APEIL. 
85 
discover anything stirring. Yonder are two black Water 
Beetles (^Colymhetes) ; see how they hurry to the top of the 
water^ then scuttle down again to the bottom^ and hide un- 
der the mud. 
C. — I have one of them. 
F. — This insect is worth a moment's observation. Ob- 
serve how smooth its surface is^ without any projections^ 
to adapt it for swift progress through so dense a medium as 
water : the elytra shut very closely^ to prevent the wings 
beneath^ which are large and long^ from becoming wet and 
unfit for use : the fore and middle feet are small and weak,, 
but the hindmost pair are very large and strong, and thickly 
fringed with hair ; they are used as oars to propel the insect 
through the water^ and we may see how admirably they 
are fitted for this purpose, by observing the effect produced ; 
for the motion of the aquatic beetles is very swift. There 
are many species of the tribe, but all agree in these particu- 
lars : some are of very large size ; this is but a small kind. 
C — Here are some larvae of the Ephemera. They keep 
the fin-like appendages at their sides constantly in a waving 
motion^, even when they themselves are at rest : what can 
be the reason ? 
F.— \ believe these fins are connected with the air-pipes^ 
and are in fact breathing organs : and as they extract the 
oxygen from that portion of the water with which they are 
immediately in contact, I suppose their constant vibration is 
necessary to produce a current, and so bring fresh particles of 
water to be inhaled. But I apprehend these likewise perform 
a double office, and are also organs of locomotion. 
C. — There is a serpentine motion of the abdomen when 
they swim, which perhaps aids them in some degree ; but 
they do not appear to swim with the same facility as most 
water-insects. I see some of these are in the pupa state^ by 
