2o6 THE INTEGUMENTAL SKELETON OE THE IMAGO. 



movement of the wing both in its descent and its ascent 

 occurs when the wing plane and the plane of movement cor- 

 respond ; hence neither materially affects the onward or 

 upward movement of the insect. The precise manner in 

 which the varied movements of the wing are brought about 

 will be understood by a study of the articulations of the 

 pterygia. The movements of the wings are capable of almost 

 infinite variations, by which the direction and velocity of 

 flight are regulated. 



Much has been written on the mechanism of flight in insects. 

 Jurine [72] first discovered that the wings are moved in the 

 Hymenoptera by an alteration in the form of the thoracic wall, 

 the depression of the dorsum raising, and its increased convexity 

 depressing the wing ; movements which he correctly attributed 

 to the action of the dorsal and sterno - dorsal muscles. 

 Chabrier [73] figured and described the wing-joints and the 

 parts of the thorax concerned in flight, but except from an 

 anatomical point of view added little to Jurine's exposition. 

 Pettigrew [91] seems to have been the first to discover the 

 curve made by the wing apex and the forward direction of 

 the descending stroke ; he also lays much stress on the rotation 

 or screw movement of the wings ; he says : ' All wings obtain 

 their leverage by presenting oblique surfaces to the air, the 

 degree of obliquity gradually increasing from behind forwards 

 and downwards during extension, when the sudden or 

 effective stroke is being given. To confer on the wing the 

 multiplicity of movement which it requires, it is supplied with 

 a double hinge or compound joint, which enables it to move 

 not only in an upward, downward, forward, and backward 

 direction, but also at various intermediate degrees of 

 obliquity.' Marey [92] demonstrated the figure-of-eight or 

 loop described by the wing. 



No one, except Lendenfeld [93], to whose work I shall 

 presently refer, has preceded me in the attempt to define 

 the exact movements of, and the part played by, the several 

 sclerites of the wing-roots. 



Marey [92] believed that the rotation of the wing results 



