MOUTH PARTS OF THE SUBIMAGO AND IMAGO. 



VI— MOUTH PARTS OF THE SUBIMAGO 

 AND IMAGO 



May-fly mouth-parts are vestigial during the life of the subimago and 

 imago. External evidences of degeneration are shown in reduced size, asym- 

 metrical form, and general lack of chitinization. Internally there is a degenera- 

 tion of the musculature. 



EXTERNAL METAMORPHOSIS. 



A comparison of the functional mouth-parts of a Hexagenia nymph (PL 

 HI, fig. 34) with the atrophied mouth-parts of the subimago of the same species 

 (PL VI, fig. 98) shows the change of form with loss of function. Molar surface, 

 canines, and lacinia of the mandible (Md) are not differentiated. The large 

 chitinized tusk (PL II, fig. 26, T) is soft, irregular and thread-like (PL VI, fig. 

 98, T). The maxillae (Mx) show no segmentation, although the labium (Lb) 

 does contain traces of a suture between the mentum and submentum. The labrum 

 is visible only after dissection. It is a tiny misshapen protuberance retracted 

 beneath the nose-like clypeus (Clp). A shapeless papilla drawn back into the 

 pharyngeal opening is the only remnant of the hypopharynx. There are no hairs 

 or spines. All the mouth-parts are diminished in size, irregular in form, and 

 very weakly chitinized. 



Atrophy of the mouth-parts is progressive during the aerial life of an 

 individual (Compare figs. 88, 90, 93, with figs. 95, 96, and 97). It also varies in 

 extent among members of a species. (Compare figs. 88, 89, 90; 91, 92; 93, 94). 



INTERNAL METAMORPHOSIS. 



Internal evidence of the metamorphosis of Ephemerine mouth-parts is 

 shown in the degeneration of the musculature. In the mouth-parts proper there 

 is a gradual regressive process during aerial life.* The part of the head capsule 

 occupied during nymphal life by the strong mandibular adductors, is filled by the 

 large compound eyes after transformation. Although these eyes have been de- 

 veloping internally since mid nymphal life, they attain the greater part of their 

 enormous size just at transformation. Specimens observed to feed one-half hour 

 before emergence show no traces of mandibular adductors one hour later. There 

 are, then, two physiological types of degeneration : a simple progressive atrophy 

 of the mouth-parts proper ; and a rapid degeneration of the mandibular adductors. 

 Histologically degeneration is the same in both cases. 



*Specimens of Hexagenia recurvata emerged in mid-afternoon May 31 ; spent two 

 days as subimago; mated at sundown two days later, and died that evening. 



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