[1] Velleius dilatatus is a large species of rove beetle being between 10 and 25 mm (0.39 and 0.98 in), and is completely black or dark brown in colour with a rounded, laterally flattened pronotum.

They have a large head with weakly convex eyes and long and curved temples, The surface of the exoskeleton is matt because there is strong microsculpture.

The pronotum is rounded, creating angles towards the front, and is flattened laterally and has a strong microsculpture which leads it to appear iridescent in low-angle lighting.

[5] Velleius dilatatus spend most of their adult lives in the nests of European hornets, typically in tree-hollows but they have also been recorded from houses.

However, they are known to leave nests on warm evenings, when the temperature exceeds 16 °C (61 °F), to prey on other insects or to visit sap-runs on oak and beech trees.