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tissues and benefiting instead of injuring their hosts. But supposing 
this food supply became inadequate. Hunger would under such cir- 
cumstances lead to the removal of all dead matter down to the quick, 
and might easily force mites to work into the living skin; and we should 
have circumstances leading to the development of a truly parasitic 
species. 
The course of development as thus marked out is not entirely imagi- 
nary. We have in the families Tyroglyphide and Sarcoptidse examples 
illustrating all the stages which have been 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 Tyroglyphidz. Its members are active mites, allied in structure to 
the Sarcoptide. 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 intermediate form, hav- 
ing the striated body of the Sarcoptide, but being in certain other re- 
spects one of the Tyroglyphide. It has been put first in one then in 
another group, and its place is not yet definitely fixed. 7 
Glyciphagus spinipes, another of the family, occurs among Canthar- 
ides. 
G. hippopodos has been found to produce severe sores about the hoofs 
of horses. 
G. cursor occurs among Cantharides, feathers, and so forth. 
G. prunorum was found, it is said, by Hering among dried plums, 
_ where it appears to feed on the sugar used in preserving 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 
indifferently, and still others of the genus Rhizoglyphus, the lowest of 
the family, which devour only vegetable matter. 
Now, when we turn to the Sarcoptide, and find examples infesting 
birds, the lowest and oldest of the two host groups, 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 Sarcoptide, 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 claspers 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. 
