Obligate essential, necessary, unable to exist in any other state or relationship, e. g. obligate parasite
Oceanic zone the region of open waters beyond the continental shelf and lying over deep areas
Ocellus/ocelli One of the 3 simple eyes of adult insects, located on or near the centre line of the dorsal surface of the head.
Odonata dragonflies and damselflies
Odour trail A chemical (pheromone) trail, on the surface or in the air or water, deposited by one animal for another animal to follow. As in mating trails and foraging trails.
Oligochaeta earthworms, pot worms, etc.
Ommatidium One of the basic visual units of the insect compound eye. The ommatidia are bounded externally by the facets that together make up the glassy, rounded outer surface of the eye.
Omnivore An animal that eats both plant and animal food
Omnivorous Feeding on a diet of plant and animal food
Ontogeny The course of growth and development of an individual to maturity
Onychophora Velvet worms
Ootheca plural = oothecae An insect egg case (see cockroach). It may have a thin covering as in cockroaches, or the eggs may be embedded in a fast-hardening foam, as in mantids.
Open circulatory system An arrangement of internal transport where the blood bathes the organs directly.
Operculum a calcified flap or plate which closes the opening of some gastropod shells
Ophiuroidea Brittle stars, a Cass in the Echinodermata.
Opiliones harvestmen
Opisthobranchia a sub class of the Gastropoda
Opportunistic species A species adapted to use variable, unpredictable or temporary environments. Usually they have a high dispersal ability, and high and rapid population growth.
Optimal most favourable
Optimal foraging a type of foraging behaviour that maximizes the ratio of energy intake to expenditure
Opithosoma the posterior body (abdomen) in arachnids and pogonophorans
Oral of the mouth
Orb web spider a spider in the Araneidae family
Order the major taxonomic division of a class
Organ A structure of the body composed of several kinds of tissue that carries out specific tasks, e.g. heart
Organic In chemistry -containing the element carbon. In nature - connected with living material.
Organelle A membrane-bound body in a cell, e.g. mitochondria. An organelle performs a specific function.
Orthoptera the Order of insects containing the grasshoppers, locusts, crickets, katydids, etc.
Osculum the excurrent opening in sponges
Osmoregulation the maintenance of a suitable internal salt/water concentration in a cell or body
Osmosis the process whereby water passes across a membrane from the weaker to the stronger solution
Ossicles the small, separate pieces of echinoderm skeleton
Ostia a pore through which water enters the body (as in sponges), or through which blood enters the heart
Ostracoda mussel shrimps, seed shrimps, a Class in the Crustacea phylum
Outbreeding The mating or crossing of individuals that are either less closely related than average in the population, or from different populations. The opposite of inbreeding.
Ovary the structure producing female gametes and reproductive hormones
Oviduct In invertebrates, a tube passing from the ovary to the vagina
Oviger the egg carrying leg in the pycnogonids
Oviparous Reproducing by laying eggs
Oviposition the act of depositing eggs
Ovipositor The egg laying tube in some female insects. In some bees and wasps it has been modified to become a sting.
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