Weevils of the genus Sitophilus are known to harbour within them certain structures (supposedly micro-organisms) that seem, on present evidence, to supply the host weevil with beneficial but not essential nutrients: the phenomenon of either symbiosis or mutualism.

The micro-organisms occur in cells called “mycetocytes”. In the larval weevil the mycetocytes are clustered together to form a mycetome. During metamorphosis from larva to prepupa to pupa to adult the mycetome breaks up into its constituent mycetocytes. The mycetocytes are found in the midguts of male and female adults and in the gonads of female adults. From the female gonads they are passed to the eggs and thence to the next generation of weevils.