Great diving beetle

Dytiscus marginalis

AKA: Great water beetle

The Great diving beetle is a very large aquatic beetle measuring up to 3.5cm in length. They have an almond shaped, stream-lined body. They are dark, olive-brown with a continuous yellowish margin around the thorax and wingcases. The wingcases of females are ribbed, but the males’ wingcases are smooth. Underneath, they are yellow, and their hind legs have a fringe of hairs which aids swimming.

The Great diving beetle can be found in ponds and slow-moving water, and very occasionally in fast-flowing. Adult beetles fly at night to look for other waterbodies to colonise.

Adults occur all year round, but tend to be less active during the colder months. They overwinter in the substrate at the bottom of ponds etc. and emerge early in the year. In spring, the female lays eggs in cavities by using her ovipositor to cut into the stems of aquatic plants.

The larvae can grow up to 60 millimetres in length and are yellowish-brown in colour. They possess large, pointed, sickle-shaped jaws which are used to capture their prey. Digestive enzymes are pumped into the body of the prey which dissolve the victim’s internal organs, and the resulting 'soup' is sucked up by the larvae. They use the damp soil by the edge of the water to pupate.

What they eat

Both adults and larvae are carnivores predating on a range of aquatic life including tadpoles, other water insects and small fish. The larvae are also cannibalistic and will eat other Great Diving Beetle larvae.

Where and when to see them

These impressive beetles can be seen all year round, but numbers peak in May and September.
They are common and occur in most still or slow-moving aquatic habitats.
Look out for them poking their abdomens out of the surface of the water. They are replenishing the air supply stored in a cavity beneath their wing cases. The larvae also tend to sit just below the surface with the end of their abdomen just poking above the water so they can breathe air.
Handle with care if you pick one up - the adults will exude a foul smelling liquid from the abdomen when alarmed, and the larvae possess a fierce set of jaws!

!Take care near waterbodies!

View a Great diving beetle 10km distribution map of Wales

Legal status

None.

Similar species

There are over 470 species of aquatic beetles that are associated with water, and there are more than 100 species of Dytiscus beetles. They are mostly around 3-15mm long so much smaller than the Great Diving Beetle.

Whirligig beetles are very common and easily identified as they swim fast in circular patterns on the surface of the water. They are much smaller than the Great Diving Beetle at around 7mm long.

Did you know?

The front pair of legs of male great diving beetles have suction cups which enable a secure grip during mating. Female beetles do not have them.