Orb-weaving spiders can spin webs of up to 1m (3ft 3in) in diameter
A new and rare species of "giant" orb web spider has been discovered in Africa and Madagascar.
In the journal Plos One, researchers describe Nephila komaci as the largest web spinning spider known to science.
Only the females of this groups of species are giants, with a leg span of up to 12cm (4.7in); the male spiders are tiny by comparison.
Scientists say the female spiders are capable of spinning webs that reach up to 1m (3ft 3in) in diameter.
Orb-weaving spiders are a widespread group which take their name from the round webs they typically spin.
The few preserved female specimens had bodies almost 4cm (1.5in) long
The new spider was identified by Matjaz Kuntner, a biologist from the Slovenian Academy of Sciences and Arts, and his colleague Jonathan Coddington, from the Smithsonian Institution's National Museum of Natural History in Washington DC.
Dr Kuntner told BBC News that the discovery was "very unusual" because Nephila spiders were so well-studied and so large.
But this species is so elusive that even Dr Kuntner has not seen one live. He was able to identify the species from a specimen he first examined in 2000.
The giant female was in a collection belonging to the Plant Protection Research Institute in Pretoria, South Africa.
"It did not match any described species," said Dr Kuntner.
In his search through more than 2,500 samples from 37 museums, no further specimens turned up and he assumed the spider must be extinct.
But when a colleague in South Africa found three more of the spiders, it became apparent that they belonged to this same new species.
Male Nephila spiders look tiny in comparison to "giant" females
The discovery will enable scientists to study the evolution of the dramatic size difference between male and female Nephila spiders.
Dr Kuntner explained that the widely accepted theory was that evolutionary pressure was causing female "gigantism", with the females increasing in size in order to produce larger numbers of offspring.
He and his colleague, Jonathan Coddington, from the Smithsonian Institution's National Museum of Natural History in Washington DC, fear the rare spider might be endangered.
"Its range is restricted and all known localities lie within two endangered biodiversity hotspots: Maputaland and Madagascar," said Dr Coddington.
Dr Kuntner named the species in honour of his best friend and fellow scientist Andrej Komac, who recently died in an accident.