Zygopauropus

Zygopauropus is a monotypic genus of pauropod in the family Brachypauropodidae. The only species in this genus is Zygopauropus hesperius, first described by J.W. MacSwain and U.N. Lanham of the University of California at Berkeley in 1948. This genus is notable as one of only four genera of pauropods in which adults have only eight pairs of legs rather than the nine leg pairs usually found in adults in the order Tetramerocerata. Before the discovery of Z. hesperius, adult pauropods were thought to have only nine or (rarely) ten pairs of legs.

Description
MacSwain and Lanham described Zygopauropus as a new genus and Z. hesperius as the type species based on twelve specimens, all small and white: an adult male holotype measuring only 0.54 mm in length, an adult female allotype measuring only 0.56 mm in length, and ten more specimens, including three adult males, three adult females, and four juveniles representing the first three stages of post-embryonic development.

This species features a head with four transversal rows of setae, temporal organs with three tube-like extensions, a fifth tergite with two submedian sclerites, and a pygidial sternum with two pairs of setae. Adults have only eight pairs of legs, and each leg has five segments. These pauropods go through the first four stages of post-embryonic development typical of species in the order Tetramerocerata, with three leg pairs in the first stage, five pairs in the second, six pairs in the third, and eight pairs in the fourth, but reach sexual maturity in the fourth stage rather than in a fifth stage and do not add the ninth pair of legs that usually appear in a fifth stage for other species in this order. Thus, adults of this species also have only eleven trunk segments and five tergites and do not acquire the twelfth trunk segment and sixth tergite that other species in this order usually add in a fifth stage.

MacSwain and Lanham also described two other new genera, Aletopauropus and Deltopauropus, along with Zygopauropus, finding no more than eight pairs of legs in all three genera. Since then, the genus Deltopauropus has been found to include adults with the usual nine leg pairs, but no adults with more than eight leg pairs have been found in the genera Zygopauropus and Aletopauropus. Since the description of Zygopauropus and Aletopauropus by MacSwain and Lanham, only two other genera of pauropods, Amphipauropus and Cauvetauropus, have been found to have adults with no more than eight leg pairs.

Distribution
All of the type specimens of this species were collected in either Contra Costa county or Marin county in California, and the distribution of this species remains limited to the western United States.