202 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



membrane, which forms two folds, one in front and the other behind the 

 cup. The anterior fold contains a spindle-shaped process {ap) covered by 

 numerous sensory hairs. The proximal extremity of this process is connected 

 by an elastic ligament with a strong apodeme, terminating in the upper 

 margin of the pre-epaulet. The posterior fold contains a shorter tooth-like 

 process, into which the tendon of a strong muscle [ap') is inserted ; this 

 muscle arises from the upper part of the great ampulla. 



When the wing is extended, the cup of the remigium is brought into rela- 

 tion with the hypopterygium, and forms a ball-and-socket joint, on which the 

 remigium rotates. 



The Hypopterygium is an erectile papilla, strengthened by a curved 

 capitate sclerite, the hypopterygial sclerite, on its lower border and at its 

 extremity. The sclerite articulates with the posterior edge of the pre- 

 epaulet, close to the upper part of the great ampulla. The head of tlic 

 hypopterygium is covered by a transparent layer of chitin, which has the 

 appearance of an elastic pad. The whole organ is freely movable, and 

 has several small muscles inserted into its base. The movements of 

 the hypopterygium may be observed when the insect is held by the 

 wings ; it vibrates rapidly, like the wing, and when the latter is extended 

 it forms a poi7it ctappui, on which the remigium moves, and its 

 capitate extremity is brought into relation with the articular cavity in the 

 head of the remigium. In this position it forms a continuation of the 

 remigium, uniting it with the strong ascending process of the episternum. 



Eelations of the Episternum to the Anterior Wing-Root (PI. VIII., 

 Fig. 7)-— The episternum extends towards the dorsum in front of and below 

 the wing — I term this the 'ascending process' of the episternum. The ascend- 

 ing process is divided into two parts by a fissure in front of the great ninpulla 

 (/)• The anterior portion of the ascending process is twisted on its long 

 axis, when the wing is extended, so that its surfaces, which look inwards and 

 outwards below, look backwards and forwards above. Its upper edge sup- 

 ports a hood-like pouch, the pre-epaulet, the margin of which bears the 

 epaulet and sub-epaulet. When the wing is flexed, the anterior part of the 

 ascending process is flat, and it is the partial rotation of its upper end 

 which extends the wing. This is effected by the closure of the meso- 

 pleural syndesmosis and the rotation of the parapteron on a vertical axis, 

 so that the outer surface of this sclerite looks backwards and its inner surface 

 forwards. The extension of the wing depends on the contraction of 

 the great longitudinal thoracic muscles, and is the result of the shortening 

 of the thoracic cavity from before backwards. The posterior part of the 

 ascending process bears the great ampulla and the hook or hamulus. The 

 increased convexity of the dorsum also causes the hamulus to descend and 

 draw the wing down by pressing on the stirrup of the unguiculus. 



The sacculus {sac) is a membranous sac which lies beneath the hamulus. 

 It is distended with air during flight so that it supports the foot of the 

 unguiculus, and keeps it in relation with the hamulus. Diminished convexity 

 of the dorsum, with flexion of the wing, releases the hamulus from the foot of 

 the unguiculus, and allows the wing to ascend by swinging on the axis of 

 the dens. 



