396 THE ALIMENTARY CANAL OF 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 (Fig. 51) are very capacious ; 
they commence as a pair of membranous vessels from the 
great cervical trachez (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 (m), which arise 
from the front edge of the gena; their contraction opens the 
Fic. 51.—The Air-Sacs of the Proboscis. a, inferior cervical air-sac ; 4, valve in the 
anterior wall ; ¢ ¢c, anterior, and dd, posterior air-sac of the rostrum ; d, air-sac of 
the haustellum ; e, azygos air-sac of the labelle ; //, tracheal vessels of the oral 
disc ;_/, the jugum ; 7, muscle controlling the valve é ; ¢, rete mirabile of the 
tentorium. 
valve and permits of the passage of air from the thoracic cavity 
into the trachez of the proboscis. Immediately in front of this 
valve the tracheal trunk dilates and gives off numerous vessels 
(ty trv) of small calibre, which are cylindrical and exceedingly 
tortuous; they form a rete mirabile in the tentorium, and 
probably communicate with the large trachez above that 
membrane. The main trunk then divides into two branches : 
an anterior air-sac (c c) which lies on the outer side of the 
fulcrum, and a posterior sac (d d) which descends behind the 
fulcrum, enters and traverses the haustellum, and terminates 
by uniting with its fellow in a trefoil-shaped azygos air- 
