The larva of Cybister lateralimarginalis looks a lot like that of the Great diving beetle, but there are differences:

• it lacks the two small "feathers" (cerci) at the end of the tail
• it's last tail segment is longer, forming a sort of spike
• it doesn't hold its body in an S shape
• it is somewhat leaner and has weaker legs
• the head is narrower, rounder and has spikes between the jaws
• the shields on the segments are very small

The larva is unable to hang freely under the water surface like the Great diving beetle larva. That's why it seeks support from the bottom or on a water plant as is shown here.

I went to a ditch where we had caught an adult beetle and seen quite a few more about a month earlier. I hoped to catch a larva there and with the first stroke of the net I caught one!