Is predation by ants driving the evolution of tarantulas? First report of tarantulas (Araneae, Theraphosidae) actively using urticating setae against invertebrates

The use of urticating setae by tarantulas from the Americas and the Caribbean has been known for centuries and is considered a defensive behavior against vertebrate predators. However, their passive use against invertebrates, such as parasitic fly maggots and ants, has also been recently proposed, but no active use of these setae against invertebrates has been observed or tested. In this study, eight tarantula specimens representing four different species from the family Theraphosidae were tested in twelve tests with the army ant Eciton burchelli and the carpenter ant Camponotus rufipes. Four spiders interacted with Eciton burchelli, and one interacted with Camponotus rufipes. The defensive behavior was distinct from that used against vertebrates, as the spider rubs the lower posterolateral region of the abdomen with downward and outward movements of the hind legs and uses silk threads. Scanning electron microscopy (SEM) demonstrated that the ants were entangled with urticating setae, primarily of type I morphology. Type I urticating setae are found exclusively in an important clade of theraphosine spiders, the tribe Theraphosini, which includes the largest known spiders. It is hypothesized here that protection against predation by ants due to the emergence of a specific type of urticating seta may be one of the factors responsible for the success of tarantulas in the New World.