There are over a hundred species of water beetles, however the diving beetles are large and easy to identify. This one is a Greater Diving Beetle, as opposed to the Lesser Diving Beetle which is much smaller. Unfortunately I've yet to see this one in its beetle form, but the Larval form here is quite unmistakable and quite a predator - look at those sharp horned jaws! They can grow up to 6cm long. This one was seen in the Lily Pond below the monument.