306 NATURAL HISTORY OF AQUATIC INSECTS CH. 



" Some Ephemera larvae make burrows ; others 

 lead a free life. The sides of the abdomen bear a 

 number of paired tufts, which are moved up and 

 down with great rapidity. They have been taken by 

 some authors for fins, but it is enough to refute this 

 notion to observe that they are moved most ener- 

 getically when the larva remains stationary. They 

 are gills, as their microscopic structure shows. In 

 some Ephemerae they stand out from the sides of the 

 body like the oars of a galley, 1 in others they are 

 upright or curved over the back. 2 There may be six 

 or seven pairs of gills, beginning at the first or second 

 abdominal segments ; the last three segments never 

 bear gills. In the larva of Ephemera vulgata each 

 gill consists of two branches of nearly equal size, which 

 are each fringed with filaments like a feather. Two air- 

 tubes traverse each branch and filament. In another 

 Ephemera larva [that of Chloeon dipterum] the gills 

 fo-rm plates or leaflets, each of which is doubled in 

 two, and the tracheae, instead of projecting, run in the 

 thickness of the leaflet. These gills work backwards 

 and forwards, and often all are moved simultaneously 

 though sometimes the last pair remain motionless 

 when all the rest are moved. 3 In the larva of Poly- 

 mitarcys virgo each gill consists of two long and 



1 This is the case, for example, with the larva of Baetis 

 (Ecdyurus) fluminum, a large species found abundantly be- 

 tween and beneath the stones of clear streams. 



2 Ephemera vulgata, Polymitarcys virgo, &c. 



3 The last pair of gills in the Chloeon larva have the leaflet 

 flat, and not folded in two. In the six other pairs the two 

 leaflets are more distinct from one another than Reaumur's 

 description implies. 



