26 Trans. Acad. Sci. of St. Louis. 



are not as a rule restricted to a single host species, also 

 that their normal hosts are always closely related forms, 

 usually species of the same genus or of closely related 

 genera. 



In regard to the fuller biological significance of the 

 distribution of the Trombidium larvae and the parasitic 

 larvae of related forms, Rhyncholophidae, Hydrachnidae, 

 and Halacaridae, several interesting facts were brought 

 to light. First, some of these larvae may sustain them- 

 selves for long periods of time when detached from a 

 host. Thus I found that the larvae of one of our com- 

 mon hydrachnids remained alive for sixty-four days in a 

 watch glass without any hosts. In this case I believe 

 that these small creatures lived partially upon minute 

 organisms found in the water, but no doubt these various 

 larvae can live for several days without food. This I 

 found to be the case with the larvae of Microtromhidium 

 muscarum Riley, the larvae of our common house-fly 

 mite, which lived for five days in confinement without 

 food. 



I made several attempts to inoculate other forms than 

 the normal hosts with the larvae of Microtromhidium 

 muscarum (Riley) and M. locustarum Walsh, but was 

 unsuccessful. The larvae soon fell from the forms on 

 which they were placed and apparently never did firmly 

 fasten the beak into their integument. I was somewhat 

 surprised at this for in some cases the alien species used 

 was very closely related to the normal host. I thought 

 that it might have been because of mechanical reasons, 

 or because that once having a square meal the appetite 

 no longer acted as a sufficient incentive for obtaining a 

 second attachment. However, I found this was not true, 

 at least with Microtromhidium muscarium Riley, as I 

 got this species to change its attachment from one in- 

 dividual of its host species (the house fly) to another 

 very easily. 



In regard to the experiments made in attempts to 

 get a suitable host for two hydrachnid larvae, I will 



