SELECTED NOTES. 179 



Blow-fly, Anterior thoracic spiracle of. —This spiracle is oval 

 and narrowest above, and is situated between the pro- and meso- 

 thorax. From the exterior free edge project hollow, arborescent, 

 chitinous rods which curve outwards, and interlock for about one- 

 third of the length of the spiracle. These rods are hollow even to 

 the minutest twigs, and have free openings at their points ; close 

 behind is a transparent membrane, the true valve, which is united 

 to the wall of the large tracheal vessel, which extends across the 

 thorax to the opposite spiracle. The free edge of the valve is 

 closely set with a chitinous fringe. A special muscle arises from 

 the integument at the lower end of the spiracle ; by the contrac- 

 tion of the muscle the free edges of the valve would be caused to 

 approach each other. From the integument another set of muscles 

 arise and are directed towards the valve, but whether they are con- 

 nected with it I have not been able to determine ; antagonistic 

 muscles are a necessary consequence for working the valve. 



The structure of the posterior thoracic spiracle is very similar 

 to the anterior one, excepting that the external chitinous rods are 

 formed into two distinct masses by the addition of a connecting 

 membrane. 



Sericomyia borealis, Posterior thoracic spiracle of.— This spi- 

 racle differs from the corresponding one of the Blow-fly in having 

 the chitinous rods free, and also that the walls behind the valve 

 are lined with walls of membrane, the edges of which are directed 

 inwards. 



Abdominal Spiracle of Blow-fly.— The spiracles of the abdo- 

 men are very much smaller than those of the thorax, their relative 

 sizes being as i to 7. The spiracle is round, fringed with fine 

 hairs, and the valve, which is placed a very short distance behind 

 it, appears to consist of a thickened membrane on the one side 

 which gradually thins out towards the free edge. The other half 

 is thinner and more flexible, and its movement is effected by a 

 curved rod or bow, hinged at one end, the other being connected 

 with a set of muscles arising from the edge of the spiracle. No 

 antagonistic muscles have been found by me, but if air is both 

 received and expelled by it, such muscles are certainly requisite. 



It is the generally accepted theory that all the spiracles are 



