About Allocyclosa bifurca (McCook, 1887)
Allocyclosa is a genus of orb weaver spiders that holds only one single species: Allocyclosa bifurca. This species was first described in 1887 by McCook under the name Cyrtophora bifurca, and was moved to its own separate genus Allocyclosa in 1999. It is the only species related to Cyclosa found north of Mexico that has a forked rear tip on its abdomen, which gives the species its specific epithet bifurca, meaning "two-pronged" in Latin. The forked abdominal tip, which carries two M-shaped humps, is a defining characteristic for both male and female specimens of this species, even though similar features appear in some Cyrtophora species. Both sexes are grey-green in color. Only females have a patch of red on their underside, located between the epigynum and the spinnerets. Females are much larger than males: female body length ranges from five to nine millimeters, while male body length ranges from two to three millimeters. Males are reported to be very uncommon, but this is likely because their small size causes them to be frequently overlooked by collectors. In a 1977 study conducted by Levi, only two out of nearly 350 positively identified Cyrtophora bifurca specimens were males. This highly skewed sex ratio represents a very unusual distribution pattern. A second, less widely supported explanation has been proposed: females of this species have less prominent genitalia than other members of the orb-weaver family, so the species may be parthenogenic, meaning females can reproduce without males. Like most other members of the Araneidae family, Allocyclosa bifurca builds orb webs that measure six to eight inches in diameter, but it uses an unusual form of protective mimicry. Females often rest in the middle of a vertical row of web decorations that includes their egg sacs and wrapped prey. Because both the spider and its egg sacs share similar color and shape, it is hard to tell the spider apart from its egg sacs.
