The tarantula, a hairy arachnid, is a spider family known for its substantial size. These impressive arthropods, which belong to the family Theraphosidae, represent the largest spiders in the world. Determining how big tarantulas can truly get requires an understanding of how their dimensions are measured and which species hold the records for both sheer mass and overall spread.
How Tarantula Size is Measured
Tarantula size is generally communicated in two primary ways: body length and leg span. Body length is measured from the front edge of the cephalothorax—the fused head and chest section—to the tip of the abdomen. This metric offers a consistent indicator of the spider’s bulk, often reaching up to 5 inches (13 cm) in the largest species.
The most common measurement used is the diagonal leg span (DLS). This involves measuring the distance from the tip of the front leg on one side to the tip of the hind leg on the opposite side, with the legs fully outstretched. While body length reflects mass, the leg span gives a sense of the animal’s true physical diameter, sometimes reaching up to 12 inches (30 cm).
The Largest Known Species
The title of the world’s largest tarantula is often split between two formidable South American species. The Goliath Birdeater (Theraphosa blondi) is widely recognized as the largest spider by mass and body length. Found in the rainforests of northern South America, including Venezuela and Brazil, this species can weigh up to 6.2 ounces (175 grams).
The body of a Goliath Birdeater can reach a length of 5 inches (13 cm), and its fangs alone can be over an inch long. While the Goliath Birdeater is the heaviest, its leg span can reach up to 12 inches (30 cm).
Another contender for the largest spread is the Brazilian Salmon Pink Birdeater (Lasiodora parahybana). Native to eastern Brazil, this species can attain a leg span of 10 to 11 inches (25 to 28 cm). Although slightly less heavy, its large size and striking appearance make it an impressive specimen. These massive spiders are ground-dwelling and are known for their dark coloration overlaid with long, reddish-pink hairs.
Biological Factors Determining Size
An individual tarantula’s ultimate size is determined by a combination of genetics and external conditions. The most significant biological factor is the spider’s sex, as female tarantulas are generally much larger and more robust than males. Females also have significantly longer lifespans, sometimes living for twenty years or more, which allows them more time to grow.
Males typically reach maturity faster, often growing quicker during the juvenile stages, but their lifespan is usually only a few years after their final molt. Since a tarantula can only increase in size by shedding its rigid exoskeleton, the process of molting is directly tied to growth. Each successful molt, or ecdysis, represents a jump in size, and the frequency of these molts is influenced by environmental factors.
The availability of food and the ambient temperature also play a role in determining how large a tarantula gets. A consistent and plentiful food supply allows for more frequent molting and greater size increases. Conversely, a poor diet or suboptimal temperatures can slow the growth rate, leading to a smaller adult size.