232 THE MECHANICAL FUNCTIONS. 



two divisions of the body. In some this pedicle is short, in 

 others long: in the former case, an exceedingly refined me- 

 chanism is resorted to for effecting the necessary movements 

 in a part so bulky compared with the narrowness of the sur- 

 face of attachment* 



Insects in their perfect state have constantly six legs, 

 which are the developments of the six proper legs of the 

 same animal in its larva condition: all the spurious legs 

 having disappeared during its metamorphosis. We have 

 seen that in the myriapoda, the result of development is an 

 increase in the number both of segments and of legs; the 

 reason of which is that, being terrestrial animals, a length- 

 ened form was more useful and accordant with their desti- 

 nation; but in winged insects, where the object is to procure 

 the means of flight, the organs require to be concentrated, 

 and all superfluous parts must be retrenched and discarded 

 from the fabric. The multiplication of organs, which, in the 

 former case, indicated the progress of a higher development, 

 would in the latter have been the source of imperfection. 

 As long as the insect remains in its larva stage, its condition 

 is analogous to that of the myriapode: but in the more ele- 

 vated state of its existence, its structure is subject to new 

 conditions and regulated by new laws. 



While the number of members is thus reduced, ample 

 compensation is given by their increased activity and power, 

 derived from their augmented length, and the more distinct 

 lever-like forms of the pieces which compose them. 



These pieces (see Fig. 150) are named, from their sup- 

 posed analogy to the divisions of the limbs of the higher or- 

 ders of vertebrated animals, the haunch (h,) the trochanter 

 (t,) the femur (f,) the tibia (s,) and the tarsus (r.) In ge- 

 neral the femur (or thigh) has nearly a horizontal, and the 

 tibia (or leg) a vertical position, while the whole tarsus (or 

 foot) is applied to the ground. 



The haunch (h,) which is supposed to correspond to the 



* For the details of this structure I must refer to writers on entomology, 

 and in particular to Kirby and Spence's « * Inti-oduction to Entomology," 

 vol. iii. p. 701. 



