MOUTH PARTS OF THE NYMPH. 



Labium (PI. V). 



Since the primary function of the labium is to prevent the escape of food, 

 and to sweep it into the pharyngeal cavity, the main adaptations are found in the 

 ligular region. The greatest changes in this region are found among the Baetinae 

 dwelling in swift water. There are two lines of specialization. In the one are 

 forms with reduced mouth-parts. The glossa (gl) and paraglossa (Pgl) lose 

 their segmentation in part as in Ephemerella (fig. 71 ) or entirely as in Tricorythus 

 (fig. 64). In the other line of specialization are forms with the mouth-parts 

 more or less enlarged and depressed. The ligula is expanded and constitutes the 

 greater part of the body of the labium. The glossa of Lachlania (fig. 76) is not 

 differentiated from the ligula. The paraglossae (Pgl) are enlarged and produced 

 anteriorly until their tips are contiguous. The whole labium resembles a scoop. 

 The same result is obtained among the Heptageninae, but by a different specializa- 

 tion. There, as in Lachlania, the ligula ( fig. 84, Lg) forms the main bulk of 

 the labium : but the glossa and paraglossa, though small, are distinct. It is the 

 basal portion of the ligula that is expanded. 



The main adaptations in the labial palp involve reduction in segmentation, 

 or decrease in size. In those swift water forms that have mouth-parts depressed 

 and the palp flattened and blade-like, there may be a consolidation of segments. 

 This condition is found in Epeorus (fig. 84). The second palp segment is small, 

 heavily chitinized, and incapable of independent movement. No musculature is 

 developed for this segment. The palp of Oligoneuria (fig. 76) is two-segmented. 

 In those swift water forms where the head is small and the mouth-parts retracted, 

 there is a reduction in the size of the entire palp, particularly the distal part. 

 The second and third segments of the palp of Tricorythus (fig. 64) are very 

 reduced, and weakly chitinized. In Bactis sp. (fig. 73), the distal portion of the 

 second palp segment is enlarged and bears an inner prolongation. The third 

 segment is knob-like and fits down into the enlarged tip. It is weakly chitinized 

 and the articulation is incompletely differentiated. Specimens of this genus which 

 do not inhabit rocky places in swift water show a less knobbed distal segment, and 

 better differentiated articulation. Greater specialization in this line is found in 

 the labial palp of Bactisca obesa (fig. 74). There the inner prolongation of the 

 second segment is longer and tooth-like. The third segment is small and pointed, 

 giving a forceps-like appearance. However, this segment is weakly chitinized 

 and shows no musculature. 



The cuticular appendages of the labrum are similar to those of the maxilla 

 of the same forms. Baetisca obesa (fig. 74) has chitinized papillae on the ligula 

 and submentum. 



The labial palp of Eaton's nameless Chilean nymph (fig. 80) is multiarticu- 

 late. The addition appears to result from secondary division of the distal segment. 



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