BIOLOGY AND CONTROL OF CHIGGERS. 



9 



ing the actions of larvae with high-power objectives after they had 

 been removed with a " slice " of epidermis. They wriggled first 

 one way, then another, pulled with all their strength backward and 

 forward, gave side twists, and in fact strained in almost every pos- 

 sible way until released. One individual was timed during this 

 process, and it took it seven minutes to free itself from the hold it had 

 obtained on the epidermis. 



These observations were repeated upon a lot of 16 individuals for 

 nine successive days. They were numbered as before, and daily ob- 

 servations made upon them. Not only did none of 

 these larvae burrow into the skin, but they remained 

 attached only by their mouthparts and ^engorged 

 like ticks. Later they released this hold and fell off. 



DO CHIGGERS ENTER THE PORES OF THE SKIN? 



Some authorities, while not believing that chig- 

 gers burrow into the skin, yet hold that because 

 of their minute size they enter the pores and thereby 

 cause much inflammation and other injury. This 

 point has been carefully investigated. Of the 26 

 numbered individuals that were observed and stud- 

 ied daily, 21 were attached to the smooth surface 

 of the skin, while 5 were attached at the bases of 

 hairs, each having the capitulum thrust into the 

 mouth of the hair follicle as shown in figure 4. 

 Not a single one had penetrated a pore or hair 

 follicle. 



The species occurring in the northeastern part of 

 the United States shows a tendency to attach at the 

 mouth of hair follicles. It may be that the larva? 

 actually try to enter. They are prevented, however, 

 from doing so under normal conditions of the skin 

 by the small diameter of the follicles themselves. 

 For this same reason it would be impossible for chiggers to enter the 

 pores of the skin, unless the latter were greatly dilated as a result of 

 some skin trouble. In diameter the pores of the skin range from 

 20 to 50 [jl, according to Piersol. The width of an unengorged larva 

 from either the western or eastern part of this country is approxi- 

 mately 150 pi. Thus it is seen that unless the pores were unusually 

 dilated the mites could not enter if they would. 



In the case of persons who have just cleaned out the pores of the 

 skin after a long period of negligence, it would be possible for the 

 mites to enter some of them, as, for example, pores dilated by come- 

 dones. The writer has observed such pores dilated until they were 

 55672°— 21— Bull. 986 2 



Fig. 4. — " Slice " 

 of epidermis from 

 the skin of calf 

 of leg showing 

 method of at- 

 tachment of east- 

 ern chigger in 

 mouth of hair 

 follicle. 



