PERLIDAE 



401 



and are able to swim well, the legs lieing provided with abundant 

 swimming hairs ; they, however, as a rule, prefer to walk at the 

 bottom of the pool, or on rocks or boulders in the water they 

 live in. 



One of , the most peculiar features of the Perhdae is their 

 respiratory system. Unfortunately the greatest differences of 

 opinion have prevailed on various matters in connexion with this 

 subject, and there are several points about which it is not possible 

 at present to express a decided opinion. 



The larvae have no stigmata; it appears to be generally 

 agreed that there is in them no means 

 of admitting air to the tracheal system 

 by means of orifices. Some breathe 

 entirely through the integument, the pro- 

 cess being aided by the accumulation of 

 tracheae at the spots where the breath- 

 ing orifices should be, and where the 

 integument is more delicate. Others, 

 however, possess gills in the form of pro- 

 truded bunches of filaments, connected 

 with tracheae in the manner shown in 

 rig. 253. These filamentous branchiae _, neo m 1, , •„ 



" Fig. 253. — Tracheal gill and 



occur in numerous species of the family, portion of a trachea of Pfero- 

 and are situate on various parts of the '"'™2'*- '^"^ Newport.) 

 body, but many species are destitute of them in genera, other mem- 

 bers of which possess the filaments. In some Nemourae instead 

 of bunches of filaments there are tubular projections on the pro- 

 thoracic segment ; and in Dictyo'pteTyx signata similar structures 

 occur even in the cephalic region, Hagen stating ^ that there exists 

 a pair on the submentum and another on the membrane between 

 the head and the thorax. In the imago state, stigmata are present 

 in the normal fashion, there being two thoracic and six abdominal 

 pairs. In several species the filaments persist in the imago, so that 

 in these cases we meet with the curious condition of the coexistence 

 of branchiae with a well-developed and functionally active system 

 of spiracles ; this is the more curious because the creatures usually 

 have then nothing to do with the water, it having been ascer- 

 tained that in these cases the species live out of the water as other 

 terrestrial and aerial Insects do. These instances of persistence 



1 Zool. Anz. iii. 1880, p. 304. 

 VOL. V 2d 



