| Habit | external appearance, aspect or growth form of an organism |
| Habitat | the place where an organism or population normally lives |
| Haemocoele | the body cavity of arthropods forming part of the blood system |
| Haemoglobin | iron-based respiratory pigment |
| Haemolymph/hemolymph | The fluid filling the haemocoel. Loosely speaking it can be referred to as blood. |
| Halictidae | sweat bees |
| Halobiont | an organism living in a saline (salty) habitat |
| Halobios | the life forms in the seas and oceans |
| Halocline | a saline discontinuity, a salinity gradient |
| Halolimnic | marine organisms adapted to live in fresh water |
| Halophilous | thrives in saline (salty) habitats |
| Halophobic | intolerant of saline habitats |
| Halteres | In true flies the 2 club-like organs that take the place of the hind wings. They act like gyroscopes during flight and help the fly manoeuvre with great precision. |
| Hamuli | The row of tiny hooks on the leading edge of the hind wings of bees and wasps. In flight the hooks zip on to the edge of the fore wings so the pair function as one unit. |
| Haplodiploidy | The genetic system where males develop from unfertilised eggs, and are haploid with a single set of chromosomes, whereas females develop from a fertilised egg and are diploid and have a double set of chromosomes. e.g. bumblebees |
| Haploid cell | a cell containing only one set of chromosomes |
| Hard tick | a tick in the Ixodidae family, e.g. the castor bean tick, sheep tick, which has a hard plate and forward-pointing head. |
| Harem | a group of females associated with a single male |
| Harvestman | Opiliones |
| Hawk moth | moths in the Sphingidae family |
| Hawking | feeding in flight as some dragonflies do |
| Hectocotylus | the specialised arm, or part of the arm, in some cephalopods that serves as the male copulatory organ. |
| Heliothermic | organisms that maintain a relatively high body temperature by basking in the sunlight |
| Heliotropic | orienting towards sunlight |
| Helminthology | the study of parasitic flatworms and roundworms |
| Hemelytra | the distinctly thickened or leathery forewings of some bugs which have overlapping, membranous tips |
| Hemichordata | worm-like marine invertebrates, e.g. acorn worms |
| Hemimetabolous | Undergoing development which is gradual and lacks a sharp separation into larval, pupal and adult stages, e.g., crickets and bugs. |
| Hemiptera | bugs, an order of insects characterised by a jointed beak, or rostrum or stylets |
| Herbivore | an animal that feeds on plants or plant-like plankton. |
| Heredity | the transmission of biological traits from parents to their offspring |
| Heritability | The capacity of being inherited. That part of variability that is genetically based. |
| Hermaphrodite | having both sexes in one individual |
| Hermaphroditic | capable of producing ova and spermatozoa, either at the same time or sequentially |
| Hesperidae | The family of butterflies that includes the skippers. The larvae are usually grass feeders, and the adults have large bodies and short wings. |
| Heterogeneous | having a non-uniform structure or composition - opposite homogeneous |
| Heteroptera | a sub-order of the Hemiptera which contains the bed-bugs, shield bugs and pond skaters etc. |
| Heterothermic | cold-blooded |
| Herterotrophic | obtaining food from other organisms, not through photosynthesis |
| Hexactinellida | the class of sponges that includes the glass sponges |
| Hexapoda | Insects |
| Hibernaculum | The place where an animal hibernates or overwinters |
| Hibernation | A period of dormancy in winter (see bumblebee hibernation). During hibernation an animal's body processes drop to a low level. |
| Hirudinea | leeches |
| Holometabolous | Undergoing a complete metamorphosis during development, with the distinct larval, pupal and adult stages, e.g. butterflies and beetles. |
| Holothuroidea | sea cucumbers |
| Homeostasis | The maintenance of a steady state, especially a physiological or social steady state by means of self-regulation through internal feedback responses. |
| Homeothermic | having a body temperature regulated independent of the environmental temperature, "warm-blooded", ability to generate heat. |
| Home range | the area that an animal, or group of animals, uses in its daily activities when foraging for food etc. |
| Homing | the act of returning to an original location, e.g. bumblebees returning to their nest. |
| Homogeneous | Of uniform structure or composition; similar throughout. |
| Homoptera | A sub-order of the insect order Hemiptera; includes the cicadas, aphids, leaf hoppers etc. |
| Honeydew | a sweet-tasting substance exuded by aphids and other sap-feeding bugs. |
| Horntail | also know as a wood wasp, but actually a saw fly! |
| Horsefly | Tabanidae |
| Horsehair worm | Nematomorpha |
| Horseshoe crab | Actually not a crab at all, but in the Chelicerata Phylum |
| Horseshoe worm | Phoronida |
| Host | an animal or plant on or inside which a parasite feeds/lives |
| Host specificity | the extent to which an adult parasite is restricted in the variety of host species it can utilize |
| House spider | Aglenidae |
| Housefly | Muscidae |
| Hoverfly | Syrphidae |
| Hunting spider | Lycosidae |
| Hydrobiology | the study of life in aquatic habitats |
| Hydroid | the polyp of a cnidarian |
| Hydrometridae | water measurer |
| Hydraulic | operated by water pressure |
| Hydrophilous | Thriving in wet, or aquatic habitats. Pollinated by water-borne pollen. |
| Hydrophobic | Intolerant of wet habitats or water. Water repellent. |
| Hydrostatic skeleton | a skeleton composed of fluid held under pressure in a closed body compartment, e.g. in Cnidaria, Platyhelminthes, Nematodes, Annelids. |
| Hydrozoa | A class of Cnidarians. |
| Hygrobiidae | screech beetles. |
| Hymenoptera | bees, wasps, ants and saw flies |
| Hyperparasitism | Parasitism of parasitic insects by other insects |