3' -4 THE TRACHEAL SYSTEM OF THE IMAGO. 



open and close like bellows, on a hinge formed between the 

 bow and the spiral prolongation of the bow which supports the 

 inner valve. 



There is a layer of muscle fibres extending between the rings 

 which support the inner and outer valves, and these, by their 

 contraction, effectually empty the spiracular sac, forcing the air 

 which it contains through the inner valve into the tracheae. 



The outer valve is opened by the contraction of the muscle 

 of the lever which depresses the bow ; as soon as this muscle 

 relaxes the outer valve closes. This is apparently effected by 

 the elasticity of the curved atrial tube and of the bow ; the 

 muscle of the spiracular sac then contracts and draws the inner 

 and outer rings together, at the same time opening the inner 

 valve and driving the air into the tracheae. The spiracular sac 

 is opened again by the spring-like action of the spiral which 

 supports the inner valves, and the depression of the lever 

 which opens the outer valve. In short, the vestibule or 

 spiracular sac is a small bellows which pumps air into the 

 tracheal tubes. 



The trachea:, which arise from the first and second pairs of 

 abdominal spiracular sacs, communicate directly with the 

 aerostats (p. 217) or abdominal pulmonary sacs; those of the 

 remaining abdominal spiracles form arborescent tracheae which 

 are not directly connected with the thoracic trachea; and with 

 each other, except by small or capillary branches. These 

 tracheae do not dilate into air-sacs in the Blow-fly, although in 

 many insects the abdominal tracheae, like those of the head 

 and thorax, dilate and form thin-walled air-sacs. 



b. Structure of the Tracheae and Air-sacs. 



The Tracheae and Air-sacs, in dissections made under water or 

 glycerine, exhibit, as is well-known, a silvery lustre, and appear 

 with reflected light like tubes or vesicles filled with mercury. 

 This appearance is due to reflection from the surface of the 

 contained air. It is very beautiful, and may be preserved in 

 dissections mounted in glycerine ; for if the cells are properly 

 sealed, the air is not dissolved out of the larger or even moderate- 



