466 



ANIMAL RESPIRATION 



is, however, one exception in this particular case, for part of the 

 blood from the last chamber but one passes back through a pair of 

 valves into the last chamber, which forces it into the three tail- rods 

 through a corresponding number of vessels (fig. 571). Each 

 of these vessels is perforated by a series of oval holes through 

 which the blood escapes into the rods, which are hollow, and 

 it is then returned to the body, presumably in a purified condi- 

 tion, having absorbed some of the dissolved oxygen from the 

 surrounding water and got rid of waste carbonic acid gas. 



We have noted in a previous chapter that the delicate gills 

 of such creatures as fishes and the higher crustaceans are pro- 

 tected by being placed in special gill-cavities. Tracheal gills may 

 also be sheltered in various ways. They some- 

 times grow out from the under sides of firm plates 

 (Oligoneuria), and in other cases the gills of either 

 side are shielded by a flat gill -cover ( Tricorythus\ 

 But the most remarkable instance is afforded by 

 the nymphs of certain May- Flies (species of 

 Prosopistomd) which are native to Europe, West 

 Africa, and Madagascar. The nymph, which lives 

 in swift streamlets, looks like, and was formerly 

 mistaken for, one of the lower crustaceans (fig. 

 572). The head and front part of the abdomen 

 are covered by a sort of shield, under which there 

 is a gill-cavity containing five pairs of tracheal 

 gills. Water flows into the cavity through an 

 opening on either side, and out again by a single 

 aperture at the back. Sharp (to whose admirable 

 volumes on Insects in The Cambridge Natural History the present 

 writer is here and elsewhere greatly indebted) says of this nymph 

 that it is " more completely adapted for an aquatic life than any 

 other insect at present known ". 



It only remains to mention that the nymphs of some May- 

 Flies appear to partly breathe by means of the rectum, like the 

 corresponding stages of Dragon- Flies which have been already 

 described. 



Alder-Flies (Sialidtz). The Alder-Fly (Sialis lutaria] is a 

 very common British insect which haunts the banks of sluggish 

 streams. The eggs are laid in regular rows on rushes or other 

 plants which grow near the water. The aquatic young which 



Fig. 572. Crustacean- 

 like Nymph of a May- Fly 

 (Prosopistoma), enlarged. 



