16 ILUNOIS BIOLOGICAL MONOGRAPHS [136 



OBSERVATIONS ON GORDIUS ROBUSTUS 



Since the investigation was begun on Gordius robustus Leidy and since 

 the series of observations is most complete in this species, it is but natural 

 that it should form the first part of the discussion. 



DETERMINATION OF SPECIES 



On account of the confusion existing in the literature in regard to this 

 and related species it is necessary to take up at this point a precise char- 

 acterization of the species and a determination of its position in the group. 

 The species was named by Leidy in 1851 and specimens were again referred 

 to it in 1879. The single female of 1851 has not been preserved but the 

 specimens of 1879 have fortunately been kept in fair condition. Leidy's 

 early characterizations are not sufi&cient for identification but his descrip- 

 tion of 1879 is fairly complete and the material is available for study. 

 Montgomery has given a somewhat detailed description of the species, but 

 he overlooked one of the most essential characters, the dorsal and ventral 

 bands. Only a general description of the species will be given at this 

 point, details of the structure being left for the discussion of the adult 

 morphology. 



Dimensions. Of the specimens collected near XJrbana the females vary 

 in length from 100 to 470 mm. and the males from 120 to 420 mm. The 

 diameter of the females ranges from 0.5 to 1.25 mm., that of the males 

 from 0.3 to 0.75 mm. Some of Montgomery's specimens from California 

 are considerably larger. The females in Leidy's collection are very short 

 and thick. 



Form. Both males and females are cylindrical, decreasing very slightly 

 in diameter toward the ends, the females more than the males. The shape 

 of the anterior end is essentially the same in both sexes. In the average 

 specimen the end is rounded in the shape of a hemisphere, separated from 

 the body by a very slight constriction and of the same diameter as the body 

 just behind the constriction (Fig. 27). In very stout-bodied females there 

 is no trace of a constriction and the body becomes distinctly attenuated 

 just before the end. This condition is at its extreme in case of the short, 

 thick females in Leidy's collection (Fig. 28). 



The posterior extremity of the female is slightly enlarged in the shape 

 of a bulb and abruptly truncated at the end (Figs. 25, 26). The cloacal 

 aperture is located at the center of the truncated area and there is no 

 trace of a dorso-ventral furrow (Fig. 34). 



