30 Illinois State Laboratory of Natural History. 



brief discussion of their comparative values may be of service 

 to beginners in. the study of this genus. 



The number of antenna! segments may be depended upon 

 as fairly constant. It never varies in adult specimens l)y 

 inore than a single segment, and then only in the case of a 

 very few species. The length of the antenna', while constant 

 in some species, is remarkably variable in others, notal)ly in 

 C. sernihttiii^. The proportionate length of antenna! seg- 

 ments does not always remain the same in species in which 

 the antenna is variable as to length. Sensory structures 

 and the hyaline plates of the distal antennal segments are 

 reliable characters. 



In certain species, C. phiiley<(tiis for instance, the propor- 

 tions of the stylets are quite constant, but in C. riridi-s, scrni- 

 Idtas, and hirti^j>i<hifiif;, the range of variation is very great; 

 consequently such measurements are not of the highest spe- 

 cific value. The apical bristles of the stylets are not very 

 variable as to comparative lengths, but the minute details of 

 their structure are not constant. This fact is illustrated by 

 the variation in the shape of the outer apical spine of ('. 

 vifidls var. hrcrlajtiiiosiis (PI. XL, Fig. 1). 



The armature of the swimming feet is of considerable value in 

 certain cases, and is constant as a rule. Sometimes, however, 

 the presence or absence of a spine or seta is not accompanied 

 by other perceptible differences. The general character of 

 the armature with regard to strength, etc., may nsually be 

 relied upon ; but I have often seen in a single specimen all 

 of the gradations between spines and sette, and it would be 

 impossible from this character to say which of the two names 

 should be applied. 



Of the easily observable structures, the fifth foot is the 

 most valuable for specific distinction. Slight variations 

 sometimes occur in the shape of the segments and in the 

 comparative lengths of spines and seta?, but, as a rule, 

 characters drawn from this appendage are quite constant. 



Of all the specific characters, the most valuable are those 

 derivable from the rvceptdcnlnm s('iii'ni}-s. Unfortunately, it 

 is often very difficult to see this organ distinctly, especially if 



