The Great Diving Beetle, Dytiscus marginalis, is a voracious predator and excellent swimmer. The beetles live, mate and lay eggs in the water. The body of the beetle is sleek and designed to slice cleanly through the water. This requires a smooth surface and tapered body. There are few rough edges on the beetles.
The beetles mate in the water and males must be able to hang onto the females while swimming through the water. Males have suction cups on their fore and middle legs (tarsi) that grasp the smooth surface of the female during mating. The suction cups on the males are only used for mating. Female beetles lack suction cups.
The fore tarsi, of male Dytiscus marginalis, have an expanded plate with 2 large suction cups and dozens of additional smaller cups. These suction cups are capable of holding onto objects that weigh over 4 times more than a female beetle.
An image of the tarsi with the suction cups is captured by French photographer, Christian Gautier and is one of the submissions to the 2011 Nikon Small World Contest. The popular vote contest is still ongoing until November 9.
In the center of the photo below, is the the adhesive organ of the foreleg of Dytiscus marginalis. The organ is divided into 3 lobes and covered with small suction cups. The red and blue colored areas are the 2 larger suction cups. The small suction cups are all at the end of flexible stalks that allow each cup to make a firm contact with an irregular surface.
Fore leg of Dysticus marginalis
Image: Christian Gautier
Nikon Small World
Living With Insects Blog 
That’s a useful appendage!
Diane D