226 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 106 



variations in relation to the environment do exist among species 

 of Euschongastin. But it is equally apparent that ecological 

 definitions and diagnoses are neither easily nor quickly acquired. 



No information of value in systematics was derived from the 

 extensive efforts to culture species of Etischongastia. That por- 

 tion of the trombiculid life cycle from unattached chigger, 

 through engorgement, the detached chigger, and the nympho- 

 chrysalis to the nymph was confirmed in the genus. 



Finally, following separation and definition of the species on 

 the basis of the available material, it might be well to consider 

 in summary fashion the biological relationships of the species 

 in the genus. According to Simpson (1945), "The genus tends 

 ... to be defined as a group of species possessing certain char- 

 acters in common." Thorpe (1940) discussed briefly the imprac- 

 ticability of defining genera on other grounds. In giving the 

 limits of the genus, Thorpe said, "For practical reasons the genus 

 . . . must in general be neither too large nor too small." The 

 genus, then, tends to be subjective. However, it is subjective 

 only with respect to the definition of its limits, because groups of 

 similar species do have an objectivity in nature, although the 

 total array of species in the genus may not make up a morpho- 

 logical continuum from one form to the next. 



Further, if similarly of morphology is evidence of relationship, 

 the genus in North America is not merely an assemblage of 

 species. It does, however, have at least three biological divisions. 

 One of these is represented by E. nunezi alone. This species, with 

 the mastitarsala on the third leg, the nude pointed seta on the 

 palpal tarsus, the two-pronged palpal claw, and unique scutum 

 and sensillae, stands apart from all other species included in 

 this paper. A second division is the "l<icerta" group. This group 

 of small mites, lacking the subterminala and the parasubterminala 

 on the first leg, with a reduced number of setae on the palpal 

 tarsus, with fine setae and setules on all parts, and with the 

 proximal placement of the seta on the coxa of the third leg, 

 is distinct. The remainder of the species comprise a third, 

 general, broad division with E. trigenuala not well fitted in. 

 This division contrasts in its morphology with the characters 

 itemized for the first two. With regard to geographic distribution 

 of these divisions, it can only be said that nothing inconsistent is 

 apparent at present. On the other hand, the distributions are too 

 insufficiently known to be discussed. 



The third division includes several morphological groups which 

 have been outlined in this paper for practical purposes. Closer 



