The beetle was laid temporarily on its back, in a Petri dish on a wooden table. Like every diving beetle in this situation, the animal was kicking with very fast and awkward movements in an effort to flip itself. Yet the short strobe of the flashlight created a sharp picture. The underside of this beetle has a dark brown colour. On the full image at left and on the middle detail picture you can see the left epipleur. That is the part of the elytron (wing case), that is visible from the underside. Here that epipleur is narrowing quickly, when following it downwards and it ends before you reach the middle of the beetle. This is true for all Agabus species, in contrast to Platambus maculatus, which has a relatively longer epipleur (van Nieukerken, 1992).
On the middle (detail) picture the bat-like figure of the metasternum was darkened, to demarcate it a bit. The yellow arrows point to one of the metasternal wings. These wings are narrow on Agabus undulatus (van Nieukerken,1992).
On the picture right above, a close-up of the left hind leg. We see rows of spines on the tibia and on the tarsus segments. These rows are double, the spines that make the second to last pair on the tibia are somewhat larger, the last pair is much larger, these are the spurs. Here, these spurs are of equal length and they are shorter than the following tarsus segment. On each tarsus segment, the last pair of spines is also larger than the rest of the row. All these spines will help the beetle when crawling, possibly they also are used when it cleans itself.
Picture right below: there are some short bristles that form a small comb at the end of the underside of the femur. This typical for Agabus and Ilybius species (Foster & Friday, 2011 and van Nieukerken, 1992), also for Platambus maculatus (van Nieukerken, 1992).
Sources:
Simons, J., Lokhorts, G.M., van Beem, A.P. (1999) Bentische zoetwateralgen in Nederland KNNV uitgeverij 1999 Utrecht
See for further sources page 1 and the Waterbeetles bibliography page.