186 PROCEEDINGS OF THE NATIONAL MUSEUM vol. los 



in identifying females. Generally, the forms which are closest in 

 body size are also quite similar in gonopod structure and are usually 

 geographically approximate. Variability of size is thus one of interest 

 in the consideration of phylogeny and relationships within the genus. 

 There is a distinct tendency for the smaller forms to be dispersed 

 around the periphery of the generic range (fig. 3). 



Shape of collum: In the subspecies of crassieutis the collum is 

 attenuated laterally and extends as much as a millimeter or more 

 beyond the paranota of the following segments (fig. l,a). This 

 character was noted by Attems in the pachydesmid which he described 

 in the "System der Polydesmiden" and is the basis of the name 

 laticollis. In clarus, however, the collum is of normal shape (fig. 1,6) 

 and thus provides a fundamental means of distinguishing between 

 the two species. No variation in shape has been noted between any 

 of the eight geographic races of crassieutis, nor does there seem to be 

 any sexual dimorphism in this character. 



Color pattern : Very little is known of the colors of living indi- 

 viduals of Pachydesmus, and therefore little can be said about the 

 taxonomic value of pigmentation. Judging from recently preserved 

 specimens, the large forms related to laticollis (retrorsus, hubrichti, 

 adsinicolus) appear to be light brown dorsally, with the paranota 

 rather testaceous. Around the periphery of the range, the colors 

 become more distinctive, clarus being black with the paranota coral 

 red or pink, duplex blackish with yellow paranota, and incursus brown 

 with either pink or yellow paranotal markings. The presence of reds 

 and yellows is almost universal in the Xystodesmidae, so that the loss 

 of such hues may be considered a specialization concomittant with 

 increase in size. 



Interzonal furrow: The interzonal furrow is quite similar in 

 both clarus and crassieutis, at least that part which crosses the dorsum. 

 Vcntrally, however, it becomes a well-defined and sharply margined 

 groove in clarus, but is reduced to a mere transverse suture across the 

 sterna in the forms of crassieutis. 



GoNOPODs: Characters afforded by the conspicuous external gono- 

 pods of many diplopods were utilized in the diagnosis of new spe- 

 cies as long ago as 1832, but did not receive real emphasis until a 

 half-century later. Since about 1884 there has developed a tendency 

 to rely more and more exclusively upon male genitalia, despite which 

 fact numerous workers have never given more than superficial sketches 

 of the appendages, accompanied by no description whatever. That 

 such important structures should have received so little close study is 

 somewhat astonishing. An attempt has been made in this study to 

 illustrate the gonopods from the same aspect for accurate comparisons, 

 and to devise a tentative terminology by which differences can be 



