118 INVERTEBRATE PHYSIOLOGY 



maintained during 85% of the wing cycle (Williams and Galambos, 1950) 

 results from the very complex pathway the wings traverse, as was noted 

 by many earlier investigators. The movement cycle, producing the neces- 

 sary precise changes in angle of attack, depends upon the proper and con- 

 stantly changing relations among the structures of the articulation in their 

 horizontal and vertical movements. The mechanical features are intimately 

 related to the physiology of the flight muscles ; for they determine the limits 

 of length change and to some degree the tension and the rate of change of 

 tension in the muscles. Each type of flight machinery encountered in insects 

 is therefore an integrated, well-adapted system. 



The following components constitute the flight machinery : ( 1 ) the 

 wings; (2) the wing articulation, including the direct muscles that con- 

 trol the setting of the articulating parts, the base of the wing, and the struc- 

 tures that relate it to the major portion of the thorax; (3) the thoracic 

 component or special parts of the thorax that serve to couple the driving 

 (indirect) muscles to the articulation ; and (4) the driving muscles. Since 

 it is not possible to treat here all of these adecjuately even for one type, the 

 wings and the aerodynamic problems of insect flight will not be discussed. 



On the basis of the presence or absence of the thoracic component, the 

 flight mechanisms may be divided into two types. In the more primitive 

 type, the muscles that furnish the power to move the wings are attached 

 directly to the articulation, as in the Odonata or dragonflies, where each of 

 the four wings possesses a set of elevators and depressors ( Sargent, 195 1 ) . 

 Although it is known that the fore and hind wings move in opposite phase 

 (Chadwick, 1940), there is no information on the nervous control of the 

 muscles or on the details of their movement. The muscle is reported to be 

 in incomplete tetanus when stimulated at flight frequency, indicating that 

 only part of the possible tension change is useful (Tiegs, 1955). The ten- 

 sion in a tetanus is little more than in a twitch. As found in a number of 

 other insect muscles, the protofibrils are organized into sheets or lamellae 

 separated by mitochondria and sarcoplasm. No true fibrils are present and 

 so these muscles are termed lamellar muscles. 



All other flying insects possess the thoracic component, coupling special 

 indirect muscles to the wing articulation. These muscles, especially in the 

 higher insects, almost completely fill the thorax and may be of three gen- 

 eral types: (1) lamellar; (2) microfibrillar in which the protofibrils are 

 organized into fibrils which have a diameter of about 1.5 ft in the freshly 

 teased preparation; and (3) fibrillar with large fibrils averaging about 

 3.0 IX in the fresh state (Pipa, 1955). This classification is certainly not a 

 rigid one but is useful for purposes of discussion. In some flight muscles, 

 in fact, the protofibrils appear to be organized into both lamellae and micro- 

 fibrils in the same cells (Tiegs, 1955). 



