THE EXO-SKELETON OF THE PROBOSCIS. 139 



and its connection with the clypeus. It has, however, been very 

 generally adopted. 



Retraction and Extension of the Proboscis. The proboscis is an 

 erectile organ. It is flaccid and folded on itself when not in 

 use, but is capable of being rendered rigid by the injection of 

 air into the extensive tracheal sacs which lie in its cavities. 

 When at rest the only rigid parts are the fulcrum, the prelabrum 

 and the theca. The two lateral halves of the flaccid stomal disc 

 are folded together, so that the stomal surface is concealed. The 

 disc is also flexed on the ventral surface of the theca. The haus- 

 tellum is folded forward on the rostrum, so that the back of the 

 labrum lies against the clypeus, and the fulcrum turns upon the 

 epistome, so that its dorsal surface looks downwards and back- 

 wards. In this position it lies entirely within the head capsule, 

 and the maxillary palpi rest one on either side of the haustellum, 

 which is also more or less completely withdrawn into the head 

 capsule. The thin integument of the ventral surface of the 

 rostrum is invaginated into itself, and lies in folds above and 

 behind the retracted proboscis. 



When the proboscis is in use, it is projected from the head 

 capsule, and rendered stiff by the injection of air into its 

 tracheae. Its varied movements are brought about by the 

 action of muscles ; the flaccid proboscis is also folded and 

 withdrawn into the head capsule by means of retractor 

 muscles. The nature and precise mechanism of the move- 

 ments of the proboscis will be further discussed in another 

 section. 



The Prelabrum is the rostellum of Fabricius, the labrum-epi- 

 pharynx of Menzbier and most recent authors. It is a hollow 

 prolongation of the mesolabrum, and lies in the groove of the 

 theca. It exhibits two pairs of sclerites, one pair on its dorsal 

 convex surface and one pair on its oral concave surface ; the 

 latter form the roof of the long tubular mouth. These sclerites 

 are all fused in the fully-developed imago, but the two dorsal 

 are readily separated from the two oral sclerites in the young 

 imago, and also in the skeletons of mature insects prepared 

 with caustic soda or potash. 



