There are certain physiological limits in biology,insects can only grow to a certain size due to the fact that too large a body and nerve impulses would become to slow for proper functioning. Size is also limited by the exoskeleton. Spiders have another issue related to size, and that is their book-lung respiratory system. A book-lung is an organ located on the abdomen of a spider which is responsible for oxygen transference. It only works up to a certain size organism. If the spider becomes too large, the book-lung becomes insufficient in carrying oxygen. Many of the larger spiders today have gotten around this problem by a book-lung trachea combination which increases blood low, and also increase speed and stamina, but only to a certain extent. So the question is, how big can spiders really get? The answer lies in Venezuela and in central Africa.
In the Congo this monster has a name and it is called J’Ba FoFi. These animals, familiar to natives, but stories didn’t reach outside of Africa until the 1890’s, at first from Uganda. Arthur Simes was an English missionary on expedition near the shores of Lake Nyasa when in horror movie fashion several of his porters became entangled in a web that stretched out between two trees. According to Simes, giant spiders preceded to quickly descends upon the porters and bite. The results were not great, swelling of the extremities, fever and eventually death.
Almost fifty years later, In 1938 to be exact, a British couple was driving in what was than the Belgium Congo when a spider that they estimated to be three to five feet in circumference walked across the road. The spider was so large that they initially though that it was a cat. They were told that these animals were rare but not unknown, and dangerous to men.
Pygmies living in the forest describe the spider as brown with a purple abdomen,five feet across. They are described as similar in behavior and appearance to tarantulas, and build web nests similar to that specie. They generally prey upon birds and small monkeys, but not adverse to attacking man if the opportunity presents itself.
The only other places where this specie, or more likely a relative, has been reported is in the Amazon Basin. There several reports out of a smaller version in New Guinea, and some intriguing but vague rumors in Vietnam and Cambodia, but all unconfirmed.
Reports in the Amazon basin are centralized near remote villages in Venezuela. These spiders are described as five feet in circumference, including the legs. The abdomen is about two to three feet in length. When the animal stands up with it’s legs up in the air, a defensive position for certain spiders such as tarantulas, these animals can also reach about five to six feet in height. In fact, on rare occasions children have been attacked, and disappearances of adults especially at night have been attributed to these spiders. While these tales may or may not be true and given a certain amount of size exaggerations, a spiders of three to four feet across would have no problem in taking down chickens or dogs. These two are indeed the victims of choice in both Venezuela and Central Africa, according to locals. As far as physical descriptions go, the Venezuelan specimens are brown and hairy, again resembling tarantulas.
Physical descriptions as well as their behavior certainly suggest an undiscovered relative of the tarantula. Tarantulas are members of the Theraphosidae family with approximately 900 species. The largest is the goliath tarantula, largest spider on the planet at 10 to 12 inches across. A far cry you say to the reported five feet monsters, but there are 50,000 specie of spiders on the planet with an estimate of at least 200,000 more to be discovered. Within that number, may lie a much bigger relative, rare, but native to the Congo and Venezuela. The Amazon rain forest is over 3 million sq. miles and the Congo basin almost 2 million sq. miles; imagine the possibilities.
Jaekelopterus rhenaniae lived 390 million years ago. It was the size of a large crocodile and was an arachnid, more precisely a sea scorpion. Arachnids grew to spectacular sizes in those days due to the fact that the environment contained 65 % more oxygen. As the oxygen decreased in the environment and nitrogen increased, arachnids shrunk due to their book lung issues I’ve priorly illustrated.
Furthermore, the fossil record is an anomaly, very few specimens fossilize. We are unaware of over 99 % of species that have populated our planet, especially when in comes to Theraphosidae. Only two extinct members are known from the fossil record. The number of giant arachnids, spiders and relatives in the past is unknown, but we do know of jaekelopterus rhenaniae. We also know that spiders the size of large crocodiles is impossible today, not enough oxygen in the air.
So, back to the original question, how big can spiders get in this current environment? We know of the improvement to the book lung organs in species such as tarantulas, ergo we can tentatively assume that based on anatomy and behavior that if the giant spider of the Congo and Venezuela exists, it is in all likely-hood a relative of the tarantula. I can conclude within then, let’s say, 75 % of spider specie yet to be discovered, exists some very big relatives. Three to five feet specimens living in the jungles of the Amazon and Congo basin is by no means a zoological impossibility or even an improbability.