''Allocyclosa'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

orb weaver Orb-weaver spiders are members of the spider family Araneidae. They are the most common group of builders of spiral wheel-shaped webs often found in gardens, fields, and forests. The English word "orb" can mean "circular", hence the English name ...

spiders that contains only one species, ''Allocyclosa bifurca''. It was first described in 1887 by McCook under the name ''Cyrtophora bifurca'', and was transferred to its own genus in 1999. It is the only ''Cyclosa'' species north of

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

to have a forked rear tip of the abdomen hence the name ''bifurca'', Latin for "two-pronged". The forked tip of the abdomen, bearing two humps shaped like the letter ''M'', is a defining feature in both males and females, though it is similar to features present in certain '' Cyrtophora'' species. Both sexes are a grey-green color, though only females have an area of red on their underside between the epigynum and the

spinneret A spinneret is a silk-spinning organ of a spider or the larva of an insect. Some adult insects also have spinnerets, such as those borne on the forelegs of Embioptera. Spinnerets are usually on the underside of a spider's opisthosoma, and ar ...

s. Females are much larger than males, ranging from five to nine millimeters, while males range from two to three millimeters. Males appear to be very uncommon, but they are probably often overlooked by collectors due to their small size. In a 1977 study by Levi, only two of the nearly 350 specimens that were positively identified as ''Cyrtophora bifurca'' were males. This is a very unusual distribution; a second, less likely explanation that has been proposed is that females of the species, which have less prominent genitalia compared with those of other members of the orb-weaver family, may be

parthenogenic Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

, are able to reproduce without the help of males. Like most other members of Araneidae, these spiders build orb webs, six to eight inches in diameter, but employ an unusual form of protective mimicry. Females often sit in the middle of a vertical row of

web decoration A stabilimentum (plural: stabilimenta), also known as a web decoration, is a conspicuous silk structure included in the webs of some species of orb-web spider. Its function is a subject of debate. Origin Likely, the use of stabilimenta evolved ...

that include egg sacs and wrapped prey. Because the spider and the egg sacs have a similar color and shape, it is difficult to distinguish the egg sacs from the spider itself.

References

Araneidae Monotypic Araneidae genera Spiders of North America {{Araneidae-stub