10 BULLETIN 986, U. S. DEPARTMENT OF AGRICULTURE. 



fully 400 or 500^ in diameter. These pores, however, are most fre- 

 quently on the face or neck— regions 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 belief 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 

 physiological. 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 equalty 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, especialW 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 explained. 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 : 



