J^54 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 10« 



record was obtained at 7° C. The rate increased with continued 

 rise in temperature. Trombicula alfreddugesi, a chigger with a 

 warm weather distribution, showed a more rapid decline in rate of 

 locomotion with falling temperature and ceased locomotion at 

 10° C. With rising temperature, T. alfreddugesi resumed loco- 

 motion at 16° C; and its rate rose rapidly as temperature in- 

 creased. 



E. peromysci chiggers were recovered alive from an infested 

 Peromyscus leucopus host which had been kept frozen at — 4.5° C. 

 for 38 days. 



E. peromysci was found attached deep in the ear or in the 

 external auditory meatus of its hosts. After death of the host, 

 the chiggers detached and wandered over the ear (pi. 3) and fur 

 of the host. 



Discussion of ecology: From the evidence assembled, there 

 could be a correlation between temperature, soil moisture condi- 

 tions, and the seasonal population peak of E. peromysci on hosts. 

 During the colder months when most plant growth has ceased, 

 soil moisture is at its highest. The combination of low tempera- 

 ture and high moisture content of the soil should produce in the 

 soil the most humid conditions of the year. During these months, 

 E. peromysci had its greatest incidence on hosts in the Duke 

 Forest area. This conforms with the results of Pearse (1946), 

 who found populations of microfauna in Duke Forest generally 

 to be highest during the colder, moister season. 



That there is a correlation between the physiology of E. 

 peromysci and its seasonal occurrence is indicated in a compari- 

 son of the temperatures at which it and the warm weather T, 

 alfreddugesi ceased locomotion under experimental conditions. 

 The same correlation is evident in the temperatures at which 

 locomotion was resumed. In both phases of the experiment E. 

 peromysci carried on activity at much lower temperatures. Ex- 

 perimental evidence showed also that E. peromysci can withstand 

 freezing temperatures for extended periods. However, the occa- 

 sional occurrence of E. peromysci in the summer indicates that 

 temperature alone is not the critical factor in its seasonal distri- 

 bution. 



The temperature at which T. alfreddugesi stopped locomotion 

 corresponds well with the findings of Jenkins (1948). In his 

 experiments T. alfreddugesi was not active below 9.5° C. at 60 

 percent relative humidity. Various authors have indicated the 

 seasonal occurrence of different chiggers. Ewing (1921) pointed 

 out the seasonal distribution of T. alfreddugesi, Audy (1947a) 



