AQUATIC INSECTS 885 



developed in the wing pads of the nymph and is comparable in 

 close detail with that of the adult. It is only necessary to remove, 

 as with a sharp razor, the wing pads from a well-grown nymph, 

 young enough so that the wings will not be already crumpled 

 within their sheaths, mount, and examine with the microscope. 

 Since, however, it is easier to get nymphs than adults, and nymphs 

 only will often be available, the following hints may be of assistance 

 in their recognition. Pteronarcys (Figs. 1355 and 1356) alone has 

 gills upon the first two segments of the abdomen. Taenia pteryx alone 

 has three- join ted, telescopic gill filaments attached singly at the base 

 of^ the coxae. Peltoperla alone has conic-pointed gill filaments, in 

 a few small clusters, concealed under the flaring, overarched mar- 

 gins of the thoracic segments. Perla and its alUes have copious 

 tufts of fine gill filaments before and behind the bases of all legs. 

 Chloroperla and its allies, and Capnia and Leuctra altogether lack 

 gills. 



Mayflies {Order Ephemerida) 



The Mayflies constitute a small group of very fragile insects, 

 all of which are aquatic in their earlier stages. They abound in 

 all fresh waters, both swift and stagnant. Some of the larger May- 

 flies are very well known, indeed, from their habit of transforming 

 all at one time and appearing in great swarms along shores of lakes 

 (Fig. 1357) and banks of the larger streams. They fly to lights 

 at night, and sometimes, under the arc lamps in city streets, they 

 accumulate in such heaps as to require removal in wagons. Such 

 concerted appearance of the adults of a single species gives some 

 conception of the abundance of individuals that may five and grow 

 up together in a restricted area; but it is to be borne in mind that 

 there are scores of other species living in the same waters, the 

 adults of which are rarely seen in numbers, of which the individuals 

 are probably quite as numerous. When their period of trans- 

 formation is extended through the summer season and their 

 habits are not gregarious, but solitary and secretive, they may 

 entirely escape the notice of the casual observer. 



Mayflies are famed for their ephemeral existence — for Kving as 

 adults but a day. They are peculiar among insects, in that they 



