1907.] PUBLIC DOCUMENT No. 73. 107 



culty of orientation, the sections made did not pass through the lesions. 

 The mesenteries, in addition to the cells already described, show numer- 

 ous stellate cells, probably fibroblasts. 



The results of these few animal inoculations show clearly one point 

 of difference between the lesions produced by the active nettling hairs 

 and the lesions produced by those which have been inactivated. The 

 lesion produced by the subcutaneous inoculation of the active nettling 

 hairs shows a much greater amount of fluid exudate than is the case 

 in the lesion produced by inactivated nettling hairs. This fact tends 

 to support the view that the action of the nettling hairs is not purely 

 a mechanical one. 



SUMMARY AND CONCLUSIONS. 



The most important facts thus far ascertained concerning the brown- 

 tail moth dermatitis may be summarized as follows : - 



The lesions are produced by the penetration into the epidermis of 

 peculiarly modified microscopic hairs, the nettling hairs, which are 

 sharply pointed and barbed for their entire length. The other hairs 

 of both the caterpillar and the moth do not produce any dermatitis. 

 These nettling hairs develop on the caterpillar, but are also found, as 

 the result of direct transference, on the cocoons, moths, egg masses, and 

 in the winter webs, and are blown about in the air. They develop on 

 the subdorsal tubercles of the fifth and sixth segments of the young 

 caterpillars, but are much more numerous as the caterpillars attain 

 their growth, being then found on the subdorsal and lateral tubercles 

 from the fifth to the twelfth segment inclusive. The caterpillars are 

 then very poisonous. 



The nettling hairs, which from their shape are perfectly adapted for 

 penetration, possess in addition an irritating substance, which is un- 

 doubtedly an important factor in the production of the dermatitis. 



This substance may be destroyed by heating the nettling hairs at 

 115 C. either in a fluid or with dry heat, or it may be extracted from 

 them by certain solvents, such as dilute solutions of alkalies at room 

 temperature, or water heated to 60 C. Nettling hairs inactivated by 

 either of these measures produce no more than a slight redness when 

 rubbed into the skin, and probably act then merely as foreign bodies. 



An index for the presence of the irritating substance is found in a 

 peculiar reaction which takes place when the active nettling hairs are 

 mingled with a drop of blood between a slide and cover-glass. This 

 reaction begins about the point of the hair, but spreads rapidly, so that 

 a large area is involved. The first change is the breaking up of the 

 rouleaux of red blood corpuscles. The corpuscles then become coarsely 

 crenated; later are spherical, with slender spines protruding from the 

 surface; and finally appear perfectly spherical and closely packed. If 

 the irritating substance has been previously either inactivated by heat 

 or extracted from the nettling hairs, they no longer give this reaction 

 with the red blood corpuscles. 



