MOUTH PARTS OF THE NYMPH. 



(fig. 65, m) is straight; while that of Habrophlebia (fig. 66) is notched. In 

 Callibaetes (fig. 67) this lobe is produced in the center. The lateral lobe (1) of 

 Habrophlebia (fig. 66) is emarginate at the anterior border. In Polymitarcys 

 (PI. Ill, fig. 36) it is notched at the posterior border. The hairs on the outer 

 border of both lobes of the hvpopharynx are incurving, and doubtless aid in re- 

 taining food. 



Maxilla (PI. V). 



The main adaptations to environment in the nymphal maxilla involve loss 

 of segmentation, and peculiar cuticular appendages. 



Reduction in segmentation in the body of the maxilla is primarily a con- 

 solidation for strength. Galea and lacinia are fused, although in some forms, as 

 Hagenulus, (fig. 85) faint traces of the suture remain. The articulation between 

 the stipes (St) and galea-lacinia (Ga-Lc) is differentiated in Lachlania (fig. 83), 

 but in most cases indistinctly (Hagenulus fig. 85; Hexagenia fig. 77; Blasturus 

 fig. 75), or not at all (Baetisca fig. 78). The palpifer also shows gradations in 

 fusion. That of Blasturus (fig. 75, Plf) or Rhithrogena (fig. 69) is well defined. 

 In Hagenulus (fig. 85) or Hexagenia (fig. 77) it is not so clearly differentiated. 

 The cardo is distinct. 



Reduction in segmentation in the maxillary palp takes place by fusion, by 

 incomplete differentiation, and by loss of segments. Where there is consolidation, 

 the parts are heavily chitinized. This condition is found in the first segment of 

 the palp in Lachlania (fig. 83). It is fused proximally with the palpifer. There 

 is incomplete differentiation of the distal segment in some forms. In these 

 (Blasturus fig. 75; Hagenulus fig. 85; Hexagenia fig. 77) the chitinization is 

 weak. Entire loss of segmentation is found in Ephemerella deficient Morgan 

 (fig. 82), where there is no palp at all. Members of this genus are found in 

 streams with gravel bottoms. The mouth-parts are small, retracted, and heavily 

 chitinized. Loss of the palp is the extreme of specialization in this line. 



The reduced multiarticulate palp (fig. 68, 72) of the nameless Chilean 

 nymph figured by Eaton is an aberrant form. 



The cuticular appendages of the maxilla mainly consist of bristles, rakers 

 and heavy spines. Bristles are best developed in those forms among the Baetinae 

 that inhabit swift water. They serve as diatom brushes and plancton strainers 

 (Hagenulus fig. 85). This is a specialization for food getting which parallels 

 the labial diatom rakers of the Heptageninae. On the ventral surface of the 

 blade-like distal palp segment of one of these (Rhithrogena, fig. 69) are countless 

 numbers of tiny toothed spines (fig. 70). Mixed with these combs at regular 

 intervals are crook-shaped hairs. Spines are present on the galea-lacinia of those 

 forms of the Baetinae that dwell in rocky streams (Ephemerella fig. 82; Baetisca 

 fig. 78). The greatest development in this line is the teeth-like spines on Eaton's 

 nameless Chilean nymph (fig. 68). 



Tracheal gills (G) are present on the maxilla of Lachlania (fig. 83) and 

 some other swift water forms. 17 



