| Facet | the external surface of an individual unit of a compound eye |
| Facultative parthenogenesis | The process by which some eggs develop parthenogenetically if not fertilised. |
| Faeces | Excrement; waste material voided through the anus. |
| Fairy shrimp | A crustacean in the Class Branchiopoda, Order Anocostraca. |
| False scorpion | Pseudoscorpion |
| Family | the major taxonomic division of an order. |
| Fan worm | Sabellida (Annelida) |
| Fat Body | a structure found in certain insects (bumblebees, cockroaches, etc.) that contains stored nutrients |
| Fauna | all the animals naturally inhabiting a certain country or region, or which have lived during a given geological period |
| Feather star | Crinoidea (Echinodermata) |
| Fecund | producing offspring, fruitful |
| Fecundity | The potential reproductive capacity of an organism or population; fertility |
| Feedback | the elements of a control system or homeostatic mechanism linked by reciprocal influences, so that the source or input is continuously modified by the product of the process in order to maintain the necessary degree of constancy. A feedback loop can be negative and have a stabilizing or inhibitory effect, or positive and have a disruptive or facilitative effect. |
| Female | the egg-producing for of an organism |
| Femur | of an insect leg, the third segment from the body between the trochanter and the tibia |
| Fertile | producing or having the capacity to produce offspring |
| Fertilisation | The union of an egg cell and a sperm. This creates a cell capable of developing into a new animal. |
| Fertility | The actual reproductive performance of an organism or population measures as the actual number of viable offspring produced per unit of time; birth rate. |
| Filamentous | like a fine thread |
| Filiform | thread-like |
| Filter feeder | an animal that gets its food by sieving it from particles suspended in water, e.g. bivalves |
| Firebrat | Thysanura |
| Firefly | Lampyridae |
| Fish louse | Branchiura |
| Fission | reproducing by splitting |
| Fitness | The measure of the contribution of a genotype to the next generation relative to other genotypes. |
| Flagella | a long, whip-like process used in locomotion |
| Flagellum | the end and most moveable part of an inesct's antenna |
| Flea | Siphonaptera |
| Fluid feeders | animals that live by sucking nutrient-rich fluids from another living organism, e.g. aphids from plants, human body louse from humans. |
| Fluke | A platyhelminth in the classes trematoda or monoginea |
| Fly | Diptera |
| Food chain | The food pathway that links 2 or more different species. Each forms the food for the species in the next link on the chain, e.g. leaf, beetle, spider, bird, human. |
| Food chain efficiency | The efficiency of transfer of energy from one level to the next in a food chain. |
| Food web | a collection of inter-linked food chains, usually from a particular habitat,see food web of the Atlantic herring. |
| Foot fringe | in slugs the upper edge of the foot circling the whole body, and marked by a continuous groove |
| Forage | to search for food |
| Fore | anterior, towards the head or front end, e. g. forewings |
| Forficulidae | a large family of earwigs (Dermaptera) |
| Formicidae | A family of ants which has around 14 000 species world wide |
| Fossil | a relic or impression of an organism from the past preserved in rock |
| Fossil fuel | a fuel derived from the fossilised remains of organisms, e.g. coal, oil or gas |
| Fossorial | adapted for digging or burrowing, e.g. the legs of a mole cricket |
| Fragmentation | A method of asexual reproduction where the parent separates into parts each of which go on to form a whole organism |
| Frass | Faecal matter and fine animal debris found in soil and on other surfaces |
| Free living | living independently of any host |
| Frenulum | the coupling apparatus joining the front and back wings of moths during flight |
| Frequency | the percentage of samples in a given area which contain the species. |
| Fritillary | A butterfly in the Nymphalidae family. |
| Frugivorous | fruit eating |
| Fruitfly | Drosophilidae |
| Fungus gnat | Mycetophilidae |
| Furcula | the forked organ attached to the abdomen of a springtail which enables it to jump. |
| Furniture beetle | Anobiidae |