310 INTRODUCTION TO ZOOLOGY CHAP. 



for they contain prolongations of blood-vessels, in place of 

 tracheae, the blood being directly purified by the air in 

 the surrounding water instead of being entirely dependent 

 on the air brought in by the tracheae. Perhaps this special 

 adaptation is necessitated because of the scarcity of air in 

 the mud in which the larva lives. 



The burrow made in the mud is U-shaped, for the larva 

 burrows down head foremost, and then works gradually round 

 and up again. A current of water is made to flow constantly 

 through it by the motion of the tracheal gills. The larva 

 feeds to some extent on organic matter present in the mud 

 which it swallows, and also on small creatures, though some- 

 times its prey is almost as big as itself, for the mouth and 

 jaws are quite well developed at this stage, though in the 

 adult so atrophied as to be practically absent. 



When ready to leave the water, the nymph 



of Habitat sw i ms ^ ^ ne surface, and with remarkable rapidity 



the skin is split and the winged form rises into 



the air ; there is no long gradual extrication of limbs and 



body nor slow growth of 

 wings, as seen in the Dragon- 

 fly, and yet the process 

 must be much the same, 

 though condensed into so 

 short a time that it is al- 

 most impossible to follow it. 



FIG. 231. Adult May-fly (Ephemera}. Imago. , P r0( ; ess 



(Natural size.) f 1S 



in that, after the first flight 



into the air, the insect again comes to rest, and throws off 

 very rapidly a second skin, changing its shape and colour 

 slightly as it does so, becoming grey instead of green, and 

 exposing wings of a rather more delicate texture, shorter 

 antennae, and longer tail filaments. 



These tail whisks are very characteristic of the May-fly. 

 There are always three in Ephemera vulgata, but two only in 

 some other species. 



The minute antennae are very inconspicuous, but the 

 front pair of limbs is turned forwards, and may be mistaken 

 for antennae until carefully examined. 



In most May-flies, two pairs of wings are present, and their 



