Ultimate: last or final: that larval stage just before pupation.
Ultramarine: an intense deep blue [cobalt blue].
Ultra-nodal sector: in Odonata, runs parallel with and between media 1 and 2, or principal and nodal sectors: = postnodal sector.
Umbilicate: navel-shaped, or resembling a navel.
Umbilicus: a navel, or navel-like depression.
Umbonate: bossed; with an elevated knob in the centre.
Umbone: an embossed, elevated knob situated on humeral angle of elytra.
Umbones: two movable spines on the sides of prothorax in some Coleoptera.
Umbrosa: shaded or clouded: a cloud or shade.
Unarmed: without spurs, spines or armature of any kind.