1. stomach of an insect; muscular digestive pouch of a bird. ventriculusven*tricu*lus , n.; pl. ventriculi (#). [l., belly, dim. fr. venter belly.] (zo?l.) (a) one of the stomachs of certain insects. (b) the body cavity of a sponge.ventriculus thick-walled muscular pouch below the crop in many birds and reptiles for grinding food [syn: gizzard, gastric mill]