| Quasisocial | Of a colony of females of the same generation, in which members co-operate in constructing and provisioning of cells. All the females are mated, have functioning ovaries and lay eggs. |
| Queen | In social insects the fully developed female in the colony which reproduces. |
| Quiescence | a slowing down of the metabolism and development in response to adverse conditions, e. g. as seen in the Tardigrada |
| Radial symmetry | mirror-image correspondence of the two halves of a body halved in any vertical plane, as seen in the Echinodermata |
| Radioactive dating | A method of determining the age of fossils and rocks by using the half lives of radioactive isotopes. |
| Radula | the rasp-like feeding tongue of some molluscs |
| Range | The geographic area in which a population lives. |
| Raptorial legs | the thick, muscular forelegs of predators used to grasp and hold prey, e.g. as in the praying mantis |
| Realised niche | The environmental resources that an organism actually uses. The niche that an organism actually occupies. |
| Rectum | the end part of the hindgut |
| Reflexed | bent upwards and away |
| Refuge | a safe place |
| Regeneration | The replacement of parts of an organism that are lost through injury, e.g. Starfish (Echinodermata) replacing an arm. |
| Replete | an individual ant with her abdomen distended by liquid food |
| Repugnatorial glands | Glands which secrete a noxious substance, used either for defence or offence, e.g. in millipedes |
| Resilience | the ability of a population to bounce back after a perturbation |
| Resilin | an elastic-like protein in some insect cuticle, e. g. fleas |
| Resistance | the ability of a population to avoid displacement from its equilibrium by a disturbance |
| Respiration | gaseous exchange between the animal and its environment |
| Respiratory tree | the internal organ for gaseous exchange in some sea cucumbers |
| Rostrum | In certain hemiptera and some other insects the set of piercing mouthparts looking like a beak. |
| Rudimentary | very imperfectly developed |
| Saltatorial | adapted for jumping |
| Sap | A fluid containing sugars etc. made in plants by photosynthesis, which is transported from the leaves to all parts of the plant through phloem tubes. |
| Saprophagous | Feeding on dead or decaying matter |
| Saprophyte | an organism which gets its nourishment from decaying organic matter |
| Satyridae | a family of butterflies containing the "browns". |
| Savanna | A tropical grassland biome with scattered individual trees, large herbivores, and three distinct seasons based primarily on rainfall, maintained by occasional fires and drought |
| Scape | The long segment in an insect's antenna, see bumblebees |
| Scavenger | An animal that feeds on waste matter. |
| Sclerite | A hard, chitinous or calcareous plate or spicule; one of the plates which make up the exoskeleton of arthropods, e.g. insects |
| Sclerotization | The hardening of the cuticle of arthropods by the formation of stabilizing cross-linkages between peptide chains of adjacent protein molecules. |
| Scolex | The attachment end (head) of a tapeworm. It is equipped with suckers and hooks to enable it to fasten to the host. If the scolex is not removed the tapeworm will continue to grow. |
| Scutellum | a triangular shield behind the pronotum in many insects, see coleoptera |
| Search image | A predator develops a knowledge of a prey type e.g. what it looks, smells like etc. and is "switched on" for that type. a) Chicks eating coloured rice grains. Green and orange grains were mixed and placed on either a green or orange background, this eliminated any preference the chicks might have for a certain colour. The chicks always took the grains on the contrasting background first. Their search image was the grains standing out against a contrasting background. b) Wood pigeons in a fields were offered around 6 types of bean or pea scattered in a field. Once pigeons were used to this an identical, but drugged version was scattered, the pigeons gathered up and their crops analysed. It was found that 72% had only one type of bean or pea , a search image of that type had been formed. In different fields different seeds were favourite, so there was probably a certain amount of imitation. |
| Secondary consumers | The trophic level consisting of carnivores that eat herbivores. |
| Secondary host | a host in which the parasite reproduces not at all, or only asexually |
| Sedentary | stationary, inactive |
| Segmentation | the subdivision of the body into a number of similar units, e.g.as in earthworms |
| Self thinning rule for plants | As plants grow the total biomass per plant increases but the population density decreases. The thinning slope is always the same, -3/2, whatever the species, from unicellular algae to giant redwoods. |
| Semelparity | dying after breeding- opposite of iteroparity = repeated breeding |
| Semisocial | A colony of females of the same generation, with some kind of division of labour, i.e. with some females tending to lay most of the eggs and functioning as queens, and with some females tending to lay few or no eggs and behaving as workers. |
| Senescence | Aging. The progression of irreversible change in an organism, eventually leading to death. |
| Sensory receptors | Specialised structures that respond to specific stimuli from an animal's external or internal environment. They transmit the information of an environmental stimulus to the animal's nervous system by converting stimulus energy to the electro-chemical energy of action potentials. |
| Septum | a partition separating two cavities |
| Serrate | toothed |
| Sessile | settled, stationary, on a solid object |
| Seta/setae (chaeta/chaetae) | a needle-like chitinous structure of annelid, arthropods etc. |
| Setose | covered with setae or short, stiff hairs |
| Sexual dimorphism | Males and females are anatomically different and may behave differently. |
| Sexual reproduction | A form of reproduction that involves fertilisation of a female cell or egg be a male cell or sperm. |
| Sexual selection | Selection based on variation in secondary characteristics, leading to the enhancement of sexual dimorphism. |
| Silica | Silicon dioxide, a very common chemical in nature, sand, used as the skeletal material by sponges. |
| Silk | A protein-based, fibrous material produced by spiders and some insects. It is liquid when it emerges from the animal's body, but it turns into elastic fibres when stretched and exposed to air. It has a wide variety of uses. In a web or a net to catch prey, as a bag or container to protect young, eggs and pupa, or as an aid to glide through the air. |
| Silk gland | Glands inside the abdomen of a spider where silk is made. The silk exits the body via the spinnerets. |
| Simple eye | an eye containing a single lens as found in spiders and in groups of three in many insects |
| Sinistral | coiled to the left, opposite is dextral |
| Sinuate | curved, wavy |
| Sipuncule | a cord of tissue running through the shell of a nautiloid connecting all the chambers with the body of the animal |
| Social insects | those species that live co-operatively in colonies and exhibit a division of labour among distinct castes, e.g. in the Hymenoptera and Isoptera |
| Sociality | the condition of living in an organised community |
| Social organization | A complex of behavioural characteristics defining the dispersion pattern of a population and the form of the inter-individual encounters within it. |
| Social parasite | a parasite that takes advantage of the social behaviour of a species, e. g. cuckoo bumblebees |
| Spatulate | having the shape of a flat spoon |
| Speciation | The origin of new species in evolution. |
| Species | the lowest taxonomic category into which living things are divided, members of a species can interbreed |
| Species diversity | The number and relative abundance of species in a biological community. |
| Species richness | the number of species in a biological community. |
| Spermatheca | Where the female stores sperm cells after mating. The sperm are released to fertilise eggs just before they are laid. |
| Spermatophore | a capsule containing spermatozoa |
| Sperm competition | In females who mate with more than one male, the syndrome whereby sperm from different males compete to fertilize eggs. |
| Spicule | small pointed calcareous or siliceous body that makes up the skeleton of certain animals, e.g. sponges |
| Spiderling | an immature spider |
| Spinneret | a finger-like projection from the posterior end of the spider abdomen through which silk is discharged |
| Spiracle | In insects, an opening on the thorax and abdomen that lets air into the tracheal system. |
| Sponge | An animal in the phylum Porifera. |
| Spongin | Fibrous, collagenous material making up the skeletal network of some sponges |
| Startle display | a display made by some cryptic insects upon discovery, involving exposure of a startling colour or pattern, e.g. eyespots on the peacock butterfly |
| Statocyst | A type of mechanoreceptor that functions in equilibrium in invertebrates through the use of statoliths, which stimulate hair cells in relation to gravity. See Bivalvia for an illustration. |
| Statolith | the small calcareous stone resting on the hairs in a statocyst |
| Sternum/sternite | the ventral plate of an arthropod body segment |
| Stigma | a small pigmented area on the front marging if the wing, e. g. some dragonflies and some sawflies. |
| Stria | a line of punctures, as seen on the elytra of some beetles |
| Stridulation | The production of sound by rubbing one part of the body surface against another, e.g. as found in the crickets and grasshoppers. |
| Stylet | a hard, pointed, dart-like structure used for penetration of cells or tissues, e.g. as found in many bugs. |
| Succession | Transition in the species composition of a biological community, often following an ecological disturbance of the community. The establishment of a biological community in an area virtually barren of life. |
| Suctorial | adapted for sucking |
| Surface tension | A measure of how difficult it is to stretch or break the surface of a liquid. Water has a high surface tension because of the hydrogen bonding of the surface molecules. |
| Suspension feeder | An animal that feeds on particles of organic matter that are suspended in water, e.g. brittle stars, cnidarians, annelids. |
| Symbiont | An organism that lives in symbiosis with another species. |
| Symbiosis | The intimate and dependent relationship of members of one species with those of another. The three principal kinds of symbiosis are commensalism, mutualism, and parasitism. |
| Sympatric | living in the same area |
| Synanthropic | sites associated with human activity, e.g. buildings, gardens, compost heaps |
| Syncitial. | Multinucleated cytoplasm without apparent division into cells. |