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TEXT-BOOK OF ENTOMOLOGY 



FIG. 109. Cross-section through tarsus of a locust 

 ch, cuticula of upper side, oh', ch", ch'", of sole; 



suckers the male is enabled to adhere to the back of its mate during 

 copulation. The fine branching hairs around the edge prevent the 



water from penetrating and 

 thus destroying the vacuum, 

 " while if the female struggle 

 out of the water, by retain- 

 ing the fluid for some time 

 around the sucker, they will 

 in like manner under these 

 altered conditions equally 

 tend to preserve the effectual 

 contact." (Tuff en West.) 



In the saw-flies (Uroceridse 

 and Tenthredinidse) and 



. , -. , other insects, there are small 



membranous oval cushions 

 ... (arolia, Figs. 109 and 131) 



oh , tubulated layer; ch , lamellate layer; ch , inner x 



projections of ch". other lettering as in Fig. ioi. See beneath each or nearly each 



p. 113. After Dewitz. J 



tarsal joint. 



The triunguline larvae of the Meloidse are so called from apparently having 

 three ungues, but in reality there is only a single claw, with a claw-like bristle 

 on each side. 



Why do insects have but six legs ? Embryology shows that the ancestors 

 of insects were polypodous, and the question arises to what cause is due the 

 process of elimination of legs in the ancestors of existing insects, so that at 

 present there are no functional legs on the abdomen, these being invariably 

 restricted (except in caterpillars) to the thorax, and the number never being 

 more than six. It is evident that the number of six legs was fixed by heredity 

 in the Thysanura, before the appearance of winged insects. We had thought 

 that this restriction of legs to the thorax was in part due to the fact that this 

 is the centre of gravity, and also because abdominal legs are not necessary 

 in locomotion, since the fore legs are used in dragging the insect forwards, while 

 the two hinder pairs support and push the body on. Synchronously with this 

 elimination by disuse of the abdominal legs, the body became shortened, and 

 subdivided into three regions. On the other hand, as in caterpillars, with 

 their long bodies, the abdominal legs of the embryo persist; or if it be granted 

 that the prop- legs are secondary structures, then they were developed in larval 

 life to prop up and move the abdominal region. 



The constancy of the number of six legs is explained by Dahl as being in rela- 

 tion to their function as climbing organs. One leg, he says, will almost always 

 be perpendicular to the plane when the animal is moving up a vertical surface ; 

 and, on the other hand, we know that three is the smallest number with which 

 stable equilibrium is possible ; an insect must therefore have twice this number, 

 and the great numerical superiority of the class may be associated with this 

 mechanical advantage. (This numerical superiority of insects, however, seems 

 to us to be rather due to the acquisition of wings, as we have already stated on 

 pages 2 and 120. 



