CHIGGERS — FARRELL 123 



of several dimensions of the scutum and the lengths of the setae. 

 All measurements involving setae or sensillae start at the mid- 

 point of the basal attachment. Often, the width of the sensilla 

 is given and is indicated by an "x" following the length. 



Following are the explanations of the abbreviations used in 

 connection with these dimensions: 



AW: Width of the scutum between the bases of the anterior lateral setae. 



PW: Width between the bases of the posterior lateral setae. 



AP : Distance between the bases of an anterior lateral seta and a posterior 

 lateral seta on one side. 



SB: Distance between the sensillary bases, the points of attachment of the 

 sensillae. 



ASB: Distance from the anterior margin to the bases of the sensillae. 

 When the anterior setae are set on anterior expansions of the 

 scutum, these expansions are included in ASB. For the purpose of 

 determining standard data, the anterior margin is considered to lie 

 at a straight, transverse line touching the most anterior points of 

 these expansions. 



PSB: Distance from the bases of the sensillae to the posterior margin, 

 which is considered to lie at a straight, transverse line touching the 

 most posterior expansions of the scutum, 



AL: Length of an anterior lateral seta. 



AM: Length of the anterior median seta. 



PL: Length of a posterior lateral seta. 



S: Length of a sensilla. 



Two pairs of simple eyes usually are present. Two species have 

 ocular plates. Two species have one pair of obscure eyes. 



Dorsal body setae are arranged in transverse rows which are 

 usually distinct, especially in partly engorged specimens. A pair 

 of humeral setae, one seta on each side of the body, is set off an- 

 terolaterally in engorged specimens ; but in unengorged specimens 

 these setae may lie at the ends of the first transverse row. By 

 convention in descriptions of chiggers the dorsal setal formula 

 consists of listing serially the number of setae in each of the 

 transverse rows, beginning with the humeral setae and proceed- 

 ing posteriorly. Frequently there are setae on the lateral edges 

 of the body not easily assignable to a row. These are inserted in 

 the formula where they occur and are usually recognizable by be- 

 ing smaller numbers between larger numbers. In a list of several 

 dorsal formulae these so-called lateral setae will not be indicated 

 in all specimens. The dorsal setae usually resemble the scutal 

 setae. The form of the posterior setae may differ from that of 

 the anterior setae. Particularly in unengorged chiggers, the setae 

 project posteriorly somewhat parallel to the body. They tend to 

 be nude on the side adjacent to the body. 



