THE THORACIC EXO-SKELETON. 205 



When the wing-tip arrives at the point d, the metapterygium 

 IS raised, and the deltoid, acting through the conus, causes a 

 rotation of the anterior part of the wing on the remigium. 

 From d to f the wing sweeps backwards, and the wing plane 

 is inclined to the plane in which it moves at an angle of about 

 45°, its dorsal surface looking upwards and forwards. The 

 anterior part of this lower back-stroke is the most powerful 

 agent in propelling the insect forwards ; but owing to the 

 inclination of the wing plane, it also assists in the upward 

 movement. The lower back-stroke is due to flexion of the 

 wing ; towards the end of it the sterno-dorsal muscles contract, 

 the meso-pleural fissure opens, and the foot of the uncinate 

 sclerite is released from the hamulus. The up-stroke,/to h, is 

 effected by the combined extension of the wing and an upward 

 movement on the axis of the dens ; during the upper back- 

 stroke the hamulus is again brought into relation with the foot 

 of the uncinate sclerite, and when the apex of the wing arrives 

 at the point a of the curve, its descent is brought about by the 

 contraction of the great longitudinal thoracic muscles, which 

 increases the convexity of the dorsum, causes the hamulus to 

 descend, and extends the wing by closing the meso-pleural 

 syndesmosis. 



It will be observed that the wing plane makes three rotations 

 on the remigium. The first at the end of the descent of the 

 wing, by which its anterior margin is depressed. The second 

 rotation occurs when the hamulus escapes from the uncinate 

 sclerite at the end of the lower back-stroke ; the anterior margin 

 of the wing is raised, so that the wing plane and the plane of 

 motion upwards correspond. The third rotation is effected at 

 the end of the upper back-stroke ; it brings the hamulus into 

 the stirrup of the uncinate. It occurs when the wings stand 

 upwards over the back, and renders the planes of the two wings 

 parallel. 



During the two back-strokes the insect is urged forwards ; 

 during the first half of the descending stroke it is raised, 

 and the sudden rotation of the wing at the commencement 

 of the lower back-stroke has a similar effect. The forward 



