22 INSECTS AFFECTING DOMESTIC ANIMALS. 
stockmen familiar with the subject. We have not had opportunity to 
personally examine a sufficient number of herds and compare the ratio 
of infestation among cattle of different colors to one ourselves that 
there can be much of truth in it. 
In one herd examined with some care the Holstein cattle were more 
generally affected than those of other breeds, but these had a pre- 
ponderance of black in their color, and on this account the ‘“‘nits” were 
especially conspicuous. In the same herd, however, one red heifer was 
quite as badly infested perhaps as any of the others. 
It is not always safe, however, to set aside the conclusions of experi- 
enced men in any branch of industry, however little foundation they 
may seem to have from a logical standpoint; and if these ideas prove 
to be supported by fact, we will no doubt in time learn the reason. for 
such selections on the part of the parasites. It is, in fact, a point of 
rather general observation that in the human family mosquitoes, flies, 
and other insects will appear to select certain individuals in preference 
to others, which we may consider as due to some peculiar condition of 
the skin or its secretions. Flies seem to take particular pains to settle 
on the exposed parts of sick people, and lice, itch mites, ete., if certain 
observations and records are to be accepted, show a decided tendency 
to infest certain persons and to be either unable or unwilling to harbor 
upon others. Certain people exhibit much greater susceptibility to 
such attacks than others, and with the harvest mites or “ chiggers” this 
amounts to practical immunity from attack by some when, under 
similar conditions, others are most seriously troubled. Whether this 
is due to some condition of the skin that attracts in one case and 
repels in the other, or simply that in one case there is particular sensi- 
tiveness, while in the other there is not, the effect on the persons is in 
one case the extreme of irritation and in the other freedom from it. If 
similar conditions exist among the lower animals, we may suppose the 
attractive influence of conditions of the skin in certain animals—and 
this in animals of unhealthy action of the skin—might act as a positive 
influence. As far, however, as difference in color 1s concerned, unless 
this is associated with some very constant difference in conditions of 
the skin (such as thickness, density of the hair or diameter of the 
individual hairs, or in the secretions, better suited to the clasping, pro- 
tection, or subsistence of the lice, there would seem to be slight founda- 
tion for the influence in selection. 
The fact that lice infesting one species of bird or mammal are in 
many cases incapable of existing upon the bodies of other species has 
doubtless a foundation in difference of the skin or its secretions or in 
the size of the hair. The thickness of the skin varies greatly in differ- 
ent animals, and consequently the proboscis of a species adapted to 
some thin-skinned species might be entirely incapable of reaching the 
capillaries from which its food supply must be drawn in a species 
having a thicker skin. The secretions of the integumentary glands 
eA See 
© one ies ap 
