2 GENERAL ZOOLOGY 



Plan of External Structure. The entire body is covered 

 with a hard, horny substance called chitin. This gives pro- 

 tection to the internal organs. The body is divided into 

 three clearly marked regions : the head, the thorax, and the 

 abdomen (Fig. 2). Each region is further subdivided into a 

 series of rings called somites, which give flexibility to the 

 body in spite of the hard chitinous covering. These rings 

 are most clearly observed in the abdomen, which is made 

 up of ten easily counted somites. The three somites of the 

 thorax are less clearly distinguishable because they have 

 become slightly modified and shoved together. Though the 

 head seems to be but a single structure without division into 

 somites, it is fairly certain that six somites have been so 

 completely combined and fused to form the head that divi- 

 sions are no longer present. The external plan upon which 

 the grasshopper is built is thus seen to be a series of somites, 

 of which the anterior (front) somites bear jointed append- 

 ages modified for various uses. 



The Head. The head of the grasshopper serves in taking 

 food and making the animal aware of its surroundings. 

 These functions are performed chiefly by appendages, or 

 paired outgrowths from the head. Certain of these append- 

 ages are purely sensory ; others aid in securing and devour- 

 ing food. The most conspicuous sensory appendages are the 

 pair of slender, jointed antennae, or feelers. In the grasshop- 

 per these are organs of touch and smell. Just behind the 

 antennae, on the sides of the head, is the pair of large 

 compound eyes. Each compound eye is covered with a 

 transparent layer of chitin which is divided into a large 

 number of six-sided divisions, each of which is called a 

 facet. Such an eye is said to be compound because each 

 facet has beneath it the necessary structures for sight. 

 Since the surface of a compound eye is convex, each facet 

 points in a different direction. It seems probable that each 



