Bolas Spiders are unusual orb-weaver spiders that do not spin the typical web. Instead, they hunt by using a sticky 'capture blob' of silk on the end of a line, known as a 'bolas'. By swinging the bolas at flying male moths or moth flies nearby, the spider may snag its prey rather like a fisherman snagging a fish on a hook. Because of this, they are also called angling or fishing spider (although the remotely related genus Dolomedes is also called fishing spider).
Bolas spiders are small nocturnal animals (females up to 15 mm or 1/2", males about 2 mm or 1/16") with oddly 'lumpy' abdomens. Several Mastophora species (e.g. M. cornigera) look like bird droppings, which enables them to rest unnoticed at day in fairly exposed places. M. bisaccata resembles the shell of a snail that is abundant in the spider's habitat. While the bird dropping mimics rest on the upper surface of leaves, M. bisaccata rests on the undersides. If removed from the surface, Mastophora females produce a pungent odor, which is highly unusual for spiders (other than bolas spiders, only one species of Cyrtarachne is known to do this). They do not attempt to flee when handled. Some species of Cladomelea also rest exposed on leaves during the day. However, the Australian Ordgarius magnificus, which displays eye spots on its back that make it resemble the moth they hunt, ties leaves together with silk to form a retreat. The females of some bolas spiders look remarkably like a bird dropping, thanks to their large, globular abdomen and brownish cephalothorax. This is a form of defensive mimicry as the animals that prey on spiders pay little attention to bird droppings.
Bolas Spiders are found in America, Africa, and Australasia. They do not occur in temperate Eurasia. About half of the known Mastophora species occur in South America. The genus is distributed from southern Chile to the extreme northern US (to 45° north latitude in Minnesota). M. archeri, M. bisaccata, M. hutchinsoni and M. phrynosoma occur widely in the US east of the Great Plains. M. cornigera occurs from Alabama to California, as well as in northern Mexico.
Every spider species produces the pheromone of only one specific moth species (or a small set) and is thus dependent on it. However, bola spiders will try and often succeed to catch any insect that is flying nearby, aided by their good eyesight. They also seem to detect prey by the sound of their approaching flight.
Spiderlings and adult males hunt without a bola. This might be because the small globule these spiders could produce would dry out rapidly. Rather, they position themselves at edges of leaves and grab prey with their frontal pair of legs. Moth flies of the genus Psychoda are prominent prey in juvenile and adult male Mastophora. Adult spiders consume the bola after at most half an hour if the hunt was fruitless.
Female Mastophora catch an average of two moths per night, but as many as six or seven catches during a single night have been observed.