The quest to identify the biggest spider in history is complex because the term “biggest” lacks a single scientific definition. Determining the largest arachnid requires distinguishing between the greatest body mass and the longest leg span, two metrics that often point to different species. This challenge is further complicated when comparing modern giants with the fragmented evidence left by extinct species in the fossil record.
The Current Reigning Champion
The title for the world’s largest living spider is split between two species, depending on the measurement criteria. The Goliath Birdeater, Theraphosa blondi, holds the undisputed record for mass and overall body size. Found in the rainforests of northern South America, this tarantula can achieve a body length of up to 13 centimeters (5.1 inches). Weighing as much as 175 grams (6.2 ounces), it is the heaviest spider known to science, possessing a dense, bulky presence.
Although the Birdeater has an impressive leg span (28 to 30 centimeters), it is narrowly surpassed by a lankier competitor. The Giant Huntsman spider, Heteropoda maxima, claims the record for the greatest leg span, reaching up to 30 centimeters (12 inches) across. This species, discovered in a cave in Laos, possesses a much smaller body length of only about 4.6 centimeters (1.8 inches). The Huntsman’s immense reach and flatter body structure reflect a different evolutionary strategy than the Birdeater’s bulk.
Defining Size: Metrics of Measurement
Scientists rely on two distinct metrics to quantify size: body mass and leg span. Body mass, or weight, is directly related to the spider’s volume and bulk. This is the preferred measure for heavily built species like the Birdeater, reflecting the amount of living tissue and overall girth.
Leg span measures the distance from the tip of the longest front leg to the tip of the longest hind leg on the opposite side. This measurement favors spiders with long, slender limbs, such as the Giant Huntsman, which uses its extensive reach for rapid movement and hunting. Using both metrics allows for a comprehensive understanding of size variation within the Order Araneae.
The Fossil Record: Seeking the Ancient Giants
To determine the biggest spider “in history,” scientists must examine the fossil record, which presents challenges due to the fragile nature of arachnid exoskeletons. The largest confirmed true fossil spider discovered is Nephila jurassica, a Jurassic-era golden orb-weaver from China that lived approximately 165 million years ago. This ancient female had an estimated leg span over 15 centimeters (6 inches), similar in size to its largest modern relatives.
The search for ancient giants is often complicated by the misidentification of other massive arthropods. Megarachne servinei was initially hailed as the largest fossil spider when discovered in Argentina. Later analysis revealed that this creature, which had a body length of 54 centimeters (21 inches), was not a true spider (Order Araneae). Instead, it was a eurypterid, an extinct group of aquatic arthropods known as “sea scorpions.”
The size of all terrestrial arthropods, including spiders, is ultimately constrained by their respiratory system. Unlike vertebrates, spiders rely on a passive system of book lungs and tracheae for oxygen diffusion. This limits how far oxygen can travel into the body, placing a cap on maximum body size. Scientists theorize that the gigantism seen in other Paleozoic arthropods, such as the two-meter-long millipede relative Arthropleura, was made possible by hyperoxic conditions, where atmospheric oxygen levels were significantly higher than today.