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. Ooeblast: the primitive germinal nucleus of an egg. Ooegenesis: the process of egg-formation. Ooelemma: the cell wall of an egg: see vitelline membrane. Ooetheca: 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. Opalinus: = opalescent; q.v. Opaque: without lustre: not transparent. Operaria: the workers in Hymenoptera. Operative: in working order or actually working. Opercula: two plates covering the vocal structure of Cicada, beneath. Operculum: a lid or covering: in Diptera, the chitinous envelope covering the lower part of the muscid mouth; the labrum-epipharynx of Dimmock: the scutes covering the meso-thoracic stigmata: in Aleurodidae, the lid-like structure closing the vasiform orifice; q.v. Ophthalmic: relating to the eye. Ophthalmotheca: that part of the pupa that covers the eyes. Opisthogoneate: having the organs of generation at hind end of body. Opisthogonia: the anal angle of the secondaries. Opposite: placed over against, or opposed to. Optic: relating to the organs of vision. Optic ganglia: are at the sides of the procerebrum and innervate the compound eyes. Optic lobes: the laterals lobes of the procerebrum in which are centered the nerves supplying the organs of vision. Opticon: the first of a series of three ganglionic swellings in the optic nerve: see epiopticon and periopticon. Optic segment: =procerebral segment; q. Optic tract: is the perceptive portion of the compound eye. Ora: a border: specifically in some Coleoptera, the lateral margin of prothorax. Ora coleopterorum: the margin of the elytra. Orad: toward the mouth. Oral: pertaining to the mouth. Oral cavity: the mouth; = buccal cavity. Oral fossa: in Mallophaga, a furrow lying in front of the mandibles. Oral segment: that ring or segment which bears the mouth. Orbicular: round and flat, the diameters of the plane equal: in sonic moths, a round or oval macula in the median cell. Orbit: an imaginary border around the eye: in Diptera the orbits are divided into vertical or superior; frontal and facial or anterior; of the cheek or inferior; occipital or posterior. Orbital sclerite: a narrow sclerite encircling some eyes. Order: one of the primary divisions of the Class Insecta, based largely on wing structure and then usually ending in -ptera. Ordure: excrement; usually applied to such as is foul or offensive. Orichalceous: = aurichalceous; q.v. Oriental: in geographical zoology as used by Wallace, that part of the earth's surface including Asia east of the Indus River, south of the Himalayas and the Yangtse-kiang watershed, Ceylon, Sumatra, Java and the Philippines.