1907.] PUBLIC DOCUMENT No. 73. 150 



cuticle is studded. (See Plate, Fig. 3.) The sockets are rather closely 

 set, but usually not in actual contact one with another. The number of 

 nettling hairs to each socket varies within wide limits; some sockets 

 hold but three, while others hold as many as twenty. Beneath the areas 

 upon which the nettling hairs develop the epidermis is modified, and is 

 represented by a large mass of cells. These cells are long and fusiform, 

 with their long axes perpendicular to the surface of the cuticle. The 

 nucleus in each cell is nearer the extremity away from the surface, and 

 the major portion of the cytoplasm is in the form of a long process, 

 which extends into the socket which holds the nettling hairs. Whether 

 these groups of cells are of the nature of poison glands, or simply the 

 formative cells of this type of hair, has not been determined. There is 

 nothing distinctive as regards the microchemical affinities of these cells 

 when compared with cells at the base of the coarse hairs. On the other 

 hand, the granular material within the nettling hairs simulates very 

 closely that found within the coarse hairs, notwithstanding that one 

 is poisonous and the other not. The homogeneous cuticular layer is 

 continuous over every part of the sockets, so that there is no apparent 

 communication between the cells on the interior and the points of the 

 nettling hairs. The significance of the collections of cells beneath the 

 areas bearing the nettling hairs might be understood if stages could be 

 obtained in which these hairs were in the process of development. 



There has been considerable conjecture as to whether the dermatitis 

 is due to the mechanical action of the nettling hairs or to a poisonous 

 substance conveyed by them. Fernald and Kirkland submitted material 

 consisting of hairs, cocoons, and molted skins to Mr. F. J. Smith, 

 chemist of the gypsy moth committee. His notes are as follows : - 



I made a number of extracts of the hairs with each of the reagents men- 

 tioned below, some of the extracts being of the hairs alone, others of the 

 molted skins, and still others of the cocoons which contained hairs in great 

 numbers. The reagents used were: water, alcohol, ether, chloroform, petro- 

 leum ether, acetone, acetic ether, dilute sulphuric acid, dilute caustic potash. 

 I tested each of the extracts after digesting for some hours, and in each case 

 they nettled the skin. On the other hand, the -filtered extracts (freed from 

 hairs) caused no irritation of any sort when applied even where the skin 

 was broken. Careful chemical tests failed to show the presence of any or- 

 ganic acids or alkaloids. Hence I am led to believe that the irritation is of 

 a mechanical nature, caused by the brittle, finely barbed hairs, and not due 

 to a toxic principle. 



This analysis would indicate that the action of the nettling hair is a 

 purely mechanical one. These results, however, appeared far from con- 

 clusive when the degree of the reaction in the tissue around these ex- 

 tremely minute elements was considered. In order to follow up this 

 question, controls of the nature of purely mechanical agents were sought. 

 Fine glass wool was comminuted between two glass slides, and then rubbed 



