Definitions A - B , C, D - E, F, G, H, I-L, M - N , O-P, Q - S , T-Z

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 Calss 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 mamimizes the ratio of energy intake to expenditure
Opithosoma	the posterior body (abdome) 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 maintenace 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 agerage 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.
Pair bonding	The formation of a close and lasting association between male and female animals; used particularly in the cooperative rearing of young.
Paleontology	the scientific study of fossils
Palp	a fleshy process from the mouthparts, see spiders and insects
Palpigradi	microwhip scorpions
Pangea	The single, huge landmass during the Permian - Triassic.
Papilla	plural = papillae A small, fleshy protuberance on an animal's body (see tardigrada). Papillae often have sensory functions such as detecting food.
Parallel evolution	the independent acquisition in 2 or more related descendant species of similar derived character states evolved from a common ancestral condition
Parapodium	plural = parapodia A leg-like flap in some worms, used for locomotion or creating a water current.
Parasite	An animal that lives on or inside another animal, and feeds on its host, or on food its host has swallowed. Parasites may weaken their host, but usually do not kill them.
Parasitic castration	the reproductive death of a host organism resulting from a parasitic infection, e. g. Mermis nigrescens
Parasitic male	A dwarf male that lives a parasitic existence attached to the body of its female, usually having well del#veloped reproductive organs, but an otherwise degenerate body form, e. g. Bonella vridis
Parasitism	Symbiosis in which members of one species exist at the expense of members of another species, usually without causing their deaths.
Parasitoid	A parasite that slowly kills the victim. The death of the victim occurs near the end of the parasite's larval development.
Parasitology	the study of parasites and parasitism
Parasocial	Bees and wasps. A colony in which all the nest sharing females are of the same generation.
Parthenogenesis	reproduction by means of eggs that develop without fertilisation
Pathogenic	capable of producing disease
Pauropoda	a Class in the phylum Uniramia
Peanut worm	an animal in the Phylum Sipuncula
Pearly nautilus	Nautiloidea
Pea weevil	a beetle in the Bruchidae family
Pectinate	comb-like, having comb-like teeth
Pectines	a sensory comb-like structure as seen in some scorpions
Pedicel	of insects, the slender part of the abdomen best seen in Hymenoptera
Pedicellaria	Minute pincer-like organs on the surface of certain echinoderms.
Pedipalps	In arachnids the second pair of appendages at the front of the body. They can be used for walking, sperm transfer, or capturing prey, depending on species.
Pelagic	living in the upper waters of the open sea
Pentastomida	Tongue worms
Pentatomidae	Shield bugs
Periodicity	the periodic or rhythmic occurrences of an event
Peristalsis	waves of contraction of circular and longitudinal muscles passing along a tubular organ (e.g. the gut) having a propulsive effect.
Permeability	Having pores or spaces allowing fluiid to pass through. The measure of the freedom of entry of new members into a community or society.
Pertubation	a disturbance: a depatrure of a biological system from a steady state
Pesticide	a chamical agent that kills insects and other animal pests
Petiolate	having a stalk
Petiole	A stalk. In Hymenoptera the narrow abdominal segment(s) that form the waist.
Petrification	a process of fossilization where the tissues are preserved by impregnation with carbonate or silicate minerals
Phagocytosis	the ingestion of solid particulate matter by a cell
Pharynx	The first part of the gut from where the gill slits open. The muscular and sometimes eversible first part of the gut.
Phenotype	the expressed traits of an organism
Pheromone	A chemical substance, usually a glandular secretion, which is used in communication within species. One individual releases the material as a signal and another responds after tasting or smelling it.
Phoresy	one animal exploiting another by using it as a means of transport, e.g. the mites that hitch a ride on bumblebees
Photic zone	The top slice of a body of water where light penetrates sufficiently for photosynthesis to occur
Photoreceptor	a cell sensitive to light
Photosynthesis	the process whereby plants build up complex organic substances from simple ones using the sun's energy.
Phototaxis	A movement away (negative) or towards (positive) light where light is the stimulus.
Phylum	one of the major taxonomic divisions of the animal kingdom
Phytophage	a plant eater
Plankton	the small plants and animals living free in the surface waters of lakes and oceans.
Plant secondary compounds	They are compounds produced by the plants that have no nutritional value to the particular plant, but often affect the health or welfare of another organism, e.g. a herbivore.
Plumose	feathery - as seen in some insect antennae
Pneumostome	the opening to the lung in pulmonate gastropods, most easily seen in slugs
Poikilothermic	Animals whose body temperature varies and fluctuates with that of the surrounding environment; cold-blooded.
Pollen	The male element of flowering plants. Usually a fine dust produced by the anthers. Pollen is eaten by many insects including bumblebees.
Pollination	The placement of pollen onto the stigma of a carpel by wind or animal vectors, a prerequisite to fertilisation.
Polyandry	mating system of one female with more than one male
Polygamy	more than one mate at a time
Polygny	A mating system involving one male and many females
Polymorphic	A population in which 2 or more morphs are present in readily noticeable frequencies
Polyp	a simple animal with upwardly-directed mouth, fringe of tentacles, and a simple gut
Polyphyletic	Pertaining to a taxon whose members were derived from two or more ancestral forms not common to all members.
Population	A group of individuals of one species that live in a particular geographic area.
Population genetics	The scientific study of gene pools and genetic variation in biological populations.
Preadaptation	A structure that evolves and functions in one envronmental context, but can perform additional functions when placed in some new environment.
Posterior	the rear or tail end
Predator	an animal that preys on other organisms
Prepupa	A restin (non-feeding) stage that some insects pass through before forming a pupa.
Prey	Any animal that is eaten by a predator
Primary consumers	Herbivores. Organisms in the trophic level that eat plants and algae.
Primary producers	Autotrophs. Photosynthetic and chemosynthetic organisms.
Primary succession	A type of ecological succession that occurs in an area where there were originally no organisms, e.g. on newly created volcanic islands.
Primitive	ancient; little evolved; of characteristics similar to those possessed by the ancestral types
Proboscis	A trunk-like process, sometimes eversible, and usually arising from the head.
Proglottid	a compartment of a tapeworm's body
Profitability of a prey item	A predator will rank items according to their size and quality, against search/handling time. The predator usually consumes those items that have the highest ratio of size, quality : search, handling time; though it will consume some less profitable items as it samples.
Prokaryote	Lacking a discrete nucleus in the cell, e.g. bacteria.
Prolegs	the soft, unjointed legs found in some invertebrates, e.g. caterpillars
Pronotum	The upper surface of the front segment of an insect's thorax. It may be enlarged to form a shield over the head as in some beetles.
Prosoma	The front part of the body in some animals, e.g. spiders, where the head and thorax are fused.
Prothorax	The front or first of the three segments of an insect's thorax.
Protist	A single-celled organism living in water or moist habitats. The cell is usually relatively large and more complex than bacterial cells. Some use photosynthesis to get their energy; others eat. A typical, much-studied protist is Paramecium.
Proventriculus	In insects, a muscular dilation of the foregut armed internally with chitinous teeth
Proximal	situated towards, or near the point of attachment
Pseudocoelomates	Animals whose body cavity is not completely lined by mesoderm, e.g. Nematoda, Rotifera.
Pterostigma	A coloured area near the leading edge of some insect wings, e.g. dragonflies.
Pubescent	whith short, fine hairs
Pulmonate	Having lungs, not gills, e.g. pulmonate snails
Punctuated equilibrium	A theory of evolution advocating spurts of relatively rapid change followed by long periods of stasis.
Pupa	The inactive instar or stage of the holometabolous insect during which development into adult form is completed.
Puparium	the case which houses the pupa
Pupate	the process of becoming a pupa
Pupation	the formation of a pupa by metamorphosis