Dytiscus marginalis - the Great diving beetle

Dytiscus marginalis
Great diving beetle, male

On the left a picture of a Great diving beetle, in a resting pose under water. The respiration spot at the hind tip of the abdomen is closed because the beetle has pressed that tip tightly to the elytra, the wing cases that cover the back. The border of the air supply, closed in under the elytra, gives the tip a a silver seam. Like most water insects, the Great diving beetle needs to come up for a new supply of fresh air. This air is taken in by bringing the tip of the abdomen to the water surface and then lowering it. The male is recognizable by it's smooth wing cases, as opposed to the grooved ones of the female. However, some females do have smooth wing cases too! But the females never have the sucking disks on the front legs that the males have. The male need these to get a firm grip on the slippery neck shield of the female when mating. More difficult to see is the difference in the fringes of swimming hairs on the hind legs: the males have these on two sides of these legs, the females only on one side.

Dytiscus marginalis
Great diving beetle, female

The picture on the right shows a female specimen. The grooves in the elytra are clearly visible. the grooves end at a distance before the apex, so the end part is not grooved but flat. The reason why the females have these grooves is not known. The females of the Lesser diving beetle, Acilius sulcatus, also have corrugated wings. One suggested explanation is the male could have more grip on the ribbed surface, but in reality the sucker discs of the male provide the necessary hold, furthermore they attach to the neck shield, never to the wings. The beetles are able to fly well: on evenings with little wind they climb out of the water and sit in a rather upright position on their legs. Then a strange high pitched humming is produced: possibly the pumping of air in the hollow veins of the second pair of wings (with which they fly), and in the body. Suddenly they take off, buzzing like a giant lady beetle. After a while flying they dive down when they spot a shining surface which may be a pond, but also swimming pools, the glass of breeding houses, shining cars or newly tarred roofs They are also attracted at night by light sources as street lamps or door lights of houses. So it is a dangerous flight nowadays, with all these confusing unnatural traps. If the ditch or pond has enough water plants and food supply, the female beetle cuts holes in the stems of some species of water plants with her ovipositor, a sort of injection needle for eggs at the tip of her abdomen, and lays an egg in every hole. In that way the long cylindrical eggs are stowed out of sight. After a few days there suddenly is a swarm of tiny, transparent larva in the water.

the Larva

Dytiscus marginalis
larva, young

The newborn little larva look like defenceless micro-shrimps and indeed they are a much appreciated snack for other insects and fishes. But they are little predators themselves: barely hatched, they catch and devour smaller creatures, or each other if they get the chance. They moult a few times and grow at an astonishing rate. The shape is obvious then: a slim, glassy caterpillar with six, long legs and two frightening sickle shaped jaws with which they snap there preys. They need a lot of them, because they must grow fast: the larvae are even more greedy then the adult beetles. On the picture at left: a specimen which is still only about two centimeters, sitting in the typical posture of the Dytiscus larva: tail bend upward, body curved in an "S" (or question mark) shape. The larva is lighter than water most of the time, because of the air it inhaled. So when letting go of it's hold, the larva slowly rises to the surface, with it's legs paddling faintly to keep balance. When the tip of the tail pierces the surface film, two small holes open, which are the ends of the main airducts (trachea). These airducts are now ventilated by breathing movements of the body. Two small appendages at the tail (cerci) support the upright position of the tail against the surface film. The larva spends a lot of time "hanging" like this at the surface, waiting seemingly lifeless for bait to pass by. But when a small fish, tadpole or insect comes near, the larva is suddenly very aware: like a cat it bends it's body backwards for the attack, the jaws standing wide out. When the prey reaches the target point the larva snatches it with a lightning fast pouncing movement of the head. The victim is dangling, pierced between the sickles, which almost meet in it's body. Then a horrifying, but also fascinating sight follows: a brown fluid is injected through the hollow jaws into the prey, that ends it's struggling after a few seconds because of the poisonous effect. The fluid contains protein-dissolving enzymes that liquifies the interior of the bait. The fluid is sucked back in, together with the dissolved nutrients. This process is repeated continuously, the streaming of the fluid is clearly visible in the jaws and in the head, where the streams gather in one duct: the digestive canal. So the the first digestion is external. After an hour most preys are sucked completely empty, a skin or external skeleton is all that remains of even the fattest caddis fly larva. The larva drops the rest and after that is ready for the next catch, the jaws opened wide. After each moult the larva expands to one-and-a-half its former size untill it is about six centimeters long. It doesn't hesitate in attacking prey larger than itself, and won't let go even if the agonised victim drags it through the water.

Dytiscus marginalis
larva, last instar

The full grown larva is fat and heavy, it doesn't hang at the surface anymore, but chooses shallow water where it still can get it's tail tip in contact with the air. At last it stops eating and leaves the water. It than digs a small hole in the mud, on the shore where it pupates. After a few weeks the beetle bursts out of the pupal skin. It is white and soft and has to stay in the pupation hole a few days to harden, during which the colour also turns dark.