Animals

Apex predator

Aplacophora

Aplacophora: worm‑like, shell‑less molluscs of the deep sea

Apodiformes

Apodiformes: swifts, hummingbirds and their specialized flight adaptations

Apostatic selection

Apterygota

Apterygota: primitive wingless insects (bristletails and silverfish)

Aquarium

Aquilops

Aquilops — an early North American ceratopsian

Arabian horse

Arabian horse: history, characteristics, and cultural importance

Arachnid

Arachnocampa

Arachnocampa — glowworm fungus gnats

Araneomorphae

Arapaima

Arapaima (pirarucu, paiche): Large Amazonian bonytongue fish

Archelon

Archelon — the giant sea turtle of the Late Cretaceous

Archosaur

Archosaurs: evolution, key features, and Mesozoic dominance

Archostemata

Arctic char

Arctic char (Salvelinus alpinus): biology, distribution, and conservation

Arctic fox

Arctic fox (Vulpes lagopus): appearance, adaptations and ecology

Arctic hare

Arctic hare (Lepus arcticus)

Arctic tern

Arctic tern (Sterna paradisaea): migration, appearance, and range

Arctiidae

Arctiidae — tiger moths, lichen moths and woolly bears

Argali

Argali (Ovis ammon): the wild mountain sheep of Central Asia

Argentavis magnificens

Argentavis magnificens — the giant teratorn of the Miocene

Argentinosaurus

Argentinosaurus — giant titanosaur of the Late Cretaceous

Argonauta

Armadillo

Armadillo: overview, anatomy, behavior, and conservation

Army ant

Aromobatidae

Aromobatidae: neotropical frogs related to poison-dart frogs

Arthrodire

Arthroleptidae

Arthroleptidae (squeaker frogs of sub-Saharan Africa)

Arthropod

Artiodactyl

Artiodactyls — even-toed ungulates

Ascaphidae

Ascaris

Ascaris: biology, life cycle, and public health impact of giant intestinal roundworms

Asian black bear

Asian black bear (Ursus thibetanus)

Asian elephant

Asian elephant (Elephas maximus)

Large mammal native to South and Southeast Asia; social, forest-dwelling herbivore used historically by people and now threatened by habitat loss, conflict, and poaching.

Asian garden dormouse

Asian tiger mosquito

Asiatic linsang

Asiatic lion

Asiatic lion (Panthera leo persica)

Asiatic salamander

Asiatic wildcat

Aspidogastrea

Aspidogastrea: primitive flukes with a distinctive adhesive disc

Assassin bug

Assassin bug (Reduviidae): predatory true bugs and their significance

Astor markhor

Astor Markhor (Flare‑horned Markhor): overview, range and conservation

Astrodon

Atelopus

Atelopus (harlequin frogs): appearance, habitat, decline, and conservation

Atelopus ignescens

Jambato Toad (Atelopus ignescens): Rediscovery and Conservation in the Ecuadorian Andes

Atelopus longirostris

Atelopus longirostris — the long-snouted harlequin frog of Ecuador