28 THE LIFE-STORY OF INSECTS [CH. 



the water in which the larva lives is foul and poor in 

 oxygen, the tail is thrust out of the water so that air 

 can be admitted directly into the intestinal chamber. 

 The aquatic life of these insects lasts for more than a 

 year, and F. Balfour-Browne (1909) has observed from 

 ten to fourteen moults in Agrion. Outward wing-rudi- 

 ments are early visible on the thoracic segments ; when 

 these have become conspicuous the insect, beginning 

 in some respects to approach the adult condition, 

 is often called a nymph. In an advanced dragon-fly 

 nymph, H. Dewitz (1891) has shown that the thoracic 

 spiracles are open, and, as the time for its final moult 

 draws near, the insect may thrust the front part of 

 its body out of the water, and breathe atmospheric 

 air through these. Thus before the great change 

 takes place the nymph has foretastes of the aerial 

 mode of breathing which it will practise when the 

 perfect stage shall have been attained. The emer- 

 gence of the dragon-fly from its nymph-cuticle has 

 been described by many naturalists from de R&uimur 

 (1740) to L. C. Miall (1895) and 0. H. Latter (1<)04). 

 The nymph climbs out of the water by ascending 

 some aquatic plant, and awaits the change so graphi- 

 cally sketched by Tennyson : 



'A hidden impulse rent the veil, 

 Of his old husk, from head to tail, 

 Came out clear plntcs of sapphire mail.' 



'From head to tail,' for the nymph-cuticle splits 



