158 PROCEEDINGS OF THE NATIONAL MUSEUM vOL. 108 



placed in the jars. Usually some medium was added. Soil, the 

 various soil mixtures, humus, and organic debris had certain 

 disadvantages. Frequently the surfaces became covered with 

 fungus. The nymphs often entered these media and were never 

 seen again. Such media were difficult and unsatisfactory to move 

 for examining the cultures. Vermiculite v/as the most satisfac- 

 tory medium. It formed a soft, loose stratum through which the 

 mites could move easily. It could be rolled about easily for ex- 

 amining the culture without damaging the mites. Apterous in- 

 sects, sometimes kept in cultures, survived and multiplied in 

 vermiculite. Nymphs seemed to survive better in vermiculite 

 than in other media. 



Fungus frequently developed in cultures but never became a 

 serious problem. When cultures were carefully moistened and 

 excess water avoided, fungus was retarded. Its control with 

 vermiculite was easier than with other media. Rolling vermiculite 

 in the culture jar broke up mycelia and reduced growth. With 

 both soil and vermiculite, adding Onychiurus sp. to cultures was 

 usually effective in controling fungus. These small, apterous 

 insects fed on the fungus. Apparently they were not harmful 

 to chiggers or nymphs. 



Cultures maintained at 30° C. seemed to follow the same pattern 

 as those held at room temperature. Nymphs steadily died out. 

 Nymphs held at 5° to 10° C. were very sluggish in their move- 

 ments when examined, but they moved more rapidly after being at 

 room temperature for a few minutes. Room temperature seemed 

 more satisfactory than other temperature ranges tried. Cultures, 

 excepting those held at definite temperatures, were kept in ai 

 darkened cabinet. 



Evidence is not adequate to determine whether a proper food 

 for nymphs of E. peromysci was found. So far as was determined i 

 by direct observation, the nymphs rejected almost all food offered. 

 Aedes aegypti eggs were offered regularly to most cultures. Under- 

 observation E. peromysci nymphs appeared to try to feed on these 

 eggs on only one occasion. On the day one culture was estab- 

 lished, two nymphs were observed manipulating eggs. Body 

 contractions and expansions, typical of feeding nymphal or adult 

 trombiculids, occurred. The efforts ceased in a few minutes. 

 Examination of one of the eggs showed no openings or any other 

 evidence of successful feeding by the nymph. Four successful 

 acts of feeding on freshly dissected Aedes aegypti ovaries and 

 eggs were observed. The first nymph to feed stopped at a scratch 

 mark made by the needle in the plaster-charcoal base in placing 



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