XXI 



INSECTA: NEUROPTERA 



313 



segment and one pair on the last segment of the tail. These 

 ' ' creepers " are carnivorous, feeding on smaller aquatic larvae ; 

 their mouth-parts are therefore well developed. This stage 

 endures for two or three years. 



When the time has 



come ' the n 3 7m P h leaves 

 the water, climbing on to 



some projecting rock or plant. The 

 skin swells and splits over the thorax, 

 and then the body is withdrawn from 

 it. The wings expand, and after a 

 few hours the tissues have become 

 dry and firm, and the insect flies off, 

 generally, however, remaining close to 

 its former habitat. It does not feed, 

 and only lives a few days. 



The Willow- fly has a 



very similar life-history 



and structure, but the 

 two long processes at the end of the 

 body, though present in the larva, are 

 lost at the last moult. The separa- 

 tion of the bases of the two pairs of FlG 2 34. Larva 

 wings is very marked in this genus. Ucaudata. 



Family 4 : SIALIDAE (THE ALDER-FLIES OR HUMPBACKS) 



This family differs from the three so far considered in 

 having a "complete" metamorphosis, i.e. one with a quiescent 

 pupal stage. There are four membranous wings, the difference 

 in size of the two pairs being but slight. They are held over 

 the back obliquely, meeting in a ridge over the middle line ; 

 the veining is fairly simple and conspicuous. The head 

 bears long antennae. The larvae of the Alder- flies are 

 aquatic, but there are other members of the same family 

 the Snake-flies which have terrestrial larvae. 



The Common Alder-fly (Sidlis lutaria). 



These flies can be readily recognised by their long black 

 antennae and darkly veined, dusky wings, the upper pair of 

 which folds over the lower pair when at rest, the hinder part 



