STING STRUCTURE. 189 



The poison bag, of considerable size, is lined with 

 epithelium, but is not muscular, as stated by 

 Mr. Hyatt, its venom being driven from it by the 

 play of the muscles giving activity to the apparatus, 

 and by a singular pump-like arrangement, presently 

 to be noticed. Indeed, a comparison of the highly 

 muscular poison sac of the wasp, which has no 

 valvular appendages, with that of the bee, is highly 

 instructive. The poison sac contracts into a strong 

 neck (pb, E, Plate VII.), which enters the upper 

 bulbous part of the sheath, in which play the valves 

 (va). The walls of the sheath are double, with blood 

 between, while it is into the cavity within the in- 

 terior lining that the poison enters, and so bathes 

 the back part of the surface of the darts, which 

 stand in the cleft in the front of the sheath, as 

 before stated. Since the darts present concave sur- 

 faces to each other, they inclose a tube-like space 

 (p, B, Plate VI.) between them, through which the 

 poison passes downwards, towards the base of the 

 wound, being driven forwards by the piston-like 

 action of the valves (va, E, Plate VII.), which 

 descend with the stroke of the darts to which they 

 are fixed, and sweep the poison before them, ram- 

 ming it onwards, when the end of the stroke is 

 reached, by the valve meeting the lower end of the 

 pouch. These valves are remarkable structures, and, 

 with regard to them, I venture to differ absolutely from 

 Mr Hyatt. It will be remembered that the bulbous 

 part of the sheath is oval in cross section. The 

 greater diameter of the oval runs from front to back, 

 and this space the two valves divide between them, 



