tJ68 THE iMECHANlCAL FUNCTIONS. 



segments, witli their accompanying legs. The 



Julus terresiris, for example, 

 ^^^ (Fig. 143) has, at its entrance 



.:^ ^fW^!\f'^"n¥ ^^ t^ ^"^^ ^^^^ world, only eight seg- 

 "^^ SSSi^ ^A ments and six feet ; but ac- 

 quires in the course of its de- 

 velopement, tifty segments and about two hundred 

 feet. The anterior legs are directed obliquely 

 forwards, and the rest more or less backwards. 

 The mandibles have the form of small feet ; as we 

 have seen is frequently the case in crustaceous 

 animals. 



§ 2. Insecta alata. 



Our attention is uow to be directed to the more 

 highly developed Insects, which have been formed 

 with a view to progression through the air. On 

 these, which compose the most extensive class of 

 the whole animal kingdom. Nature has lavished 

 her choicest gifts of animal powers, as far as they 

 are compatible with the diminutive scale to which 

 she has restricted herself in their formation. The 

 model she has chosen for their construction is that 

 which combines the greatest security against in- 

 jurious impressions from without, with the most 

 extensive powers of locomotion ; and which also 

 admits of the fullest exercise of all those faculties 

 of active enjoyment which are characteristic of 

 animal life. She has provided for the first of these 

 objects by enclosing the softer organs in dense and 

 horny coverings, which perform the office of an 

 external skeleton, sustaining and protecting the 



