Measuring some 3.5 mm Allacma fusca is a large species and the largest representative of its family Sminthuridae. It is dark brown, round in shape and the legs and antennae are a lighter shade of brown compared to the body. The hairs on the body are brown as well, even though those at the base may be ochreous. Under a microscope one can see that the 4th segment of the antenna is divided into 16 tiny segments. The complex eye is made up of 8 single eyes. Allacma fusca is a very common species preferring moist conditions. It can be seen in woodlands and parks, but is common in gardens too. Present all over Britain and Ireland it may be encountered all year long.