Pandiculation documented in a spider

After a long period of waiting in ambush on the hub of its orb web, an araneid spider (Cycloso argenteoalba) was found to stretch out its body and legs, exhibiting a distinctive behavior known as pandiculation. Pandiculation is a natural and instinctive activity, often accompanying yawning, that is performed broadly by mammals (including humans) and birds, while only yawning is known in reptiles, amphibians, and fish, excluding sharks and rays. However, along with other arthropods, spiders belong to the Protostomia, which is phylogenetically very distant from the Deuterostomia, whose members include vertebrates. Whether pandiculation has evolved once or separately in Mammalia/Aves and Araneae is still uncertain, but the discovery of pandiculation in spiders suggests that this behavior may be exhibited by other arachnids and perhaps by other arthropods or even other Protostomia as well. Within the Arthropoda, the possibility of yawning via spiracles (exoskeletal openings associated with respiration) should also be considered.