There are 15,000 species in this family world wide ranging in size from 0.2 - 5.0 cm, around 600 species in Europe, and 40 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. Adult length is 12 - 16 mm for U. K. species.
Above 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. Adult body length is 12.0 - 17.5 mm.
Below is another member of the Tenebrionidae family, Tribolium castaneum, the flour beetle. It is a pest in flour and dried foods such as rice, maize and cocoa. Its life span can be as long as ten years, and it can fly. Adult body length is 2.3 - 4.4 mm.