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-flv, 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 
