158 THE LOCUST 



branes, sense organs whose function is supposedly auditory. The ven- 

 tral part of the first somite is only indistinctly separated from the 

 metathorax. The dorsal surfaces of the ninth and tenth somites are 

 very narrow and only partially separated from each other. At the 

 posterior end is the supra-anal plate, beneath which is the anus. At 

 the sides of the supra-anal plate are the two triangular podical plates. 

 At the base of each of these is a small projection, the cercus, which 

 is sensory in function. In other respects, the terminal portion of the 

 abdomen is different in the two sexes. In the female, the abdomen 

 terminates in two pairs of stout, pointed structures which form tlie 

 ovipositor, used for digging the holes in the ground in which the eggs 

 are laid. In the male, the ventral surface of the ninth somite is pro- 

 longed backward and upward as the genital plate. 



(f) On each of the first eight abdominal somites is a pair of spira- 

 cles, the openings of the tracheae, or respiratory tubes. These are 

 seen along the ridges on the sides of the abdomen. The spiracle on the 

 first abdominal somite is just in front of the tympanic membrane. 

 There are two pairs of spiracles on the thorax, located between the 

 somites. These may be difficult to see but may be located by squeez- 

 ing the specimen gently to press out the preserving fluid. 



(g) Make a drawing of the insect as seen from the side (X 4). 

 Spread out the wings above the back in such a position as to show their 

 size and shape; arrange the legs in the position they assume when the 

 insect is at rest; number the somites of the abdomen, and label all 

 structures. 



Exercise 3. — The Mouthparts. 



(h) Remove the labrum and clypeus and thus expose the mandi- 

 bles, or jaws, of the insect. Each mandible is a heavy, strong struc- 

 ture with black toothed edges. The jaws of insects have been evolved 

 from structures corresponding to the legs; in development they arise 

 in the same manner as the legs and later assume their characteristic 

 form. Is there any apparent advantage in having right and left in- 

 stead of upper and lower jaws? Separate the two mandibles a little 

 at the tips with a needle or point of a scalpel ; then remove the right 

 mandible by cutting with the point of a scalpel between the gena and 

 the base of the mandible. Sketch the appendage as seen after removal 

 (X 10), orienting it with the former attached end toward the top of 

 the page (c/. Fig. 69). 



(i) Posterior to the mandibles are the maxillae, a pair of more 

 delicate jaws, and the labium, which serves as the lower lip. Each 

 maxilla has a basal portion which bears three other parts: (1) a short 



