Ontogenetic: relating to the development of the individual.
Ontogeny: the development of the individual as distinguished from that of the species: see phylogeny.
Onyches: claws of tarsi.
Onychium -ia: small processes between the tarsal claws in many Diptera; see empodium: a more or less retractile process on the feet of some beetles: in Hymenoptera, the apical tarsal joint bearing the claws: see also arolium and pulvillus.
Oöblast: the primitive germinal nucleus of an egg.
Oögenesis: the process of egg-formation.
Oölemma: the cell wall of an egg: see vitelline membrane.
Oötheca: the covering or case over an egg mass, as in certain Orthoptera: see egg case.
Opacus: opaque; a surface without any lustre.
Opalescent: with a bluish white lustre, as in opals.