68 



The May-flies and Stone-flies 



through. And here is an opportunity for some keen-eyed amateur ento- 

 mologist to add needed facts to our knowledge of insect life. 



The breathing-organs of the nymph are of interest, as special adaptations 

 to enable them to take up oxygen and give off carbon dioxide without com- 

 ing to the surface, as do the water-beetles, water-bugs, mosquito-wrigglers, 

 and many other familiar aquatic insects. Each plate-like gill (Fig. 102) 

 is a flattened sac, with upper and lower membranous walls which run into 

 each other all around the free margin. Inside this sac is an air-tube 



(tracheal trunk) with numer- 

 ous fine branches. By osmosis 

 an interchange of gases takes 

 place through the walls of the 

 tracheae and of the sac car- 

 bonic dioxide passing out, and 

 air from that held in solution 

 in the water passing in. If a 

 nymph held in a watch-glass 

 of water be watched, at times 

 all the gills will be seen rap- 

 idly vibrating, thus setting 

 up currents and bringing fresh 

 aerated water to bathe the 

 gills. 



In the adult winged stage 

 (Fig. 103) the May-flies are 

 extremely frail and delicate- 

 bodied. The wings are fine 

 and gauzy, consisting of 

 the thinnest of membranes 

 stretched over a perfect net- 

 FIG. 103. May-fly, from California. (Natural size.) WO rk of veins. The fore 



wings are always markedly larger than the hind wings; in some species 

 the latter are very small indeed, or even wanting altogether (Fig. 104). 

 The body- wall is weakly chitinized, and collected specimens almost always 

 shrivel and collapse badly in drying. The abdomen usually bears two 

 or three long filaments on its tip; the head is provided with compound eyes 

 and short awl-like antennae. The often-repeated statement in text-books 

 that adult May-flies have no mouth nor mouth-parts is not literally true 

 of all species, as weakly developed jaws and lips are present in some. But 

 they are in such weak and atrophied condition that they can hardly be func- 

 tional. It is probable, therefore, that no adult May-fly takes food. In 

 the males of some species the compound eyes present a very interesting 



