152 PROCEEDINGS OF THE NATIONAL MUSEUM vOL. 106 



The population peak coincided generally with the season of least 

 rainfall (fig. 6,/) and highest soil moisture (fig. Q,9). 



Laboratory experiments showed tlie rate of locomotion of E, 

 peromysci to be influenced by temperature (fig. 6,/i). The rate 

 declined with falling temperatures until locomotion ceased at 

 0'' C. As temperature rose, locomotion was resumed; the first 



Explanation of figure 6 



a, Distribution of setules on the galeal seta of chiggers in the "rubra" group. Vertical 



axis represents frequency; horizontal axis, number of setules per seta; light solid 

 line, Euschoiigastia rubra; light broken line, E. magna; heavy solid line, E. diversa 

 acuta; heavy broken line, E. diversa diversa. 



b, Distribution of setules on the lateral seta of the palpal tibia of chiggers in the "rubra" 



group. Vertical axis represents frequency; horizontal axis, number of setules per 

 seta; hea\'y broken line, Euschongastia diversa diversa; heavy solid line, E. diversa 

 acuta; light broken line, E. magna. 



c, Seasonal distribution of Euschongastia peromysci in 91 collections of Peromyscus 



leucopus in the Duke Forest area, September 1947 to December 1949. The 91 col- 

 lections included 392 specimens of P. leucopus. Horizontal axis represents months 

 of the year, with the total number of collections of P. leucopus per month; vertical 

 axis, percentage of collections positive for E. peromysci. 



d, Seasonal distribution of Euschongastia rubra m 91 collections of Peromyscus leucopus 



in the Duke Forest area, September 1947 to December 1949. The 91 collections 

 included 392 specimens of P. leucopus. Vertical axis represents percentage of col- 

 lections positive for E. rubra; horizontal axis, months of the year, with the total 

 number of collections of P. leucopus per month. 



e, Mean monthly temperatures in Duke Forest area, September 1947 to March 1950. 



Vertical axis represents temperature in degrees centigrade; horizontal axis, months 

 of the year; light broken line, air temperature; light solid line, surface soil tempera- 

 ture; heavy solid line, subsurface temperature at three inches depth. 



/, Mean monthly precipitation in Duke Forest area from September 1947 to March 

 1950. Vertical axis represents inches of precipitation; horizontal axis, months of the 

 year; light line, mean monthly precipitation in inches (yearly average for period, 

 44.33 inches); heavy line, mean monthly precipitation for central North Carolina 

 for the 61-year period from 1887 to 1947 (yearly average for period, 46.76 inches). 



g, Monthly soil moisture in Duke Forest locality typical of areas in which hosts of 

 Euschongastia peromysci were trapped for the present study. Vertical axis rep- 

 resents soil moisture as a percentage of dry weight; horizontal axis, months of the 

 year; light broken line, percentage of soil moisture in the Ai horizon, to 2 inches 

 depth; light solid line, percentage of soil moisture in the As horizon, S to 7 inches 

 depth; heavy solid line, percentage of soil moisture in the Ba horizon, 14 to 16 

 inches depth. The bar diagram gives the monthly rainfall in inches. The graph of 

 soil moisture and the rainfall record covers the period from May 28, 1946, to March 

 16, 1947. There are no soil moisture records for December 1946 and no rainfall 

 records for May 1946. Data obtained from the thesis of Mr. John B. Sharp, Jr., 

 in the School of Forestry, Duke University, 1947. 



h. Behavior of E. peromysci and T. alfreddugesi during falling and rising temperatures. 

 Vertical axis represents rate of locomotion in millimeters per second; horizontal axis, 

 temperature in degrees centigrade; light line, rate of locomotion of E. peromysci; 

 heavy line, of T. aljreddugisi. Rates above .80 mm. i>er second for T. alfreddugesi 

 have been omitted. Calculations by Dr. G. W. Wharton. 



