196 LECTURE XVI. 



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

 called ^' hemelytra." When the hinder pair of wings is wanting, it is 

 replaced by a pair of rudimental appendages called balancers : other 

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

 and " patagia." 



The muscular system is, as may be supposed, developed in relation 

 to the several kinds and powers of locomotion indicated by the modi- 

 fications of the extremities. As a necessary corollary of the cylin- 

 drical form and external position of the principal parts of the skeleton, 

 the joints are for the most part ginglymoid, and restricted to move- 

 ments in one plane. The muscles of the legs are consequently 

 simply flexors and extensors. The coxae have a round head inserted 

 into cup, and the movements of the hip-joint are rotatory ; the head 

 is usually connected with the thorax by a similar joint, which, from 

 the greater freedom of the movements, may be termed arthrodial. 

 In insects of flight, the cavity of the thorax is almost entirely 

 occupied by the muscles of the wings. The muscular fibre is trans- 

 versely striated, and is also characterised by a second series of trans- 

 verse indentations at regular but wider intervals. 



The Orders of Insects are founded upon the modifications of the 

 wings ; those in which the first pair serve as sheaths, and the second 

 alone are used for flight, and are folded transversely when at rest, 

 constitute the order Coleoptera : they undergo complete metamor- 

 phosis, 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 the 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 includes the most 

 voracious and destructive Insects, as the Locust, Cockroach, &c. 



Those Insects which have both pair of wings membranous, trans- 

 parent, strengthened by numerous nervures, and finely reticulated, 

 form the order Neuropiera, which includes the highest organised insects, 

 as the predatory dragon-flies. 



The Insects which have four membranous wings simply veined, and 

 crossing each other horizontally when at rest, form the order Hyme- 

 noptera : they undergo a complete metamorphosis, and include the 

 most useful of insects, as the bee. 



The Insects with four wings, more or less clothed with minute 

 scales, are called Lepidoptera : they undergo complete metamorphosis, 

 and include the most beautiful species of the class, as the butterflies : 

 in one family of this order the wings are divided lengthwise into a 



