The teguments are soft or subcoriaceous. The striae of the cephalothorax and of the three last abdominal segments are very distinct; those of the anterior segments are scarcely or not at all distinct. The anterior and lateral borders of the cephalothorax are smooth. The eye eminence is relatively small; smooth or, rarely, provided with small, slightly distinct, tubercles; widely separated from the cephalic border. Lateral pores small, oval, and marginal. Anal piece large, transverse-oval or semicircular, much wider than long, and much wider than the reflected borders of the eighth segment. Mandibles short, similar in the two sexes ; first joint furnished at the base below with an acute tooth. Palpi simple ; femur, patella, and tibia without any process and without projecting angles ; maxillary lobe provided at the base with two strong, conical teeth. Maxillary lobe of the second pair of feet very long, nearly straight from the base, not attenuated, directed mesad nearly horizontally, and united on the ventro-meson to the lobe from the opposite side without forming a sensible angle; the two together lightly arched on the cephalic border, and forming an even curve. Sternal piece large, slightly contracted between the fourth pair of coxae, gradually enlarging and obtusely truncate cephalad. The feet are very long and slender; tibia of the second pair with a few false articulations. Palpal claw denticulate.
Many Leiobunum species tend to form clusters of several, but some up to 1,000 and more individuals.
An as yet undescribed species of Leiobunum was first found in the Netherlands in October 2004, although reports date back to at least 2002. Since then it has been identified from Germany, Switzerland and Austria. It is distinct from all known central European species and was probably introduced. This species has a strong tendency to group together, most of the time protected from wind and direct sunlight, and stay in one place for weeks. They swarm out at night to hunt on their own. When disturbed, they move their bodies up and down in a fast rhythmic motion, with individuals moving away from the disturbance. The first juvenile stages seem to live on the ground below rocks and debris. The largest observed aggregation counted 770 individuals.