58 INTERNAL ANATOMY OF INSECTS. 
differ in form. In the larva and pupa of Ephemera vul- 
gata there are six of these double false gills on each side 
of the abdomen, the three last segments being without 
them ; each branch consists of a long fusiform piece, ra- 
ther tumid and terminating in a point, which is fringed 
on each side with a number of flattish filaments, blunt 
at the end. An air-vessel from the trachea enters the 
gill at its base ; is first divided into two larger branches, 
each of which enters a branch of the false gill. These 
branches send forth on each side numerous lesser rami- 
fications, one of which enters each of the filaments a . In 
another species (E. vespertina) each false gill presents 
the appearance of a pair of ovate leaves with a long 
acumen, and the air-vessels represent the midrib of the 
leaf, with veins branching from it on each side b ; and, to 
name no more, in E.fusco-grisea, one branch represents 
the leaf of a Begonia, the sides not being symmetrical, 
with its veins, while the other consists only of numerous 
branching filaments c . In other aquatic larva?, as in that 
of the common May-fly (Sialis lutaria), these appendages 
consist of several joints d . 
By the above apparatus these aquatic animals are en- 
abled to separate the air from the water, as the fish by 
their gills; but how this separation is made has not been 
precisely explained. The false gills in many species are 
kept in continual and intense agitation. When they 
move briskly to one side, Reaumur conjectures they may 
receive the air, and when they return back they may 
a Plate XXIX. Fig. 5. De Geer ii. 624—. 
b Ibid. Fig. 4. De Geer Ibid. 647—. 
c Ibid. Fig. 3. De Geer Ibid. 653—. 
«• Ibid. Fig. 6. De Geer Ibid. 727-. 
