224 TOPOGRAPHICAL ANATOMY OF THE 



It is very generally admitted that the great thoracic muscles 

 are the most powerful agents of flight, and that they act upon 

 the wings by altering the convexity of the dorsum, and its 

 relation to the mesosternum which supports the wings through 

 the intervention of the pleural plates. It is generally believed 

 that the dorsales depress and the sterno-dorsales elevate the 

 wings. 



I have already stated (p. 187) that it does not appear that 

 the two sets of muscles are direct antagonists, nor is it apparent 

 why the elevators of the wings need to be as powerful as the 

 depressors. The latter movement raises the whole weight of 

 the insect, whilst the elevation of the wing does no work in 

 flight, but only prepares it for the downward stroke. I think 

 it more probable that the sterno-dorsales give power to the 

 back stroke, and urge the insect forwards. The wing-joints 

 and their relations to the thorax are so complex that it is not 

 easy to ascertain exactly the movements of the wing each 

 slight alteration of the position of the parts produces, as these 

 depend upon the angles which the articulating surfaces make 

 with each other. 



The Respiratory Organs. — The tracheae of the head and thorax, 

 with the exception of their smaller branches, are all membranous 

 sacs ; those of the abdomen, except the great air-sacs at its 

 base, are all cylindrical vessels, which exhibit a spiral arrange- 

 ment of the intima. The existence of membranous tracheae 

 and dilated tracheal sacs is characteristic of all aerial insects, 

 but I know no other group in which the cylindrical tracheae are 

 more completely replaced by thin-walled tubes of variable 

 calibre than the Muscidae and the allied Diptera. 



It is usually stated that the air is forced out of the tracheal 

 vessels by the contraction of the abdominal muscles, and that 

 they are refilled by the elasticity of the spiral thread in their 

 walls. Such an explanation is totally inadequate to explain 

 the filling of the tracheae with air in the Diptera, where the 

 sacs have no tendency to expand — indeed, they collapse as soon 

 as an opening is made for the escape of air ; neither is there 

 any tendency for the abdomen to expand by the elasticity of its 



