38o 



HISTOLOGY 



it emerges from the thick cuticle of the horn-like process. This cuticle 

 on the horn is very thick indeed and is lined internally by a well-developed 

 hypodermis, inside of which is some connective tissue and a few blood 

 cells and other cells. Where a poison hair takes its origin the hypoder- 

 mis is evaginated to form a cylindrical layer extending obliquely up 

 through the cuticle and finally emerging at the surface as a hair-like 

 structure covered with its own thinner cuticle which is, of course, con- 

 tinuous with the thick layer. This whole structure is extended within 

 the entire length of the hair, which is closed on the end except for a 

 number of fine openings through which the poison can pass out. 



FIG. 346. Three poison cells from the epithelium lining the poison gland of the ground hornet, 

 Scolia dubia. p.n., poison cell nuclei; CM., cuticle on distal surface; hyp.n., nuclei of the 

 hypodermal cells that form the cuticle. At d. is the duct-like invagination of the cuticle 

 which ends in the poison sieve or secretion sieve at s. Parts of others may be seen in the 

 other cells. X 1200. 



The poison is produced by a large cell lying inside the hair and its 

 hypodermis, at about the level of the hair's emergence from the horn. 

 It is a very large and thick cell, with a large branched nucleus, and it 

 often looks to be, and may be, a syncytium. Its large cytoplasmic body 

 is hollow at its distal part, and this hollow end is produced into a tube 

 which carries the poison up into the hair inside of the hypodermis. 



The presence of openings in the hair to permit the poison to escape 

 has been questioned or even denied by some writers. The present writers 

 have not been able to see them and, therefore, think that the great irri- 

 tation (which has certainly been felt) is due to the breaking off of the 

 spines in the flesh, and the consequent introduction of the poison. No 



