The list of diseases associated with mites and especially with the liponyssid mites has been reviewed.
A method for rearing and maintaining mites that permits the continuous observation of individual mites during their entire life has been described.
The observations on the effect of temperature and relative humidity appear to establish within rather narrow limits the conditions of the micro-environment under which the mite can multiply either in the laboratory or in nature. Since the eggs are always laid away from the host, the natural environmental temperature determines their hatching. Egg laying, and therefore reproduction, does not occur at 6° to 8°C., although the adult may live for more than a month at these temperatures. Eggs are laid and protonymphs develop at 12° to 14°C. Protonymphs in nature feed on the body of the rat deep within the fur and protected from a cold environment. It appears likely that 12° to 14°C. represents the lowest temperature at which L. bacoti can reproduce, and then only with a great lengthening of the cycle and an increase in mortality.
At low and intermediate temperatures, these mites can tolerate a wide range of relative humidity. At 24° to 26°C., they withstood a relative humidity of 18 to 20 per cent. However, with increasing temperatures, increasing humidity is necessary to permit survival. Slightly over 60 per cent relative humidity is necessary to permit a complete cycle at 34° to 36°C., while even at 100 per cent relative humidity all mites died at 36° to 38°C.
At least for the temperatures and relative humidities studied, the conditions of the open laboratory (24° to 26°C. and 47 per cent relative humidity) gave the best over-all opportunity for development and permitted rearing to the adult stage of 62.5 per cent of some 627 eggs studied under those conditions. There was, however, some evidence that humidities higher than 47 per cent are more favorable to survival of the egg. In the open laboratory, the average life of the adult female was 61.9 days and the average number of eggs per female 98.8; a little over ¾ of the mites reached the adult stage between the 11th and the 16th day. These figures suggest a considerable reproductive ability in L. bacoti and, since they undoubtedly reflect some mortality due to handling, it may be that the mites reproduce even more efficiently in nature under favorable conditions.
Observations in January and February 1948 at Savannah showed the lowest temperature of even the accessible portion of both protected and unprotected Rattus norvegicus burrows was 7.2°C. while the highest temperature was 20.5°C. The relative humidity of these burrows was usually over 90 per cent but never as high as 100 per cent. Temperature changes in the burrows always lagged behind and were less marked than those of the environment. The average burrow temperature in winter was warmer than the environmental temperature. These observations suggest that, even in the coldest months of winter, many days occur in the Savannah climate which could permit some reproduction of L. bacoti.
The laboratory studies suggest some of the factors which may be expected to favor the appearance of excessive numbers of L. bacoti or to greatly reduce these ectoparasites in nature.
No significant difference occurred in the total egg production and in the length of the adult period of fertilized and unfertilized females. Unfertilized eggs developed parthenogenetically into male offspring which as adults, were capable of fertilizing female adults.