222 AMERICAN HYMENOPTERA. 



being open below to a slight extent from base to apex. The 

 dorsal piece is known as the sheath. It consists of a promi- 

 nent and elongate basal swelling, which contains a large 

 cavity and is known as the bulb, of a terminal tapering shatt, 

 and of two elongate, curved, foliaceous basal arms or roots. 

 The two ventral pieces are long slender rods, called the lan- 

 cets or darts, which slide freely upon two tracks on the ventral 

 edges of the sheath and diverge upon grooved tracks run- 

 ning the entire length of the basal arms. These lancets have 

 several small teeth or barbs on one side near their distal 

 ends. The cavity of the sheath bulb, in a narrowed exten- 

 sion, continues through the entire length of the shaft of the 

 sting as a channel running between the sheath and the lan- 

 cets. This channel is the poison canal of the sting. 



Several paired plates of definite shape and arrangement 

 are attached to and overlap the base of the sting. None of 

 these plates appear to have any taxonomic value, and only 

 two of them need to be considered in detail. One of these 

 paired plates maybe called the palptis plate, for to each plate 

 of the pair is attached an elongate hairy organ, these organs 

 being known as the sting palpi. These palpi are very promi- 

 nent and are dorsal to the sting. The plates which bear 

 them also support the basal arms of the sheath and are con- 

 nected by a membrane which lies against the bulb of the 

 sting. The plates of the other paired plate, which deserves 

 attention, bear spiracles and appear to represent a part of 

 the eighth abdominal segment (the propodeum being con- 

 sidered as the first), most of the other plates present repre- 

 senting the ninth segment. These spiracle bearing plates 

 (figs. 24. 104, 106, 116 and 118) show considerable variation 

 in form between the various species and also, to some extent, 

 between different individuals of the same species. 



A membrane extends across between the two basal arms 

 of the sheath of the sting. The inner margin of each of 

 these arms is usually gradually extended more or less into 

 an inconspicuous lobe. In the species of the Terrestris 

 group, this lobe is divided into two well developed teeth in 

 the queen and sometimes in the worker also. These lobes 



