254 THE MECHANICAL FUNCTIONS. 



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 commence- 

 ment 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 suspended, 

 being firmly braced by their articulations, enable 

 the muscles to act with great mechanical advantage 

 in accelerating the progression of the body. Hence 

 these animals are enabled to run with great swift- 

 ness, and to spring in all directions and from con- 

 siderable distances on their prey ; powers which 

 are necessary to those tribes that live altogether by 

 the chase. 



Progression on a plane surface is effected, in 

 these animals, by the alternate advance of two sets 

 of feet ; the one set consisting of the first and third 

 feet on the right side, and the second and fourth on 

 the left ; the body resting on the other four, which, 

 on the former set having been brought to the 

 ground, are, in their turn, raised and brought for- 

 wards : so that four feet are always in action, and 

 four at rest, at the same moment.* 



The greater number of species, however, as is 

 well known, are provided with a curious apparatus 

 for spinning threads, and for constructing webs to 



* Muller Diss. Inaiig. Physiol, de Phoronomia animalium. Bonn. 

 1822. 



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