We report on the larviposition behaviours and life cycles of what appear to be the first known insect parasitoids of a marine mollusc. In the field, Sarcophaga megafilosia Pape, McKillup & McKillup and Sarcophaga meiofilosia Pape, McKillup & McKillup were seen depositing larvae close to live individuals of the intertidal snail, Littoraria filosa (Sowerby), which lives on the upper leaves and branches of mangroves. Once a larva had entered the shell, by wriggling between the operculum and the spire, it began burrowing into the foot of the snail. This caused the snail to retract violently and produce large amounts of mucus that entrapped and prevented other larvae present outside from entering the shell. Each snail died within an hour of larval penetration and its shell, within which the larva and pupa developed, became glued to a leaf or branch with dried mucus. S. megafilosia only larviposited on snails with shells 10 mm or longer, while S. meiofilosia only larviposited on those with shells from 4 to <10 mm long. Both flies were reared in the laboratory and had similar life cycles with a minimum generation time of about 50 days. Adults lived for up to 7 months and overwintered as adults; there was no evidence of a larval or pupal diapause.

The larviposition behaviours of these flies are contrasted with a sarcophagid parasitoid of terrestrial snails. We suggest that other sarcophagid parasitoids of littorinid snails may have been overlooked, and that visual selection of hosts by S. megafilosia and S. meiofilosia may be at least partly responsible for maintaining the shell colour polymorphism shown by L. filosa.