The new sp. (holotype 6, allotype 9: Ecuador, Zamora Chinchipe prov., grassy marsh 5.5 km SE of Zamora, ca 3000 ft, 4°10’S, 78°56’W, 5-XI-1997; deposited at FSCA, Gainesville, FL, USA) is described and compared with M. hypodidyma Calvert. M. sympriona differs in the low, laterally rounded transverse ridge on the venter of abdominal segment 1 which bears 0 to 3 widely spaced black denticles on each side of the median depression, the tips of the outer arms of the hamules surpassing the anterior laminae, and segment 9 all black. Females have abdominal segment 9 sternite convex instead of flat as in M. hypodidyma.