VII STONE FLIES 2S3 



late summer and autumn. They hide themselves 

 during winter, but reappear early in spring. From 

 this time they feed actively, capturing and devouring 

 all the weaker inhabitants of the stream, until the time 

 comes to quit the water. I have found in the stomach 

 as many as five empty heads of Chironomus larvae. 

 The jaws of the larva resemble those of a Beetle or 

 Cockroach. The mandibles are stout, and armed 

 with small pointed teeth ; both pairs of maxillae are 

 well developed, and provided with good-sized palps. 



It would naturally be expected that a larva so 

 entirely aquatic as that of Perla would possess re- 

 spiratory organs of the nature of gills, and this is 

 actually the case. At the base of each leg, and just 

 above it, is a tuft of filaments, supplied by fine, 

 branching air-tubes. A little behind each of these 

 bunches of tracheal giils, and in the flexible skin 

 which unites the thoracic segments, or unites the last 

 thoracic segment to the first abdominal, is another 

 bunch. There are thus twelve thoracic gills (six 

 pairs) in the larva of Perla. Finally, just inside the 

 base of each tail-filament is another gill.^ It will be 

 noticed that, as in Crustacea, the gills are placed where 

 the movements of the appendages can either wave 

 them to and fro, or set up a swirl in the water. The 

 living larva rises and sinks continually (when not 

 travelling about) by a slight movement of the legs, 

 and this movement is probably of use in respiration. 



In an Ephemera larva the tracheal gills are com- 



' Palmen, Morph. d. Tracheen-systevis (1877). The descrip- 

 tion in the text applies to certain large species, such as Perla 

 bipunctata. 



