642 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.53. 



ONYCHIURUS SUBTENUIS, new species. 



Plate 68, fig. 3; plate 72, figs. 29-38. 



White, elongate; abdomen but slightly broader than the thorax, 

 tapering posteriorly (fig. 3). Postantennal organs (figs. 29, 30) each 

 with 8 to 14, usually 11 to 13, tubercles, simple or occasion- 

 ally biramous. Pseudocelli of antennal bases 2+2 or 3 +3, arranged 

 side by side (fig. 31). Base of head without pseudoceUi. Antennae 

 shorter than the head, with segments about as 3:3:5:8 in relative 

 lengths. Sense organ of third antennal segment (fig. 32) with five 

 elongate papillae, five guard setae, a pair of subclavate sense rods, 

 and two capitate sense clubs, coarsely tuberculate. Dorsal pseudo- 

 celli of body (fig. 3) typically 0, 1 + 1, 1 + 1, 1 + 1, 1 4-1, 0, 2+2,3+3, 

 0. Thus pseudocelli are absent normally on the prothorax and on 

 the third and the sixth abdominal segments. One of the two pseudo- 

 celli is frequently absent on the first or the second abdominal seg- 

 ment (see beyond under variation). The pseudocelh of the fourth 

 abdominal segment are in two obhque pairs. Those of the fifth 

 abdominal segment are sometimes 4 + 4 instead of 3+3, the pseudo- 

 celli of each group being close together and in obhque ahgnment. 

 Lateral and ventral pseudocelh are absent. Unguis (fig. 33) stout, 

 curving, with inner margin unidentate one-third from the base. 

 Ungiculus extending two-thirds as far as the unguis, subeUiptically 

 lamellate basaUy and acuminate apically. Anal spines (figs. 34, 35, 

 36) two, half as long as hind ungues, curving, on stout contiguous 

 papillae about one-fourth as long as the spines. Supra-anal lobe 

 (fig. 34) tapering, projecting far behind the infra-anal lobes. Cloth- 

 ing (fig. 37) of numerous feebly-curving setae of moderate length, 

 interspersed with fewer longer and stiffer setae. Cuticular tubercles 

 small (fig. 38). Maximum length, 2 mm. 



Variation. — One small specimen, 1 millimeter in length, had 8 

 postantennal tubercles on the right side; another of the same length 

 had, however, 11. 



The pseudoceUi of the antennal bases are 2+2 in all the Illinois 

 specimens that I have seen ; but in some of the specimens from New 

 York and Pennsylvania there are 3+3 (fig. 31). Furthermore, the 

 pseudocelli of the fifth abdominal segment, typically 3+3, show the 

 variation 4 + 4 in some of the material from New York and Penn- 

 sylvania. 



The pseudocelli of the first and second abdominal segments vary 

 persistently in number, one of each pair being usually absent. The 

 variations in 50 specimens taken at random fall into the following 

 categories : 



