Obtusilingues: short-tongued bees with the tip obtuse or bifid: see acutilingues.
Occipital foramen: the opening in the occiput, opposed to a similar opening in the prothorax: = foramen magnum.
Occipital margin: in Mallophaga, the posterior margin of the head.
Occipito-orbital bristles: in Diptera; situated on posterior orbit of eye.
Occiput: that part of the head behind the vertex: in Diptera, the whole posterior surface of the head: in bees, the space between the vertex and the neck.
Occlusor: applied to muscles which close an opening; e.g. spiracles.
Occult -us: hidden; concealed from superficial view.
Ocellar bristles: in Diptera, are situated close to the ocelli, usually directed forward: often absent.
Ocellar ribband: a crescent-shaped, smooth thin belt across the eye region in butterfly chrysalids.
Ocellar triangle: a triangle, indicated by grooves or depressions, on which the ocelli are situated; Diptera.