TR AC HEAL GILLS OF LARVAE 



467 



Jolia there is a pair on the under side of the first thoracic segment at 

 the insertion of each of the legs. In certain genera (Heptagenia, 

 Oligoneuria, and Jolia), they are in the form of a flat cover, under 

 which lies a tuft of respiratory tubes, or 

 (Ephemerella) a small bifid cluster of very 

 delicate leaves (Fig. 437, A). In Coenis and Tri- 



FIG. 439. Inner side of Fio. 440. Nymph of Baetisca: FIG. 441. Nymph of 



a gill-cover of the first pair, III, section of abdomen; a, gills; Prosopistoma pnncti frail 6 : 



of Ephemerella, with the 6. flap ; 1-9, abdominal segments. 0, upper orifice of the respira- 



tracheal gills. After Vays- After Walsh. try chamber. After Vays- 



si<ire. siere. 



corythus the tracheal gills of the second pair are modified to form 

 plates covering all the succeeding pairs, those of the first pair being 

 nearly atrophied and well-nigh functionless. 

 (Fig. 437, B.) 



Finally, in the highly modified forms 

 Baetisca and Prosopistoma the tracheal gills 

 are entirely concealed and protected by 

 mesothoracic projections so as to form a 

 true respiratory chamber, to which the 

 water has access either by an opening be- 

 hind, as in Baetisca, or by three openings, 

 two ventral and one dorsal (Fig. 441), as in 

 Prosopistoma. 



The slender cylindrical tracheal gills of 

 Heptagenia in the third or fourth nymphal 



., . . ii-i F IG - 442.-Filamentous 



Stage are J-JOinted, and the first abdominal tracheal gill and part of a trachea 



. , , T\ i ' i of Pteronarcvs. After .Newport, 



pair in Caenis are said by Palmen to be from sharp. " 



finger-shaped and 2-jointed. In Polymitar- 



cys virgo the gills do not appear until the eighth or tenth day after 



hatching. 



