124 INVERTEBRATE PHYSIOLOGY 



and an inward force on the mesopleural process resulting from its struc- 

 tural connections and the contraction of the pleurosternal muscle. At the 

 beginning of the next stroke, whether an up or a down stroke, k is moved 

 by the action of the scutellar lever acting through the 1st axillary sclerite. 

 The hinge of the parascutum and the mesopleural process are forced apart 

 to accommodate the full width of the parascutum and 2nd axillary, storing 

 potential energy by straining the tergum and the mesopleural process. At 

 the critical point, where k moves past the direct line between the hinge h 

 and the hook articulation of the 2nd axillary sclerite on the mesopleural 

 process, the union point k is forced strongly toward its extreme position 

 by the recoil of the strained elements, releasing the energy stored at the be- 

 ginning of the stroke. By the wing articulation and the thoracic component, 

 therefore, changes in length of the indirect muscles are magnified 400-600 

 times and the muscle can operate almost isometrically. 



The foregoing is a simplified and brief statement of the mechanical 

 events of the wing cycle of Sarcophaga hullata Parker as analyzed in CCI4- 

 treated flies. Other factors, such as the secondary wing articulation, the 

 posterior notal process, the basalar and the subalar, may also play im- 

 portant roles. 



To obtain further information and to check the conclusions made on 

 CCU-treated flies, studies were made of the flight mechanism in action. If 

 the description given above holds for normal flight, the following can be 

 expected : ( 1 ) the anterior notal process must move in and out for each 

 up stroke or down stroke ; (2) the mesopleural process must also move in 

 and out each stroke ; ( 3 ) the movements of the scutellum should accurately 

 reflect, as a built-in isotonic lever, the changes in length of the driving in- 

 direct muscles during flight. 



Tiny pieces of mirror silver were fastened to the thorax of flies with 

 wax, one on the side of the mesopleural process, one on the tergum just 

 above the parascutum, to indicate the in and out movement of the anterior 

 notal process, and one on the side of the scutellum to show wing position. 

 A light beam, reflected by the three mirrors, was brought to focus on 

 moving film in a Grass kymograph camera. By careful adjustment the 

 three spots could be brought close together, but not in a vertical line. 

 Therefore, some means of obtaining simultaneous ordinates in the three 

 records was necessary. A two-bladed fan driven at high speed was mounted 

 to cut the light beam before it reached the mirrors. Blanks appearing in 

 the records could then be lined up and the instanteous position of each 

 structure during a cycle determined. A typical result is shown in Fig. 4A. 

 The dotted lines indicate the positions of the blanks made by one blade of 

 the fan on the three records. It is apparent that not only are there in and 

 out movements of the tergum and mesopleural process but also anterior- 



