DYTISCUS MARGINALIS. THE GREAT WATER BEETLE. 155 
for the business of life. Similar modifications are found in the 
beetle family. The wing-cases fit closely at the margins and en- 
close air underneath, enough to supply the tracheze for some time 
when under water. 
The shape of this beetle is much more fish-like, and presents a 
smooth surface to the water, thus lessening the friction, which is 
much greater in the water than in the atmosphere. The legs are 
much modified, the anterior pair being small but well armed, the 
intermediate pair are much smaller also than the corresponding. 
pair in a land beetle. A marked difference is found, however, in 
the posterior pair, which in land beetles are about the same size as 
the pair in front of them; but in the Dytiscus marginalis they are 
about three times the size of the intermediate pair, being flattened 
and edged with strong hairs, to be used as swimming paddles, the 
flattened surface and the hairs converting each into the blade of an 
oar. 
Another feature worth notice is the position of these swimming 
paddles which are placed well back, as are the flappers of seals and 
diving birds. The attachment is of such a nature as to allow the 
utmost possible space for the powerful muscles which work them. 
While every provision is made for the best means of locomotion, 
you will notice that, although much reduced, the formidable claws 
are present in the intermediate and posterior pairs of legs as well 
as in the anterior, being found both in the male and female. The 
wings as we have seen, are large and powerful, and of great service 
in transporting the owner to new hunting grounds, for food must 
often fail in one pond if they exist in great numbers, owing to their 
rapacity. They are pretty fairly distributed over the country, 
though in some districts they are much more plentiful than in 
others. I know many ponds where they exist within a few minutes 
walk of my house, but do not know of any place literally swarming 
with them as was a pond near Bristol, which a friend of mine came 
across by accident, when, of course, he was without net or bottle, 
and compelled to hurry on owing to urgent business. A find of 
this kind is really valuable to a microscopist, as each male beetle 
furnishes eight really interesting slides. I mention the male beetle 
as producing eight good objects, because his better half only pro- 
duces six, owing to the absence of the dilations of the tarsi (fig 26) of 
the front pair of legs, which form, to my mind by far the most interest- 
ing portion of this beetle. In Geodephagous beetles the males 
have a pad on the under surface of this dilation, but the Dytiscus 
has a most wonderful array of suckers, which, although I have 
briefly referred to, is worthy of closer attention. One of the suckers 
is very large, the next about half the size, and the rest like long- 
stalked champagne glasses. The two large ones seem to be rather 
like the flower of the common coltsfoot, the centre depressed, and 
