Solifugae many species also are used for stridulation. Unlike scorpions, solifugids do not have a third tagma that forms a "tail". Most species of Solifugae live in dry climates and feed opportunistically on ground-dwelling arthropods and other small animals. The largest species grow to a length of , including legs. A number of urban legends exaggerate the size and speed of the Solifugae, and their potential danger to humans, which is negligible. Solifugae are moderately small to large arachnids (a few millimeters to several centimeters in body length), with the larger species reaching in length, including legs. In practice, the respective
Opabinia family tree is that modern tardigrades may be "Opabinia"s closest living evolutionary relatives. Although Zhang and Briggs (2007) disagreed with Budd's diagnosis that "Opabinia"s "triangles" were legs, the resemblance they saw between "Opabinia"s lobe+gill arrangement and arthropods' biramous limbs led them to conclude that "Opabinia" was very closely related to arthropods. In fact they presented a family tree very similar to Budd's except that theirs did not mention tardigrades. "Opabinia" made it clear how little was known about soft-bodied animals, which do not usually leave fossils. When Whittington described it in the mid-1970s, there was already a vigorous debate about