Mysmenopsis is a kleptoparasitic genus of tiny tropical and subtropical American spiders in the family Mysmenidae. Most live in the funnelwebs of spiders in the family Dipluridae. M. archeri lives on webs of a species in the family Pholcidae, M. capac and M. cienaga have been observed living in Cyrtophora (Araneidae) webs. One reason why diplurid webs are preferred seems to be that they are persistent in time and space, sometimes spanning several years.

The monotypic genus Isela is closely related.

Mysmenopsis furtiva from Jamaica lives as a kleptoparasite and commensal in webs of Ischnothele xera. It steals portions of its host's prey, but also consumes minute trapped insects that are not sought after by the host. In order not to be recognized, it moves only slowly when the host does not move; else it synchronizes its rapid movements with the movements of the host. M. furtiva has been observed to feed on one end of a prey animal, while the host feeds on the other. When feeding, its legs and pedipalps remain motionless, but its abdomen sways slowly and slightly. One feeding bout can double its abdominal volume. The host shows anti-kleptoparasite behavior, amongst others by adding silk between the kleptoparasite and the feeding site.

The closely related M. furtiva and M. monticola parasitize two spider species that are also closely related, and it is believed that the two groups coevolved.

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