The title of the world’s biggest spider is not a simple designation, as the answer depends entirely on the metric used for comparison. Spiders that are the longest are typically not the heaviest, and vice versa. Exploring the maximum size spiders can achieve requires defining the different ways “largest” is measured and understanding the biological limits that cap their growth.
How Spider Size is Measured
Scientists rely on two distinct metrics to quantify spider size, which often leads to confusion when comparing species. The first focuses on the spider’s overall reach, known as the Diagonal Leg Span (DLS). This measurement is taken from the tip of the front leg on one side to the tip of the hind leg on the opposite side, measuring the animal’s maximum diameter.
The second primary metric is a combination of body length and mass, which determines the animal’s physical bulk. Body length is measured from the front of the prosoma (cephalothorax) to the end of the opisthosoma (abdomen), excluding the legs. This measurement, paired with the spider’s total mass, provides a sense of its girth and weight. The difference between these two approaches explains why the record for the largest spider is split between two very different arachnids.
The Heaviest Spider: The Goliath Birdeater
The Goliath Birdeater (Theraphosa blondi), a member of the tarantula family, holds the record for sheer mass and body length. Found in the upland rainforest regions of northern South America, this terrestrial species inhabits deep burrows. Its robust body structure makes it an extremely heavy arachnid, with some specimens weighing up to 175 grams (6.2 ounces).
The Birdeater’s body alone can reach up to 13 centimeters (5.1 inches) in length, which is comparable to a small smartphone. Its coloration is typically tan to light brown with golden hues, giving it a velvety appearance due to the covering of fine hairs. While its leg span can also be substantial, reaching close to 30 centimeters (12 inches), its defining characteristic is its bulk, making it the most massive spider known.
Despite its common name, the Birdeater rarely preys on birds. Its diet consists mainly of ground-dwelling animals like insects, frogs, toads, lizards, and sometimes small rodents or snakes encountered near its burrow. The sheer size of this spider means it can overpower and consume small vertebrates, though its venom is not considered a threat to humans.
The Longest Leg Span: The Giant Huntsman
In stark contrast to the Birdeater’s bulk, the Giant Huntsman (Heteropoda maxima) holds the record for the world’s largest leg span. This species can stretch its legs up to 30 centimeters (12 inches) across, roughly the diameter of a dinner plate. Its body, however, is significantly smaller and more slender, measuring only about 4.6 centimeters (1.8 inches) in length.
The Giant Huntsman belongs to a family of spiders known for their speed and agility, which they use to actively pursue prey rather than relying on a web. Its body plan is designed for rapid movement, featuring long, spindly legs that twist forward in a crab-like fashion. This structure allows it to cover ground quickly, a necessity for a hunter.
This spider is believed to be a cave-dweller, a habitat suggested by its pale, yellowish-brown coloration and extremely elongated appendages. Although it possesses the greatest reach of any known spider, its mass is considerably less than the Goliath Birdeater. The difference highlights the distinct evolutionary paths taken by the world’s largest spiders, one prioritizing sheer size and weight, the other prioritizing length and speed.
Why Spiders Don’t Get Bigger
The size of the largest spiders is constrained by fundamental biological and physical laws that apply to all arthropods. One main limitation is their respiratory system, which is based on organs called book lungs. These structures facilitate gas exchange, but they rely on passive oxygen diffusion from the atmosphere into the hemolymph, or “blood.”
This diffusion-based system becomes inefficient as body size increases, making it difficult to supply oxygen to the inner tissues of a large body. A second limitation is the spider’s exoskeleton, which is shed in a process called molting to allow for growth. During this time, the spider is soft, immobile, and vulnerable to predators and injury.
Furthermore, the weight of a large body with an exoskeleton is subject to the square-cube law. As the spider’s dimensions increase, its volume and mass grow much faster than the cross-sectional area of its supporting legs. Beyond a certain size, the exoskeleton would not be strong enough to support the animal’s weight, particularly when climbing or falling. These factors impose a ceiling on how large terrestrial arachnids can grow.