| Ichneumonidae | Solitary, parasitic wasp. |
| Imago | The adult insect. |
| impunctate | lacking punctures |
| inbreeding | the mating or crossing of individuals more closely related than average mating pairs in the population |
| inchworm | the caterpillar of a moth in the Geometridae family, so-called for its manner of locomotion |
| incipient | about to become or happen; used of the initial stages of an event or structure |
| incompatibility | the inability to unite, fuse or form a viable association |
| incompatible | the failure of cross- or self-fertilisation because of genetic, structural or physiological barriers |
| incubation | the maintenance of eggs under conditions favourable for hatching, e.g. as in a bumblebee nest |
| indeterminate growth | growth that continues throughout the life of the individual so that body size and age are correlated |
| indicator | a organism, species or community characteristic of a particular habitat or set of environmental conditions, e.g. the presence of certain insect aquatic larva (e.g. stoneflies and caddis flies) indication or high water quality. |
| indicator species | a species whose presence or absence is an indicative of a particular habitat, community or set of environmental conditions, e.g. stoneflies of high water quality |
| Indigenous | Organisms that are native to a particular region - not introduced. |
| industrial melanism | an increase in dark morphs as a response to changes in selection in habitats blackened by industrial pollution |
| inert | inactive, quiescent |
| infertile | not fertile, non-reproductive |
| infestation | invasion by parasites or pests |
| ingestion | the action of taking in food, feeding |
| inheritance | the transmission of genetic information from ancestors or parents to descendants or offspring |
| inhibition | any process that acts to restrain actions or behaviour |
| innate | inherited, present at birth, not learned behaviour |
| Inoculation | the insertion of a pathogen into a host organism by a vector, or of an organism into an experimental culture |
| Inorganic | pertaining to, or derived from non-biological material; used of non-carbon compounds |
| Inquilinism | symbiosis in which one organism (the inquiline) lives with another (either inside its body, or outside, or in its nest) without causing any harm to the host, e.g. certain mites which live in bumblebee nests |
| insecta | the insects |
| insecticide | a chemical used to kill insects, but commonly used of any chemical used to kill invertebrates |
| Insectivora | an order of small mammals including hedgehogs, moles, shrews which include insects as a large part of their diet |
| Insectivorous | feeding on insects |
| Insemination | the introduction of sperm into the genital tract of the female |
| Instar | the stages between moults of arthropods |
| Instinctive behaviour | complex, stereotyped behaviour in response to an environmental stimulus |
| Integument | an external covering or layer |
| intensity of infection | the number of individuals of a parasite species in each infected host |
| interbreeding | mating or hybridisation between individuals, species, populations etc. |
| intermediate host | the host in which some development of a parasite occurs, but not sexual reproduction or maturity |
| Internal fertilisation | In reproduction a form of fertilisation that takes place inside the female's body. Internal fertilisation is a characteristic of many land animals. |
| Interstitial | living in the spaces between sand grains, small crevices, cells. |
| Intertidal zone | the shallow zone where land meets water, also known as the littoral zone. |
| intraspecific | within a species; between individuals or populations of the same species |
| Introduced species | A species that humans have accidentally or deliberately brought into an ecosystem in which it does not naturally occur. |
| Intromittent organ | a male copulatory structure that is inserted into the body of the female |
| Introvert | a retractable body region, as seen in the peanut worms |
| Invagination | an infolding of a layer of tissue to form a sac-like structure |
| invasion | the mass movement of organisms from one area into another |
| Invertebrate | lacking a backbone |
| Isolation | the separation of 2 populations so that they are prevented from interbreeding |
| Isopods | a group of crustaceans that includes woodlice |
| Isoptera | termites |
| Ixodida | a group of ticks which includes the castor bean tick or sheep tick |
| Juvenile hormone | In arthropods a hormone secreted by the corpora allata which promotes the retention of the larval state. |
| Keystone predator | the dominant predator (often the top predator), in a food web; this predator will have a major influence on the community structure |
| Keystone species | A species whose disappearance from a community transforms the populations of the other species in the community. |
| Kin selection | Selection acting on one or more individuals and favouring or disfavouring the survival and reproduction of relatives (not offspring) that possess the same genes by common descent. As happens in selection for altruistic behaviour between genetically related individuals, e.g. the social insects. |
| Kingdom | The highest category in taxonomic classification |
| Kinorhyncha | A phylum of tiny marine invertebrates |