64 ELEMENTARY ZOOLOGY. 



The "ear." Examine the first segment of the abdomen for 

 the ear sac. It has a membrane or drum stretched over it. 

 Structures are said to be homologous when they are funda- 

 mentally the same, even though they may have very different 

 appearance and uses. According to this definition, would 

 you consider the segments homologous ? Are the three pairs 



FIG. 5. A diagram showing the digestive tract and some of the other internal 

 organs of the locust. The digestive tract is represented as sectioned. The regions 

 of the body are: head (hd), thorax (th), and abdomen (ab). a, antennae; b, brain and 

 its chief nerves; c, colon or middle part of intestine; ce, cement glands; d, duct of the 

 salivary glands; e, excretory tubules which open into intestine; g, gizzard; g.c., gastric 

 caeca; h, heart; *', ileum or front part of intestine; m, mouth; o, oviduct; ov, ovaries; 

 p, pharynx; r, rectum or hinder portion of intestine; 5, stomach; s.g., salivary glands; 

 v.g., ventral line of ganglia; w, body wall, made up of cuticle, skin, and muscle. 



Questions on the Figure. Enumerate regions of the digestive 

 tract, and study the differences. Can you make out the probable 

 special functions of any of the regions? What would you suppose 

 to be the function of the gastric caeca? Evidences? What is 

 believed to be the function of the tubes (e) opening at the union of 

 stomach and intestine? Compare the diagram with your dissections 

 and with other figures so as to realize that the organs are not so 

 clear in reality as in a diagram. What do you understand by 

 "diagram"? What are its strong and what its weak points? 



of legs homologous? Are the antennae homologous with legs? 

 Are the mouth parts? Are the wings? Give the reasons 

 for your answer in each case. Compare your conclusions 

 and reasons with those of the writers of the books. 



d. Internal Organs. It it is desired to dissect the locust and 

 study the internal anatomy, the student will find detailed 

 instructions in the reference text-books. 



