There are 670 species in the world, mainly in the tropics, but just three species of Cossidae in the British Isles. As adults the Cossidae do not have functional proboscis. They have long wings that are held like a steeply pitched roof over the body. Fore wing length ranges from 4 - 1245mm. The adults are nocturnal and usually drably coloured, either gray or black, sometimes with brown or orange patches. The females lay their eggs in groups on the stems of the host plant - always woody. On hatching the larvae bore into the host plant, with some species living communally. The larvae feed in the wood and usually take at least a year, and some as much as four years before pupating.
The eggs are brown, and laid on the bark of ash, elm, willow, oak, birch, apple and poplar.
The caterpillar feeds on the inside of the trees. It takes three or more years of feeding on this low-quality food before it can pupate.
During the cold winter months it makes a kind of nest for itself and rests out the winter. When fully grown it can be up to 100 mm long. It gets its name from the rather unpleasant smell the caterpillars have.
It exits the tree to pupate in the ground making a hole as large as 20 mm in diameter.
The adults have a wingspan of 60-80 mm and and both front and rear wings are the same colour. They fly in June and July, usually around the food plant trees, and the females are larger and fatter than the males. It is found throughout the UK, but is more common in the south, in Europe and eastwards as far as central Asia.