MOVEMENT 57 



elevators of the wing are muscles with fibres running almost 

 vertically (Fig. 19, el) between the ventral and dorsal 

 aspects of the thorax ; when they contract they pull the dorsal 

 wall down, the wing-base is necessarily pulled down and the 

 wing- tip rises. The depressors of the wing (Fig. 19, A, dp) 

 run obliquely along the thorax so as to pull part of its wall 

 backwards ; this raises the dorsal region with the wing-bases, 

 and so the wing generally is depressed. In dragon-flies the 

 great wing muscles are attached to the sclerites of the wing- 

 bases so that they act directly on the wings as on levers. 

 Their arrangement, three elevator and five depressor muscles 

 to each of the four wings, has been described in detail by 

 R. von Lendenfeld (1881). H. R. A. Mallock (1919), 

 commenting on this remarkable contrast in the working of 

 an essential group of muscles in dragon- flies as compared 

 with other insects, writes : " The question arises as to why 

 has this compHcated and indirect method prevailed ? If the 

 problem were set of designing a mechanism for flapping 

 wings, the dragon- flies' solution would certainly be the first 

 to suggest itself ; yet it evidently must have some dis- 

 advantages since it has not been generally adopted." It may 

 be suggested that the flight muscles, attached between 

 various regions of the thoracic wall, though acting indirectly 

 on the wings are perfectly correlated with the articulation 

 of these to the thorax, and that such mode of attachment is 

 less delicate and liable to derangement than the direct action 

 which characterises the dragon- flies. In Locusts, Grass- 

 hoppers and their allies there are direct elevators and 

 depressors inserted at the wing-bases, as well as the indirect 

 muscles attached to the thoracic wall. It is noteworthy 

 that dragon-flies, while remarkably powerful, swift and 

 accurate fliers, have no provision for coupling the two wings 

 of one side together when flying ; the forewings and hind- 

 wings are worked by independent sets of muscles and the 

 co-ordination of the wings of the two pairs depends upon 

 an internally placed lever connecting their basal sclerites, 

 and correlation in nervous control. In the more primitive 

 insects of other orders also, such as Orthoptera (Cockroaches 



