1907.] PUBLIC DOCUMENT- -No. 73. l/>7 



irritation. Furthermore, by serial sections of lesions produced cx|icri- 

 mentally, I have been able to demonstrate the nettling hairs imbedded 

 in the skin. The most important result of this investigation is the proof 

 that the action of the nettling hair upon the tissue is not a purely me- 

 chanical one, as the observations of Fernald and Kirkland and others 

 tend to indicate, but that there is in addition an irritating substance. 

 When this irritating substance is removed from the nettling hairs they 

 are then practically innocuous, and, when inoculated, act merely as 

 foreign bodies in the tissue. 



The nettling hairs are of the form of straight, tapering, needle-pointed 

 shafts, barbed for their entire length, after the manner of a certain 

 form of African spearhead. They vary from .07 to .2 millimeter in 

 length, the average is about .1 millimeter, or ^- of an inch, and 

 are not over .004 or .005 millimeter in thickness at the thicker extremity. 

 They possess a thin, chitinous wall, from which project three rows of 

 recurrent barbs; while the interior of the shaft consists of material 

 which appears finely granular, and stains with the ordinary anilin dyes 

 after fixation in Zenker's fluid. When the nettling hairs are thoroughly 

 dried, they often contain air. No pore or opening is visible in these 

 hairs, even on high magnification ; but if they are dried, and then placed 

 on a slide in some such stain as Loeffler's alkaline methylene blue solu- 

 tion, the dye is seen first to penetrate the point of the hair, and after- 

 ward gradually to diffuse itself through the remainder of its length. 

 From this phenomenon it appears that there is a minute opening at the 

 point of the nettling hair, although it can not be visually distinguished. 

 When suspended in a fluid, individual nettling hairs appear to the 

 unaided vision as brownish scintillating points. In the dry state large 

 masses of them form a fine brown powder, which is very light and 

 easily blown about. 



The nettling hairs develop upon the caterpillar. Although Fernald 

 and Kirkland and others state that the nettling hairs are present upon 

 the caterpillar only after the last two molts in the spring, they are, 

 nevertheless, demonstrable much earlier. The two velvety brown spots, 

 which appear on the dorsal aspect of the fifth and sixth segments after 

 the first molt, and while the caterpillars measure but 4 or 5 millimeters 

 in length, are found to consist of nettling hairs. Sections of these small 

 caterpillars show the anatomical relations of these hairs, and when the 

 latter are rubbed upon the skin a dermatitis is produced. The caterpil- 

 lar is thus demonstrated to be poisonous at a very early stage in its 

 development. The young caterpillars hibernate in colonies in the winter 

 webs which are found on the tips of twigs. These webs contain, in 

 addition to the young caterpillars, the skins of their various molts, and 

 may produce irritation if torn open. The two brown spots situated on 

 the back of the caterpillar are in reality two pairs of subdorsal tubercles. 

 They are likewise found on the fifth and sixth segments after each suc- 

 ceeding molt up to the last two spring molts, when they are present on 



