CHIGGERS — FARRELL 217 



body setae is useful in other species. Observation indicates that 

 the structure of the dorsal setae is more constant than their 

 numbers. 



The ventral setae, except the sternals, are not easily assignable 

 to rows. Usually they are rather numerous in Euschongastia, 

 and counting them becomes laborious. Some descriptions 

 (Womersley, 1944) list the setae by rows when the number is 

 small. When the number is large, the approximate total number 

 may be given (Wharton, 1948, and Brennan, 1947, 1948). The 

 latter method is used in this paper. Usually the poststernal setae 

 increase in size posteriorly. Brennan (1948) has referred to a 

 difference in form between the setae anterior to the anus and 

 those posterior to the anus, with the posterior setae frequently 

 resembling the dorsal setae. This difference has been found in 

 certain species in the present study, being most clearly shown 

 in E. nunezi. In other species, such as E. peromysci and E. i-ubra, 

 the dorsals and all poststernal ventrals have the same form. 



The scutum, with its associated setae and sensillae, is a very 

 remarkable structure. It has been much used by all workers in 

 defining and diagnosing species. In E. nunezi, E. trigenuala, E. 

 peromysci, E. rubra, E. hamiltoni, and possibly others the outline 

 and markings of the scutum are so distinctive that they alone may 

 serve to diagnose the species. In all species they are characteristic 

 and helpful. The markings usually consist of punctae and ridges. 

 Caution must be exercised in using these structures, since ridges 

 frequently are obscured on wide scuta, such as occur in the 

 "blarinae" group, and punctae may be obscured in any species. 

 The sensillae also are distinctive in many species. E. nunezi, E. 

 criceticola, and E. blarinae are examples. Sensillae are always 

 useful but, regrettably, are often lost. Both length and structure 

 of the scutal setae are useful systematic characters in certain 

 species, such as E. pipistrelli and E. hamiltoni. 



Womersley and Heaslip (1943) introduced the series of meas- 

 urements known as the standard data of the scutum. These data 

 are an expression of size and proportion of the scutum and as 

 such are useful in systematics. Womersley and Kohls (1947) 

 treated the standard data statistically in describing new species 

 of Euschongastia from New Guinea. In the present study no 

 effort was made to use statistical methods with the data. 



From observation of numerous specimens, it seems that the 

 scutum is subject to change or deformation in many species. 

 Particularly in species with wide scuta, larger scuta are asso- 

 ciated with well engorged specimens. This is in accord with 



