Evolução e morfologia comparativa dos pés em aranhas Mygalomorphae e raptoriais

Abstract

Spiders are among the most diverse animals on the planet and have developed different morphological and behavioral characteristics for capturing prey. Recently, it has been suggested that the microstructures of spiders' feet could reflect their way of capturing prey. Mygalomorphae composes one of the first clades of spiders to branch, with more than 3000 known species, with the microstructures of the feet little studied, but with well-defined and diverse hunting behaviors. Another group of spiders little studied regarding foot morphology and hunting behavior are those with raptorial foot configuration. We studied the anatomy and functionality of Mygalomorphae and raptorial spider feet using 3D reconstruction modeling, scanning electron microscopy, and light microscopy. Evolutionary reconstruction of Mygalomorphae foot features using a composite tree of spiders indicated (1) that they exhibit two character states for the metatarsal-tarsal joint, (2) a putative evolutionary novelty with two pairs of foot sensilla in two rows, (3) another possible synapomorphy in Theraphosoidina is the morphology of the subungual tufted plates that are partially separated from the tarsus by a dorsal suture. In the case of spiders with raptorial feet, evolutionary reconstruction of their features indicated that similar traits arose three times independently in Trogloraptoridae, Gradungulinae, and Doryonychus raptor (Tetragnathidae). The defining features of the raptorial feet are (1) a complex formed by the base of the elongated prolateral superior claw connected with the sclerotized ring of the pretarsus, with (2) the first pressing against the tarsus. This complex can even flex over (3) stout raptorial macrobristles forming a scaled-down version of a trapping basket on the tarsus to capture prey during hunting. Furthermore, our results show that taxa previously compared with raptorial spiders, Celaeniini (Araneidae) and Heterogriffus berlandi (Thomisidae), have other configurations of spider feet, lack the characteristics that distinguish raptorial feet, and lack the tarsal trapping basket. We also make predictions about the possible behavior of the aforementioned taxa that will need to be tested through observation of live specimens. We conclude that multiple tarsal morphological microstructures are required to define any spider foot and recommend a comprehensive assessment before asserting a foot configuration for any spider taxon.