198 THE BOOK OF NATURE STUDY 



to the species, be spotted with yellow. The whole surface of 

 the thorax is covered with bristles of some length, which are 

 probably tactile in function. 



The thoracic exo-skeleton is exceedingly hard, and further, 

 has hard processes extending into the internal cavity. Now, the 

 chief function of a skeleton, whether external or internal, is to 

 afford firm points of attachment for muscles, and to constitute 

 a series of rigid rods which may play the part of levers in bringing 

 about motion of the several parts, or locomotion of the entire 

 individual. Hence the great hardness of the thorax to which 

 the organs of locomotion, the legs and wings, are attached is 

 a mechanical consequence of the presence of the powerful muscles 

 by which these organs are moved. 



The wings are thin membranous expansions of the exo- 

 skeleton, supported by a system of firmer " veins " or " ner- 

 vures " of definite arrangement. The " veins " along the front 

 margin of the extended fore- wing are much stouter and less 

 flexible than the remainder ; moreover, the fore and hind-wing 

 of each side are united, as already stated (vide p. 192). Hence, 

 when a down-stroke is made by the wings the front edge remains 

 rigid, but all the posterior portion yields to the pressure of the 

 air below and is bent upward, so that a forward thrust results 

 upon the under surface of the wings, and so the insect is driven 

 forward. The action can be imitated by folding a sheet of paper 

 two or three times along one of its edges, so as to render that 

 edge stiff, and then dropping it (flat side horizontal) from above 

 the head : the paper will not fall vertically, but will travel 

 obliquely forwards and downwards in the direction towards 

 which the stiffened edge is turned. The main muscles for moving 

 the wings up and down are not attached to the wings themselves, 

 but to portions of the thoracic skeleton itself. Their action is 

 such that when the thorax is made shorter fore and aft, but 

 deeper dorso-ventrally, the wings are depressed ; and conversely 

 when the thorax is made shallower dorso-ventrally but longer 

 from neck to waist, the wings are elevated. This movement can 

 be brought about in a freshly killed wasp by removing the head 

 and abdomen, then (i) place the scar whence the head has been 

 removed on the table and press with a pencil point upon the 



