14 The Crasjyedosomatidae of North America. 



where they would be likely to become soiled they are alwaj^s 

 clean to a degree. But when not to be cleaned the antennjB are 

 frequently bent at, the fourth joint and the end held near the 

 mouth, for no reason apparent. 



In males the posterior pair of legs of the seventh segment 

 are two-jointed, the distal joint being thick and clavate, and 

 curved up against the side of the body. In walking, this modi- 

 fied foot which cannot touch the ground, is waved back and 

 forth in unison with the legs on either side. The copulation 

 of these animals has not been observed, but that this struc- 

 ture can have any part in the process is hard to believe or 

 imagine, and that it is to be looked upon as a merely rudimen- 

 tary structure seems more reasonable. In American C rasped o- 

 somatida3 are found the transitional stages between the condition 

 existing in the Polydesmid?e and Lysiopetalidse, where onh^ 

 the anterior pair of feet of the seventh segment have been modi- 

 fied for copulatorj^ purposes, and the condition present in the 

 lulidse and Polyzonidae, where both pairs are thus modified. As 

 might be expected, the modification in form and the modification 

 in function are apparently taking place gradually, and in the 

 present case a part of the leg aids in copulation while the rest 

 waves idly, " from force of habit." 



Even in the C rasped osomatidse which do not have the usual six 

 bristles of each segment well-developed on the anterior part of 

 the body, as in Pseudotremia and Chordeuma, the posterior 

 median bristles of the last segment are well developed and to all 

 appearances alike in all the species. Thej^ are curiously modified, 

 consisting of a cylindric, or slightly conic, enlarged base, from 

 the distal end of which projects a long, exceedingly slender, flex- 

 ible bristle, or in this case more properl}^ a hair. This is suffi- 

 ciently stiff' to remain nearly straight, but is much finer than the 

 bristles of the preceding segments, and not brittle. 



Attached to the ends of these bristles have on several occasions 

 been noticed fine threads like spiders' web dragging out behind 

 the animals as they walked, that is in TrichoiDetalum. And in 

 the living specimens it has also been noticed that these modified 

 bristles were sometimes carried vertically, and in others were 

 nearly horizontal. The peculiar conformation of the bases of 

 these bristles and of the apical portion of the segment suggests 

 the possibility that these bases are articulated with the segment, 



