Illaenus has moderate or large size, smooth or linear exoskeleton. Cephalon is rounded, strongly convex, with steep frontal part, rounded or sharpened or sometimes spinulosous genal angles. Thorax has 9 to 10 segments, thorax axe is wider than pleurae which have blunted edges. Pygidium is rounded and sub-equal to cephalon in shape and size. Over 20 species are known in St.Petersburg region.