The size range within the order Araneae is vast, spanning from creatures barely visible to the naked eye to heavy-bodied giants. Spiders are arachnids, possessing eight legs and a body divided into two main segments: the cephalothorax and the abdomen. Establishing the true extremes of spider size requires understanding the specific metrics scientists use to define bigness.
How Spider Size is Measured
Measuring a spider’s size is complicated because scientists and enthusiasts often use two different metrics. The first measurement is Body Length, which is the primary scientific measure of mass and volume. This length is quantified from the front edge of the cephalothorax to the posterior tip of the abdomen, excluding the chelicerae (fangs) and the silk-producing spinnerets.
The second measurement is the Diagonal Leg Span. This metric is determined by stretching the spider out and measuring the distance from the tip of the first leg on one side to the tip of the fourth leg on the opposite side. Leg span can be deceptive; a spider with a small body may still possess an impressive span due to disproportionately long legs. This distinction explains why the record for the world’s largest spider can be claimed by two different species, depending on the chosen measurement standard.
The Absolute Largest and Smallest Spiders
The title for the largest spider by mass and body length belongs to the Goliath Birdeater (Theraphosa blondi), a tarantula native to the rainforests of South America. This species can weigh up to 175 grams (6.2 ounces), which is heavier than a small puppy, and its body alone can reach lengths of 13 centimeters (5.1 inches). The Goliath Birdeater’s leg span is typically around 28 to 30 centimeters (11 to 12 inches) in the largest recorded specimens.
The record for the greatest leg span is held by the Giant Huntsman Spider (Heteropoda maxima), discovered in Laos in 2001. Its long, spindly legs can stretch up to 30 centimeters (12 inches) across, roughly the size of a dinner plate. Although its diameter is enormous, its body length is comparatively slender, measuring only about 4.6 centimeters (1.8 inches), making it lighter than the Birdeater.
At the opposite extreme are the miniature members of the family Symphytognathidae, which include the smallest known spiders. One contender for the record is Patu digua, found in Colombia. The males of this species reach a body length of a mere 0.37 millimeters (0.015 inches), a size smaller than a typical grain of sand.
Biological Limits on Spider Growth
The size of the largest spiders, such as the Goliath Birdeater, represents a near-maximum size possible under current physical and atmospheric conditions. A primary constraint is the spider’s external skeleton, or exoskeleton, which must be shed during growth in a process called molting. As a spider gets larger, the weight and structural demands on the exoskeleton increase.
During molting, the spider loses its structural support and is extremely vulnerable, often having to lie on its back while its new, soft cuticle hardens. For very large species, gravity can cause a rupture of the abdomen if they fall from even a small height, limiting their mass. This reliance on an external, rigid skeleton places a cap on maximum achievable weight.
Another limitation is the spider’s respiratory system, which is inefficient for very large bodies. Spiders use specialized organs called book lungs, which consist of alternating air pockets and tissue layers that resemble the pages of a book. This system relies on passive diffusion to move oxygen from the air into the body fluid.
As a spider’s body volume increases, the surface area of the book lungs does not keep pace, creating a challenge for oxygen delivery to internal tissues. This reliance on passive diffusion, especially with current atmospheric oxygen levels around 21%, means the respiratory system cannot sustain the metabolic needs of an organism much larger than the current record holders. If atmospheric oxygen levels were higher, the maximum size limit for spiders would theoretically be greater.