18 



BULLETIN 168, UNITED STATES NATIONAL MUSEUM 



Eyes absent. Postantennal organ (pi. 3, fig. 28) elongate, a little 

 shorter than the width of the first antennal segment, straight or 

 slightly curving, with almost parallel sides, usually emarginate at 

 the middle of the anterior margin, six to eight times as long as broad, 

 with a posterior row of strong guard setae seated on a cuticular 

 thickening. Antennae shorter than the head (as 5:7), with segments 

 in relative lengths about as 4 : 7 : 6 : 10. Sense organ of third antennal 

 segment (pi. 3, fig. 29) uith a pair of rods immersed in a deep pit. 

 Fourth antennal segment with olfactory setae. Body subcylindrical; 

 abdomen slightly dilating posteriorly. Prothorax not unusually 

 long (one-fifth to one-half as long as mesothorax); fourth abdominal 

 segment longer than the third (as 6:5); anal segment not evident. 

 Anus ventral. Claws largest on third pair of feet. Unguis (pi. 3, 

 fig. 30) slender, curving, untoothed. Unguiculus extending less than 

 half as far as unguis, lanceolate, pointed, untoothed. Tenent hairs 

 absent. Furcula appended to the fourth abdominal segment, short 

 (pi. 3, fig. 27), extending scarcely to the middle of the third abdominal 

 segment. Manubrium (i)l. 3, fig. 31) stout, longer than dentes (as 5:3), 

 with several dorsal setae and two pairs of ventral subapical setae. 

 Dens stout, narrowdng but little, with unusually thick ventral wall, 

 without dorsal crenulations, vnth a few setae dorsally and ventrally, 

 and with the pair of basal hooks exceptionally large. Mucro half 

 as long as dens, subequally bidentate (pi. 3, figs. 31, 32); apical tooth 

 feebly hooked; anteapical tooth suberect. Rami of tenaculum (pi. 3, 

 fig. 33) quadridentate, slender, tapering; corpus with a single long 

 stout curving seta. General clothing (pi. 3, fig. 34) of short, stiff, 

 simple setae, becoming longer and more curving toward tlie end of the 

 abdomen; erect sensory setae short and simple. Integument mi- 

 nutely granulate. Maximum length, 1 mm. 



Remarks. — Table 2 gives the relative lengths of the body segments 

 of four individuals. The considerable variation in the length of the 

 prothorax is due to the fact that this segment is normally capable of 

 being lengthened or shortened considerably by muscular action and, 

 furthermore, elongates more or less in preserved specimens under 

 the influence of potassium hydroxide. 



Table 2. — Relative lengths of the body segments of four specimens of Isotomodes 



tenuis, new species 



