Mealworms seen in pet shops everywhere are the larva of a species of darkling beetle, the yellow mealworm beetle, Tenebrio molitor. They are heavily exploited and easy to breed – sold live for feeding fish, reptiles, and as bait, and dried for wild bird food. People also eat them in restaurants, baked, toasted or fried.
In the wild, however, the adult beetles – pale brown to black and shiny – are in short supply. Like so many other insects, numbers are declining. The beetles are 12-18 mm long and can be found between May and September occurring over most of the northern hemisphere, although in pockets. For example, in the UK they can be found in the Midlands, East Anglia and the south-east and only occasionally elsewhere. They are attracted to light and fly so may be found in houses before disappearing into dark corners.
The eggs, 200 to 600 per female, can be laid singly or many at a time depending on the supply of food. The larva can be a pest in badly stored cereal or food left in damp places but mostly spread out looking for organic material. They can survive in low temperatures and adverse conditions but generally will overwinter, pupate in the spring, and spend the summer mating and distributing their eggs.