212 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 106 



variation is considerably greater than that usually found in 

 species of Euschongastia. The unusual extent of the variation is 

 easily apparent in the first posthumeral row of setae for the two 

 forms. The lowest count of the number of prongs on the palpal 

 claw of E. sciuricGla coincides with the highest count determined 

 for Euschongastia sp. When the data for the number of prongs 

 on the palpal claw of the two forms are put together, the range 

 of variation becomes extreme, but it is continuous. 



The variation in the number of prongs on the palpal claw of 

 E. sciuricola is not remarkable. However, the variation exhibited 

 by the five specimens of Euschongastia sp. has no precedent. The 

 number of prongs on the palpal claw of all other North American 

 Euschongastia is very constant and has been useful in arranging 

 the chiggers into apparent natural groups. The striking variation 

 of the number of prongs on the palpal claw of Euschongastia sp. 

 cannot be explained until a longer series of specimens has been 

 examined. 



The sample available at this time is not adequate for making 

 a determination of the position of Euschongastia sp. 



Discussion 



A systematist engaged in separating and defining species must 

 have some clear concept of these populations. Any definition 

 which he may develop Vvill be shaped by what he finds in his ma- 

 terial and should be more clear the more intimately he knows the 

 material. The many diverse definitions of species with which stu- 

 dents are familiar all tend to reflect the varying interests of the 

 authors. It follows that, whatever its scope, the definition of a 

 working systematist must be useful. On these grounds, a species 

 of Euschongastia in this paper is a population of chiggers which 

 conforms to a morphological pattern, normally variable within 

 limits, which is found in a certain ecological pattern within a 

 geographic range, and which is biologically continuous. The 

 morphological pattern of the species can be observed and charted. 

 The ecological pattern, including the distribution of the chiggers 

 before attachment, the host distribution and the localization on 

 hosts during attachment, and the seasonal distribution is deter- 

 minable. The geographic range can be plotted. Chiggers are not 

 the reproductive stage of the trombiculid life cycle, but their bio- 

 logical continuity is evident in collections of succeeding years. 

 Accordng to this concept, a species of Euschongastia is natural 

 and definable. 



Much of the basic work on systematics is concerned with mor- 



