284 THE MECHANICAL FUNCTIONS. 



their outer skin several times, and at regular 

 periods. In the earlier stages of their existence, 

 although they have the general form of the 

 mature insect, yet they have a smaller number of 

 legs : the last pair not making their appearance 

 till after the spider has attained a certain size. 

 We may here trace the commencement of that 

 system of metamorphosis, which, as we shall 

 afterwards find, is carried to so great a length in 

 winged insects. 



Spiders are endowed with extensive powers of 

 progressive motion, and display great activity 

 and energy in all their movements. The long 

 and elastic limbs on which the body is sus- 

 pended, being firmly braced by their articu- 

 lations, enable the muscles to act with great 

 mechanical advantage in accelerating the pro- 

 gression of the body. Hence these animals are 

 enabled to run with great swiftness, and to spring 

 from a considerable distance on their prey ; 

 powers which were necessary to those tribes that 

 live altogether by the chase. The greater number 

 of species, however, as is well known, are pro- 

 vided with a curious apparatus for spinning 

 threads, and for constructing webs to entangle 

 flies and other small insects. Every species of 

 spider weaves its web in a manner peculiar to 

 itself: and, besides the principal web, they often 

 construct in the neighbourhood a smaller one, 

 in the form of a cell, in which they conceal 



