| Gaian hypothesis | that the development of living organisms brings about major modifications in the chemical and physical conditions on the planet, and that subsequently the organisms mediate the climate and biogeochemical cycles. |
| Gall | A swelling or abnormal growth in plants usually produced in response to microbial or fungal infection, or to attack by insects, e.g. gall wasp, gall midge, mites or other invertebrates. |
| Gall midge | Cecidomyiidae |
| Gall wasp | Cynipidae |
| Gastropoda | snails |
| Gastrotricha | A phylum of tiny transparent worms. |
| Gastrovascular cavity | A body cavity that functions both in digestion and circulation, and has a single opening serving as both mouth and anus. |
| Gene | The part of a chromosome that is the hereditary determiner, and is transmitted from one generation to another. |
| Gene flow | The exchange of genetic factors within and between populations by migration or interbreeding. |
| Gene frequency | The proportion of one allele to the total of all alleles at the same locus. |
| Gene locus | the position of a gene on a chromosome |
| Gene mutation | A point Mutation. Any heritable change in a single gene. |
| Gene pool | A collection of all the alleles of all the genes in a population. |
| Generation time | The average length of a lifecycle between birth and reproduction. The mean period of time between the reproduction of parents and the reproduction of the following generation. |
| Genetic | Pertaining to genes |
| Genetic altruism | the instinctive parental care of the young, e. g. in the social insects, some beetles, earwigs, etc. |
| Genetic code | The biochemical basis of heredity. The sequence of nucleotide base pairs on the DNA polynucleotide chain which encodes the genetic information. Within the code 3 successive nucleotide base pairs (codon) code for a single amino acid. |
| Genetic drift | Changes in gene frequencies by chance processes in the evolutionary process of animals. |
| Genetic engineering | The alteration of the genetic code of an individual. |
| Genetics | the science of heredity, the study of heritable information |
| Genetic system | The organisation of the genetic material and the reproductive strategy of a species. |
| Genital capsule | the copulatory organs of the male, e. g. in Bumblebees or scorpion flies. It is sometimes used to identify down to species level |
| Genome | all the genes in a haploid set of chromosomes |
| Genotype | The hereditary or genetic constitution of an individual. The genetic material of a cell. |
| Genus | the major taxonomic division of a family. |
| Geographical variation | Any differences between spatially separated populations of species. |
| Geological timescale | See the geological table. |
| Geophilous | Thriving or growing in the soil |
| Gerridae | Pond skaters |
| Gill | An organ used fro extracting oxygen from water. Unlike lungs gills are located outside the body. In aquatic insects, for example, they are usually found down the sides or at the end of the abdomen. |
| Gizzard | the muscular region of the gut in which food may be ground |
| Glabrous | smooth, bald, without hair or scales |
| Gland | a small sac secreting liquid |
| Glass sponge | Hexactinelliad |
| Global stability | the tendency of a population to return to its equilibrium after a large perturbation. |
| Glow worm | actually a beetle in the Lampyridae family. |
| Glyco- | Refers to the presence of carbohydrates |
| Gnathostomulida | Jaw worms. Graze on fungi and bacteria. |
| Golden eye | Chrysopidae, lace wing |
| Gonads | the male and female sex organs |
| Graminivorous | feeding on grass |
| Granivorous | feeding on seeds |
| Gravid | full of eggs |
| Greenbottle | A fly in the Calliphoridae family |
| Greenfly | Aphidae |
| Greenhouse effect | The warming of the Earth due to atmospheric accumulation of carbon dioxide and other molecules which absorb infrared radiation and slow its escape from the irradiated Earth. |
| Gregarious | living in groups or flocks, e.g. locusts. |
| Gross primary production | the total assimilation of organic matter by an autotrophic individual, population or trophic unit |
| Growth | increase in size, number or complexity |
| Grub | soft-bodied, legless larval stage of some flies, beetles, bees and some other insects |
| Gyne | A reproductive female hymenopteran, a queen |
| Gyrinidae | Family containing the whirligig beetles |