396 



THE ALLMEXTARY CAXAL Ol- THE IMAGO. 



Kraepelin [70], and these authors have all correctly ascribed 

 the exsertion of the organ to the inflation of the tracheal air- 

 sacs, although the manner in which this is effected has hitherto 

 remained unexplained. 



The Air-Sacs of the Proboscis tFig. 51) are very capacious; 

 they commence as a pair of membranous vessels from the 

 great cervical tracheae (a) and descend in front of the jugum (/), 

 beneath the tentorium. Each trunk has a slender rod of 

 chitin (b) in its anterior wall, which closes the tube by pressing 

 against the jugum. This forms a valve capable of being 

 opened by a small bundle of muscle-fibres (>;/), which arise 

 from the front edge of the gena ; their contraction opens the 



Fir.. 51. The Air-Sacs of the Proboscis. /. inferior cervical air-sac ; />, valve in the 

 anterior \\all ; < f, anterior, and </</, posterior air-sac of the rostrum ; </, air-sac of 

 the haustellum ; < , a/yyos air-sac of the labelkv ; / /, tracheal vessels of the oral 

 disc ; /, the ju^um ; /, im^cle controlling the valve /' ; /;-, rete mirahile of the 

 ttntorium. 



valve and permits of the passage of air from the thoracic cavity 

 into the tracheae of the proboscis. Immediately in front of this 

 valve the tracheal trunk dilates and gives off numerous vessels 

 (tr tr) of small calibre, which are cylindrical and exceedingly 

 tortuous; they form a rete mirabile in the tentorium, and 

 probably communicate with the large trachea: above that 

 membrane. The main trunk then divides into two branches : 

 an anterior air-sac (c r) which lies on the outer side of the 

 fulcrum, and a posterior sac (</</) which descends behind the 

 fulcrum, enters and traverses the haustellum, and terminates 

 by uniting with its fellow in a trefoil - shaped azygos air- 



