10 BULLETIN 986, U. S. DEPARTMENT OF AGRICULTURE. 
fully 400 or 500y in diameter. These pores, however, are most fre- 
quently on the face or neck—yregions seldom attacked by chiggers. 
In all the observations made, including many hundred, of chigger 
attacks, it has always been possible during the early stage of attack 
to locate the chiggers themselves or their evident places of attach- 
ment, and this has always been on the surface of the skin or in the 
mouths of hair follicles. 
DIFFERENCE IN SUSCEPTIBILITY. 
Another common belef among the public and entomologists is 
that a great difference exists between persons in susceptibility to 
chigger attacks. Such a difference usually has been assumed to be 
physiclogical. Observations were made to ascertain the foundation 
for such a belief, if any existed. Upon several occasions it was 
observed that there was a difference in injury to people who appar- 
ently had all been exposed equally to the attacks of chiggers. It 
was found in most cases, however, that although all members went 
on the same picnic, or collected berries in the same patch, or made 
the same journey, they were not equally exposed to the attacks of 
the mites. Particularly three fundamental differences were found: 
First, a great variation in the clothing, especially about the feet and 
ankles; second, a variation in the actions of the persons, some never 
sitting or reclining on the ground; and third, a great variation in the 
intensity of chigger infestation even over a small area. Observations 
clearly show that these are usually the reasons why some members of 
a party are but slightly attacked while others are driven almost 
frantic. : 
Laboratory tests show that chiggers attack by preference where 
the skin is very thin and the flesh wrinkled or tender. Field observa- 
tions also have brought out the fact that women and children suffer 
more from a given number of chiggers than men do. In other words, 
a correlation exists between thin skins and seriousness of chigger 
attacks. This, however, is the only way in which certain differences 
in the seriousness of chigger attacks between individuals equally 
exposed could be expiained. Although hundreds of people were 
found susceptible to chigger attacks, no one was found who was 
clearly shown to be immune. 
LOCAL INJURY. 
Since there has been so much confusion in regard to chigger in- 
jury, a careful tabulation was made daily in the case of two lots of 
infestations. The first lot of 10 individuals, located on various parts 
of the leg below the knee, were numbered and notes made daily upon 
the appearance of the local area around each point of attachment, 
with the following results: 
it i i 
