| Labium | the lower lip of an insect |
| Labrum | the upper lip of an insect, also the outer lip of a gastropod shell |
| Lacewing | an insect in the Neuroptera Order |
| Ladybird/ladybug | a beetle in the Coccinellidae family |
| Lamarkism | Inheritance of acquired characters. The theory that changes in use or disuse of an organ result in changes in size and functional capacity of the organ, and that these modified characters are transmitted to the offspring. |
| Lampyridae | glow worms or fire flies. |
| Lampshell | Brachiopoda |
| Lancelets | a group of animals in the phylum Chordata. Often used to study the primitive features of animals with notochords. |
| Land bridge | a land connection between 2 or more adjacent landmasses which can be a route for migration or dispersal of terrestrial organisms. |
| Larva | An immature stage which is radically different in form to the adult, e.g. caterpillar. |
| LD50 | the dose required to kill 50% of the organisms in a test population per unit of time |
| Leaf beetle | Chrysomelidae |
| Leaf miner | a feeder on the mesophyll layer between the upper and lower epidermis of a leaf |
| Leatherjacket | Tipulidae |
| Leech | Hirudinea |
| Lek | an area where animals assemble for communal courtship display and mating, e.g. male midges dancing in the air in a tight group while the females sit below and watch |
| Lepidoptera | butterflies and moths |
| Life cycle | The sequence of events from the birth of an individual to its death. The stages through which an organism passes between the production of gametes by one generation to the production of gametes by the nest, e.g. the bumblebee life cycle. |
| Life history | The significant features of the life cycle through which an organism passes, with particular reference to strategies influencing survival and reproduction. |
| Life span | Longevity. The maximum or mean duration of the life of an individual or group. |
| Life tables | tables of data summarising mortality in populations |
| Limiting factor | Any environmental factor, or group of related factors, which exist at sub-optimal level and so prevent an organism from reaching its full biotic potential. |
| Limnology | The study of lakes, ponds and other standing waters and their associated organisms. |
| Limpet | A mollusc |
| Lineage | a line of common descent |
| Linguatulida | a.k.a Pentastomida or tongue worms |
| Linnean classification | the system of hierarchical classification and binominal nomenclature established by Linneus |
| Lithosphere | the rocky component of the earth's surface |
| Litter | Recently fallen plant material which may be partly decomposed |
| Littoral | living between the low tide and high tide marks |
| Local stability | the tendency of a pop to return to its equilibrium after a small perturbation |
| Longevity | The average lifespan of individuals of a population under a given set of conditions. |
| long-horned beetle | Cerambycidae |
| longitudinal | running from front to back |
| looper | aka inch worm, the caterpillar of Geometrid moths, so-called because of the looping locomotion |
| Lophophore | the circlet of eversible ciliated tentacles surrounding the mouth of certain sedentary aquatic animals |
| Lorica | the protective, external case found in some rotifers, etc. |
| Loricifera | a phylum of microscopic invertebrates |
| Lucanidae | Stag beetles |
| Luciferous | Light producing, luminescent, e.g. the European piddock, glow-worms |
| Lug worm | A polychaete worm |
| Luminescence | chemical production of light, e,g, as in the European piddock, glow-worms |
| Lungs | the evaginated respiratory surfaces of terrestrial animals that connect to the atmosphere by narrow tubes, e.g. book lungs in spiders |
| Lycaenidae | The butterfly family which contains the Blues, Coppers, and Hairstreaks |
| Lycosidae | Wolf spiders and hunting spiders |