1S5 



tissues and beiietitiiij>- iiistojul of injiiiiiij;- tlioir hosts. But su])posingf 

 this food supply became iuade<piate. Hunj;er would under such cir- 

 cumstances lead to the removal of all dead matter down to the (piick, 

 and might easily force mites to work into the living- skin; and we shouhl 

 have circumstances leading to the development of a truly parasitic 

 species. 



The course ot development as thus marked out is not entirely imagi- 

 nary. We have in the families TyroglyphidtB and Sarcoptidae examples 

 illustrating all the stagt^s which liave l)een indicated — vegetable feed- 

 ers, mixed feeders, scavengers, commensals, and parasites — and I be- 

 lieve something like this was the course taken by these mites in assum- 

 ing the parasitic mode of life. 



As illustrating the stages mentioned I would call attention again to 

 the Tyrogly])hidie. Its members are active mites, allied in structure to 

 the Sarcoptidie. Cheyletus, a predaceous genus of the family, often 

 taken among animal refuse, as hairs, feathers, and even occasionally on 

 the body of man, may fairly be considered an intermednite form, hav- 

 ing the striated body of the Sarcoptidjie, but being in certain other re- 

 spects one of the Tyrogl,\q)hid;e. It has been put first in one then in 

 another group, and its place is not yet definitely fixed. 



Glyciphagus spinipes, another of the family, occurs among Canthar- 

 ides. 



O. h ipjwpodos has been found to produce severe sores about the hoofs 

 of horses. 



G. cursor occurs among Cantharides, feathers, and so forth. 



O. prunorum was found, it is said, by Hering among dried plums, 

 where it appears to feed on the sugar used in i)reserving the fruit. It 

 is known to produce a transitory inflammation by attacking the hands 

 of shopmen. Here we have a species which might readily assume the 

 parasitic habit. 



There are others of the family which take vegetable and animal food 

 indifi'erently, and still others of the genus Rhizoglyphus, the lowest of 

 the family, which devour only vegetable matter. 



Now, when we turn to the Sarcoi)tidie, and find examples infesting 

 birds, the lowest and oldest of the two host grou])s, which appear to 

 feed only on the waste of the skin, the series seems quite complete. 



A few words may be added concerning those peculiarities of form and 

 structure among Sarcoptidie, which have special relation to a life in 

 and on the skins of mammals and birds — though I must premise that I 

 have nothing new to present on this part of my topic. 



The characters are of two kinds, namely, (1), those which subserve a 

 useful purpose in the economy of the species, such as the highly devel- 

 oped tarsal daspers of Myobia, the tarsal suckers and forcipate man- 

 dibles of Sarcoptes, and the striated and otherwise roughened skins of 

 all the species ; and (2) the degradational characters, such as the absence 

 of ocelli and of functional limbs. 



