CHIGGERS — F AERELL 213 



phology. According to Simpson (1945), "It is not useful to set 

 up a classification in which groups with different names cannot 

 be distinguished morphologically." All species of Euschongastia 

 included in this paper have been defined morphologically. This 

 includes an enumeration of the various features which character- 

 ize each species, but it does not imply a static definition. Given a 

 series of specimens sufficiently long, it is doubtful if there is any 

 morphological character which caimot be found to vary. Varia- 

 bility is an expression of the genetics of the species, and is itself 

 a character of the species. An adequate morphological definition 

 should include these variations. However, it is standard practice 

 to identify specimens initially by means of diagnostic or sys- 

 tematic characters, those characters which differ between known 

 species. Use of these practical tools of the systematist saves both 

 time and effort. But these characters are only systematic aids, 

 and a species of Euschongastia is not a "nude lateral seta on the 

 palpal tibia" or a "mastitarsala III." Following a tentative iden- 

 tification on the basis of diagnostic or key characters, the speci- 

 men should be compared with the complete description, with other 

 identified specimens, and, when possible, with type material. This 

 final comparison is important in Euschongastia. Undoubtedly, 

 many species remain undescribed. 



Study of North American Euschongastia reveals that at present 

 it contains four Vv^ell-defined groups of species, three somewhat 

 less well-defined groups of species, and four ungrouped species— 

 a total of 24 species (excluding Euschongastia sp.), with one of 

 these divided into two subspecies. These species have certain 

 relationships with each other which can be used to segregate 

 either groups of species or the species themselves. 



When attention is given to particular systematic characters, 

 it is found that fundamental cleavages can be made in the genus 

 by use of the specialized, or striated setae of the legs. The value 

 of these setae in descriptions and diagnoses was pointed out by 

 Wharton (1947a). In particular he emphasized the striated setae 

 of the third leg as being important in separating m.orphological 

 groups of species in the genus Trombicula. In Euschongastia 

 these setae have a similar importance. On the clear basis of the 

 presence or absence of the tibiala on the third leg, the genus in 

 North America can be split through the middle, so that each half 

 contains groups of morphologically related species. Also, the 

 presence or absence of other striated leg setae distinguishes defi- 

 nite groups. Thus, the absence of subterminalae and parasub- 

 terminalae separates the "lacerta" group from all others. The 



