CH. V] WASPS 137 



and slightly upward and the needle itself is darted along 

 the groove of the director and projects beyond it, escaping 

 at a spot just short of the extremity. 



In the act of stinging the first incision is made by the 

 point of the director, which is itself darted out by the 

 action of levers LD, LD' and muscles : on its dorsal sur- 

 face is a strong piece, S, which at first holds the director 

 down, but eventually, when full extrusion has been reached, 

 causes the director and needles to turn abruptly upwards 

 towards the dorsal surface; then by a rapid alternate 

 movement of the levers the needles are driven deeper into 

 the wound. The two concave inner faces of the needles 

 are pressed firmly against each other, partly by the taper- 

 ing sides of the director and partly by the course imparted 

 by the muscles, and so form a closed tube between them. 

 Down this tube the poison is driven by the contractions 

 of the muscular wall of the bag in which the poison is 

 stored. An enlargement NP, NP f on each needle near 

 the anterior end perhaps acts as a piston on reaching the 

 narrower part of the director: the two pistons virtually 

 sweep the poison in front of them. Their surfaces are 

 covered with numerous fine scales like those of fish. 

 The two together may serve to close the triangular cleft 

 between the edges (N) of the director when the needles 

 have been thrust out. In this position they would prevent 

 the liquid poison from escaping through the triangular 

 cleft. It is possible that they perform both duties. 



The duct from the poison bag discharges close against 

 the anterior enlargement of the director between the 

 diverging anterior portions of the needles which are thus 



