MOUTH PARTS OF THE NYMPH. 



SURVEY OF THE MOUTH-PARTS. 



A survey of the mouth-parts of representatives of the various genera* 

 reveals striking variations in form, and diverse adaptations to similar ecological 

 situations. 



Labrum (PI. IV). 



The main variations in the labrum of May-fly nymphs are in the form of 

 the anterior border, and in the cuticular appendages. Judged from embryological 

 evidence, the more primitive form of border is straight, like that of Ephemerella 

 (fig. 57) and Chirotenetes (fig. 42). There are two lines of deviation from this 

 condition. In the one, the border is notched in the center (Callibaetes, fig. 43; 

 Baetis, fig. 44). In the other, it is extended laterally and incurved like that of 

 Ecdyurus (fig. 46) and Hagenulus (fig. 47). The cuticular appendages consist 

 of hairs and spines. Some forms possess both hairs and spines (Polymitarcys, 

 fig. 41) ; some like Chirotenetes (fig. 42) have spines only; and others but hairs 

 (Oligoneuria, fig. 45). Spines are characteristic of those forms exposed to swift 

 water. Hairs are peculiar to burrowers or sprawlers in the mud. 



The labrum of Hagenulus (fig. 47), one of the Baetinae, is strangely 

 adapted for life in swift water. It is flattened and has an extraordinary lateral 

 extension far beyond the head capsule, simulating the flaring, depressed head of 

 the Heptagenine associates. Among these, the mouth-parts are retracted beneath 

 the outgrowth of the head ; in Hagenulus it is the labrum that protrudes, and the 

 head capsule is small. The backward projecting spines at the free posterior 

 margins in Hagenulus doubtless serve for protection. 



Mandible (Pis. II, IV). 



Molar surface. The primary function of the May-fly mandible is to grind 

 food. For this purpose a molar surface is developed. Among the Ephemerinae 

 there is one prevailing type. The left molar surface of Ephemera (fig. 31) bears 

 eight transverse ridges, the anterior edge of each provided with teeth. Between 

 these ridges are deep gutters. The right molar surface (fig. 32) has seven 

 transverse ridges toothed at the anterior edge. The molar surfaces (fig. 28) 

 move in a dorso-ventral plane, rubbing together the transverse ridges. The 

 teeth extending out into the gutters doubtless serve as strainers for the food. 

 Cast skins show many of the transverse ridges worn smooth, but the teeth still 

 sharp, and clogged with ground food. Among the Baetinae and Heptageninae 

 are two main types of molar surface. In one, the transverse ridges are irregularly 

 serrated (fig. 33), with stiff bristles acting as strainers across the gutters. In 

 the other, the transverse ridges are rounded and knobbed (fig. 29, 30). They 



*The exotic specimens were very kindly loaned me by Dr. J. G. Needham. 



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