Introduction 3 



portion of the preantennal region is called the frons, while the 

 lower portion is known as the gena, but there is no distinct line 

 of demarcation between the two areas. 



In many genera the frons is notched or tuberculated, the an- 

 terior margin of the head bearing a notch or tubercle called the 

 frontal notch or frontal tubercle. Frequently the gena is pro- 

 longed below and posterior to the eye into a heavily pigmented, 

 highly sclerotized genal process, which may in some cases be 

 difficult to distinguish from a spine of the genal ctenidium. 

 Simple eyes, one on each side, are located near the third anten- 

 nal segment; they vary in size and in prominence, being heavily 

 pigmented and conspicuous in some groups, while vestigial or 

 absent in others. When the preantennal region is armed with 

 two rows of bristles, the lower row is called the ocular (genal) 

 row, and its first bristle is termed the ocular bristle; the upper 

 row is frequently designated as the frontal row. A conspicuous 

 structure of the head, when it is present, is the genal ctenidium 

 which consists of heavily pigmented spines. With the exception 

 of the labial palpi, the mouthparts are of little systematic value. 

 Each labial palpus (there are two) is variable in length and in 

 the number of its segments; but the value of this variation is 

 not the same in all groups, being of only specific importance in 

 some cases while generic in others. 



The thorax is of systematic importance because of the chaeto- 

 taxy of its sclerites which are formed as follows. Each segment 

 (pro-, meso-, and metathorax) is divided into a dorsal notum or 

 tergite and a ventral sternite. A vertical, rodlike sclerotization 

 divides the meso- and metasternite into an anterior sternal por- 

 tion and a posterior epimeron. The sternal portion is further 

 divided into a lower part, the sternite, and an upper part, the 

 episternum. The epimeron of the metathorax is broad and 

 overlaps the abdomen supplanting the first abdominal sternite. 

 The legs, with the exception of the tarsi, are of only occasional 

 taxonomic importance. Each tarsus is armed on its fifth seg- 

 ment with plantar bristles whose position, whether lateral or 

 ventral, and number are significant in generic delineation. A 

 conspicuous feature of the thorax is the pronotal ctenidium, 

 present in most groups but absent in some, which usually con- 

 sists of long slender spines variable in number according to the 

 group. 



The abdomen is of importance to taxonomists largely because 

 of the number of rows of bristles on each segment, the an- 

 tepygidial bristles, and the genitalia. Each segment consists of 



