The beetle on the right is a male Dytiscus marginalis, the great water beetle or carnivorous water beetle, a member of the Dytiscidae family. There are 4 000 species world wide in this family, some up to 7 cm long. There are 118 species in the UK. U. K. species range in length from 0.2 cm to 5.0 cm. Both larva (see below left) and adult are carnivorous, and the UK species have jaws powerful enough to puncture human skin.
All the adults in this family have the same oval-shaped body and fringed and flattened hind legs, which makes them powerful swimmers. Their air supply is carried under their hard fore wings (elytra), and they stick the tip of their abdomen out of the water to take in air. Usually the adults emerge in the late summer or early autumn. Pupation takes place out of the water. The larva (below) digs a chamber in the bank. In the summer pupation can take just 2 or 3 weeks, but in the autumn or winter the adult will not emerge until the spring.
The adults have spurs on their tibia (see the drawing above left), which are sharp enough to draw blood if the beetle is mishandled. The larvae have mandibles strong enough to puncture human skin, so should be handled carefully or not at all.
Dytiscidae, steroids and breast enhancement
When mishandled adults secrete a fluid from their thoracic glands as a defence. This fluid contains steroids, in fact one secretion contains as much deoxyxorticosteron as a whole herd of cows. Consequently pharmaceutical companies use Dytiscids in their R and D. Even more remarkable is the practice followed by an east African tribe. The young women collect adult Dytiscids, and then induce them - probably by mishandling them - to bite their breasts. The defensive hormone-like steroids (estrone, estradial and testosterone) secreted by the beetles stimulates breast growth!
Larva spend their time eating, adults stop eating only to mate. The larva has hollow jaws/mandibles. It passes digestive juices down the hollow mandibles into the prey turning the inside of the prey into a soup which the larva then sucks up as if through a straw. All that is left is the exoskeleton and hard parts.
Dytiscus marginalis, the Great water beetle
Dytiscus marginalis is probably the commonest member of the Dytiscidae family in the U. K., and is found in a wide range of aquatic habitats throughout the country (weedy ponds, ditches and canals), but rarely in fast flowing water.
AdultDytiscus marginalis are dark brown/black with a greenish sheen and a yellow margin. Although they are large, 27-35 mm in length, they are strong fliers. It is mainly the males that fly, and nearly always at night. They are searching for females. Unfortunately they often mistake greenhouses, shiny cars and even wet roads for water and crash land, sometimes with such violence that they die.
The females are similar to the males, but they do not have the wide front tarsi with suction pads. The male is very glossy, and the female less so. The female has longitudinal striations (grooves running lengthwise). Male Dytiscus marginalis have one large, nine medium and 160 small suckers on the undersides of three joints of their front pair of tarsi (see the drawing on the left). These help them hold on to the female while mating. These suckers can support up to 13 times the beetle's own weight. While mating the pair can swim around for hours. Adults hibernate in mud at the bottom of ponds during winter. They are quite long-lived and can be as much as 3 years old. Newly emerged adults are most commonly seen between July and October.
In the spring the female cuts a slit in a submerged plant stem with her ovipositor and lays an egg inside each slit.
The larva is lighter than water, so must actively swim to stay submerged, or hold on to something. It breathes through 2 spiracles at the very tip of its abdomen. Larva can reach 5 cm long when fully grown.
Pupation may take just a few weeks, or the whole winter depending on temperature.
When I built my first pond we had a Dytiscus marginalis which we named Ghingis because it ate or attacked anything, even a human finger. Grabbing things with its front legs and pulling them towards its jaws. And it swam much faster than anything else in the pond. Then we found out she was a female, but the name stuck. And if we thought the adult was voracious that was nothing compared to the larvae, see above, which can reach 50 mm long. However I couldn't help liking Ghingis, and as we had a fairly large pond we just left her to eat knowing there were more than enough tadpoles for even her. However whatever you do do NOT have one of these in a small aquarium along with other animals, as pretty soon you will just have one hungry beetle.
On the left is Deronectes elegans, also in the Dytiscidae family. The adults are brown and yellow. It is found in stagnant ponds among algae. The larval diet is mainly water louse and other insects. And on the right is the underside of Deronectes sp. it is about 5 mm long.
Below right is the larva of Hydroporus sp. They can grow up to 10 mm long, and have a relatively fat body for a Dytiscid, with longer tails and a narrow, pointed head. They are fairly common in weedy ponds and canals.