vmj SOME STRUCTURAL FEATURES 113 



muscular walls of the bag in which the poison is 

 stored. Near its front end each needle bears an 

 enlargement whose surface is covered with numerous 

 fine scales like those of a fish ; when, during the pro- 

 trusion of the "needles," these enlargements reach 

 the narrower part of the "director" they probably 

 act as pistons and sweep the poison along towards 

 the hinder, pointed extremity. It is possible that 

 they also serve to close the triangular cleft between 

 the edges of the director in front of the region where 

 these actually touch each other. 



The tube from the poison-bag discharges close 

 against the forward enlargement of the "director' 

 and between the divergent front portions of the 

 "needles," which are thus conveniently placed for 

 receiving the fluid between them. The poison 

 escapes from between the needles at their apices, 

 and also through five minute canals that pass ob- 

 liquely from the poison groove through the bases of 

 the first five barbs. Neither the "director" nor the 

 "needles" are solid structures; tracheal (breathing) 

 tubes are plainly traceable within their substance, 

 and distinct cavities are visible in transverse sections. 

 It is not probable that the poison produced by all the 

 stinging Hymenoptera is of the same quality and com- 

 position : that of some is very deadly in its effect on 

 other insects, whereas in other cases paralysis and not 

 death is caused. Both Oarlet and Bordas, who have 



L. B. W. 8 



