20 W^RTHROrODS 



Make a drawing of the inner wing slightly folded to show 

 the markings (X2). 



The Abdomen. — (i) Behind the thorax are the mov- 

 able segments of the abdomen. Move them back and forth 

 to show how they are joined together. How do these joints 

 differ from those in the thorax? Each segment is called a 

 somite. 



Examine the tergum, the pleurum, and the sternum. How 

 do they differ from the corresponding parts of the thorax? 

 Is the covering of a somite continuous around the body? 



Count the segments on the ventral and dorsal sides. Is 

 the number the same ? Explain. Count the segments in the 

 abdomen of three individuals. How many on each ? 



(2) On the segment next the thorax find an indented 

 ear-shaped surface which contains a silvery membrane, the 

 tympanum, or ear drum. This is supposed to be the organ 

 of hearing. State the advantages and disadvantages of 

 having it located here. 



Look at the pleurum just above the fold for a spiracle, 

 or breathing pore. How does it compare with the one on 

 the thorax ? Count the number on each side of the abdomen. 

 Is there one for each segment? Is there one on the first 

 segment of the abdomen? If so, where? Can you see these 

 open and close in a living specimen? 



Do any of the somites of the abdomen seem to be special- 

 ized? 



(3) Observe the posterior somites in several specimens. 

 They vary with the sex. The females are more abundant 

 than the males and may be recognized by the presence of 

 four blunt points extending from the ventral side of the last 

 segments of the body. 



