavity open the salivary glands.* In those forms 

 which eat solid food like the crickets and grasshoppers a 

 1 chewing-stomach ' with hard horny teeth occurs. Behind 



FIG. 66. Diagram of hexapod anatomy, b, brain; c, crop; h, heart; m, Mal- 

 pighian tubes; r, reproductive organs; s, stomach; sg, salivary glands; v, 

 ganglia of ventral chain. 



this comes the true stomach, and following this the intes- 

 tine, to which are attached a varying number of Malpighian 



* The ' molasses ' of the grasshopper is the saliva. 



