. An introduction to zoology, with directions for practical work (invertebrates). 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 mad
. An introduction to zoology, with directions for practical work (invertebrates). 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 HabUat ^^™® ^'^ *^® 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. , process (Natural size.) . , ^^ also pecuhar m that, after the flrst 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 fllaments. These tail whisks are very characteristic of the May-fly. There are always three in Ephemera mdgata, but two only in some other species. The minute antennae are very inconspicuous, but the front pair of limbs is turned forwards, and may b
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