(botany) A minor, nonsupporting vein in a leaf of a plant; a branch vein of a nervure (supporting vein) or of another nervule.
(entomology) A minor vein in a wing of an insect.
2003, W. J. Holland, Cassia B. Farkas (editor), Field Guide to Butterflies, Revised and Updated Edition, page 12,
The branches of these compound veins are known as nervules. The median vein always has three nervules. The nervules of the subcostal veins branch up and outward, with from four to five subcostal nervules, toward the costal margin and the apex of the forewing.