352 LECTURE XVI. 



movements, may be termed arthrodial. In Insects of flight, the 

 cavity of the thorax is almost entirely occupied by the muscles of the 

 wings. There are two extensor and several small pairs of flexor 

 muscles, which arise from the meso- and meta-thorax, and are in- 

 serted into a process at the base of each wing. The muscles of 

 the legs have similar fulcral processes, but longer, like tendons ; 

 they are, however, chitinous apodemata of the external skeleton. 

 The muscular fibre is transversely striated, and is also charac- 

 terised by a second series of transverse indentations at regular but 

 wider intervals. 



The wings of Insects are essentially flattened vesicles, sustained by 

 slender but firm hollow tubes called "nervures," along which branches 

 of the tracheae, and channels of the circulation, are continued. The 

 wings never exceed two pairs, which are developed from the meso- 

 notum and metanotum. Sometimes one or other of these pairs is 

 wanting. The wings present many varieties in their shape, their 

 consistence, and their teguments. When they subserve flight, they 

 are thin and transparent ; or, if opake, are rendered so by an imbri- 

 cated clothing of most delicate scales, which, when detached, resemble 

 the pollen of flowers. In certain insects, especially those that bur- 

 row, the first pair of wings become thick, hard, and opake, forming a 

 kind of shield to the back ; they are called " elytra," and cover the 

 posterior pair of membranous wings when these are not expanded 

 for flight. Sometimes the anterior wings are membranous at their 

 extremities, hard and opake at their base, when they are called 

 *' hemelytra," When the hinder pair of wings is wanting, it is re- 

 placed by a pair of rudimental appendages called balancers : other 

 modifications of, or appendages to, the wings have been called "alulas" 

 and "patagia." 



The Orders of Insects being, as before remarked, founded upon 

 the modifications of the wings, the chief of these are best exemplified 

 by recapitulating the ordinal characters. Those Insects in which the 

 first pair of wings are hard, inflexible, and serve as sheaths (elytra), 

 and the second alone are used for flight, and are folded transversely 

 when at rest, constitute the order Coleoptera : these insects undergo 

 complete metamorphosis, and are subdivided according to the number 

 of joints of the tarsi. Beetles and most burrowing Insects belong to 

 this order. 



Those Insects in which the anterior pair of wings are converted 

 into elytra, of less density than in the Coleoptera, and in which tlie 

 posterior wings are folded longitudinally when at rest, constitute the 

 order Orthoptera : they are said to undergo a semi-metamorphosis, 

 the chief change being the acquisition of wings. This order in- 



