There are 17 000 species in this family world wide ranging in size from 0.2 - 5.0 cm, and 47 British species.
Most adults have heavily striated (lines and ridges) or bumpy elytra (wing cases). The adults (see below) are mainly dark brown or black, and if you look closely, the compound eye of the adults is not round, but usually kidney shaped or notched. Most are nocturnal scavengers, and range from 2 - 35 mm long. Adult length is 12 - 16 mm.
Tenebrio molitor, Mealworm
Above, right and below show the 3 typical stages of a holometabolous insect; the larva, pupa and finally the adult. In this case it is the mealworm, Tenebrio molitor, a female can lay up to 576 eggs either singly or in groups. They are bean-shaped and sticky, so soon become covered in debris.
Note the tiny antennae of the larva above. It really does look like a worm, but when it is turned on its back the three pairs of articulated legs are clearly seen (above), as are its very powerful jaws. Its long cylindrical shape and yellow colour are typical of larvae in this family. A fully grown larva can reach 28 mm long.
Larvae of this beetle are commonly sold as live bird and lizard food. If you feed these to your pet and your pet swallows the larva whole then it is a good idea to cut off the larva's head before feeding. The powerful mealworm jaws will continue to work in your pet's stomach and there is a real danger of it harming your pet or even eating its way out of the stomach before the digestive acids kill it!
The entire life cycle of mealworms takes between 9 months and 2 years depending mainly on temperature. They are most commonly found where grain products are stored. Unlike most other species in this family the adult beetle can fly.
Tribolium castaneum, the flour beetle
On the right is another member of the Tenebrionidae family, Tribolium castaneum, the flour beetle. It is a pest in flour and dried foods. Its life span can be as long as ten years, and it can fly.
All the 19 British beetles in this family are small - 5 mm long. They are easily recognised if you turn the adult on to its back you can see that the first joint of the hind leg is enlarged to form a large plate, see Haliplus sp. on the left. Both adult and larva (see below) are found among thick algae, especially Spirogyra, other aquatic vegetation, or under stones in still and running water. The adult is yellow or reddish brown. The larva feeds on Spirogyra and has long triangular spines down its back and a long tail.
The 20 British species in this family have terrestrial adults that do not stray far from the larval habitat, and aquatic, filter feeding larvae. On the left is a larva of Helodes marginata. When fully grown it is 10 mm long. It is found on the undersides of stones in small rivers and mountain streams.
9 British species. All British beetles in this family are vegetarian, small (adults are usually less than 5 mm long), and are found in fast-flowing rivers and streams. The adults have, long, sharp claws (see Helmis sp. right) enabling them to hold on to stones etc. Both adults and larvae require a similar habitat, and so are often found together.
In the UK there is just one species in this family, Hygrobia tarda also known as Hygrobia hermanni, the screech beetle or squeak beetle, right and left. The adult (right) is about 10 mm long, and the larva (left) 12mm long. Both are found in muddy ponds and ditches.
The larva tends to stay near the bottom of the pond where it feeds on Tubifex worms. It pupates out of the water, and the whole lifecycle takes less than 4 months - very short for a beetle.
The adult has a very convex, reddish brown body, with darker elytra and a black edge round the thorax, and prominent eyes. The beetle gets its common name from the sound the adult makes when picked up. It makes this sound by rubbing the tip of its abdomen against its elytra.