MAY-FLIES 



433 



specimens, so that it is not certain that what he calls ^ the first 

 stage is really such. He found no tracheae in the earliest stages ; 

 the small first rudiments of the gills became visible in the third 

 stage, when there were no tracheae ; the fourth instar possessed 

 tracheae, and they could be seen in the gills. The wing rudi- 

 ments could first be detected in the ninth and tenth stages. The 

 changes of skin during the winter months are separated by 

 longer intervals than those occurring at other periods of the year. 

 The nymphs differ gteatly in the structure and arrangement 

 of their tracheal gills, and display much variety in their general 

 form and habits ; some of them are very 

 curious creatures. Pictet ^ divides them 

 in accordance with their habits into four 

 groups: (1) Fossorial larvae: these live 

 in the banks of streams and excavate 

 burrows for shelter; they are of cylin- 

 drical form, possess robust legs, abundant 

 gills at the sides of the body, and 

 frequently processes projecting forwards 

 from the head : examples, Ephemera (Fig. 

 278) and Pcdingenia. (2) Flat larvae: 

 these live attached to rocks, but run with 

 rapidity when disturbed ; they prefer rapid 

 streams, have the breathing organs at- 

 tached to the sides of the body and not 

 reposing on the back ; they are exclu- 

 sively carnivorous, while the fossorial 

 forms are believed to obtain their nutri- 

 ment by eating mud : example, Baiitis. 

 (3) Swimming larvae : elongate delicate 

 creatures, with feeble legs, and with strongly 

 ciliated caudal setae : example, Glo'eon (Fig. 

 276). (4) Climbing larvae: these live in 

 slowly-moving waters, especially such as 

 have much slimy mud in suspension, and fig. 278.— Adult nymph of 

 they have a habit of covering them- Ift"^^"' '^**'' 

 selves with this mud sometimes to such 

 an extent as to become concealed by it : example, Potamanthus. 



1 Tr. Linn. Soc. xxiv. 1863, p. 62, and xxv. 1866, p. 477. 

 2 Sist. Nat. Neuropt. Eph6mirines, 1843, p. 2 J. 

 VOL. V 2 F 



