AMPHIBIOUS INVERTEBRATES 



465 



and dilates, so that water is forced out and drawn in rhythmically. 

 When the nymph develops into the adult dragon-fly, spiracles are 

 acquired by the system of air-tubes, the water is abandoned, and 

 ordinary air is breathed in the manner usual among insects. 



May -Flies {^EphemeridcE). — The adult insects are here thor- 

 oughly aerial, as in the groups of Net-Wings so far described, 

 but the nymphs present a maximum amount of adaptation to 

 an aquatic mode of life. They possess 

 tracheal gills of various kind, a typical case | 



being that of the Common May- Fly {Ephe- 

 mera vulgata"), in which these structures are 

 in the form of a double 

 series of tufted out- 

 growths running along 



int. 



i.ap.' 



Fig. 569. — Dragon-Fly Nymph 



Dissection to show air-tubes {t.t) 

 which supply rectal gills; tfii., intes- 

 tine (rectum): /.rt/., intestinal aper- 

 ture. Enlarged. 



9- 



¥ 



Fig. 570. — Nymph of 

 Common May - Fly 



(^Ephemera znclga (a), 

 enlarged, to show 

 tracheal gills at sides 

 and three tail -rods. 



Fig. 571. — Hinder part 

 of Nymph of a May- 

 Fly {Cloeon dipterian), 

 enlarged, a, Last three 

 segments and bases of 

 tail -rods. Last cham- 

 ber (l.ch.) of heart gives 

 ofT vessels to the tail- 

 rods. Arrows indicate 

 direction of blood-flow. 

 B, Part of a tail-rod, 

 showingperforated walls 

 of central blood-vessel. 



the abdomen, and traversed by numerous air-tubes (fig. 570). 

 Breathing is also helped, it would appear, by three feathery rods 

 which project from the hinder end of the body, and which differ 

 greatly both in structure and mode of action from tracheal gills. 

 The arrangement has been worked out in detail in the nymph of 

 one kind of May- Fly {Cloeon dipteruin). As in Insects generally, 

 the heart is here a slender tube situated in the middle line close 

 below the upper surface of the body. It consists of a series of 

 chambers which receive blood by means of paired valvular aper- 

 tures and pump it forwards. To this direction of blood-flow there 



Vol. II. 



62 



