122 INVERTEBRATE PHYSIOLOGY 



This lever-like structure, moved by the action of the indirect muscles on 

 the tergum and on the joint of the scutellum, is termed the scutellar lever. 

 By the action of this lever, the wing-tip movement is amplified about 20 

 times. The two wings move together since the lateral arms are quite rigidly 

 connected through the scutellum. Reducing this structural rigidity, by 

 simply removing the soft cuticle forming the top of the scutellum, destroys 

 the ability for sustained flight. 



Fig. 2. Horizontal section through 

 the notum. Structures labeled as in 

 Fig. 1, with the following additions : 

 V, prescutal ridge ; w, chitinous sup- 

 X"-^ 3 — ^^ :^-^ m ~z: lH-M^ porting structure ; X, anterior hard- 

 ened plate; 1, first dorsoventral 

 muscles ; 2, second dorsoventral 

 muscles ; 3, oblique dorsal muscles ; 

 4, tergal remoter muscles ; 5, longi- 

 tudinal muscles. 



Anterior Notal Process: The anterior notal process (/?, Figs. 1, 2) 

 in flies is supported by and moves in conjunction with the parascutum 

 (d and e, Fig. 2) and so these must be treated together. The anterior notal 

 process is strongly supported by the termination, /, of the prescutal and 

 transverse ridges. The parascutum is hinged to the rest of the tergum be- 

 tween / and g so that the anterior notal process can be freely rotated up 

 down but cannot twist. The presence of this hinge makes it impossible for 

 the tergum to produce up and down movements of the anterior notal pro- 

 cess except through the scutellar lever. 



Mesopleural Process : The mesopleural process is a pleural deriva- 

 tive to which is attached one end of the second axillary sclerite (Fig. 3) . It 

 serves as a fulcrum for the rotation of the sclerite. At the end of each stroke 

 it moves in closer to the tergum, thereby increasing the wing-stroke 

 amplitude. 



The second mechanical component is the wing articulation, consisting 

 of the 1st and 2nd axillary sclerites, a number of other structures of 

 secondary importance for wing movement, and the direct muscles of flight. 

 The articulation contains elements for folding the wings as well as those 

 concerned in the wing cycle. The basic features of its operation, however, 

 depend upon the movements of the 1st and 2nd axillary sclerites and their 

 relations to the elements of the thoracic component described above. These 

 relations are illustrated in Fig. 3. The space between the hinge h, connect- 

 ing the parascutum to the lateral border of the tergum, and the mesopleural 

 process a (Fig. 3) is bridged by the parascutum (including the anterior 

 notal process) and the 2nd axillary sclerite. At k the radial vein of the 



