Where They Come From:
Food in your cupboards may have been contaminated with insect eggs which were sealed in the package. When the eggs hatch, you can spot worms or insects in the food container. If you leave the package in the cupboard for a long time, the insects will escape and infest other foods and, eventually, the entire cupboard.

Source:W.L. Gojmerac
[Professor of entomology]

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This cosmopolitan insect feeds on a large variety of food products. Although coarse grades of flour are preferred, Indian meal moth larvae are often found feeding in whole grains, cereal, dried fruits, nuts, seeds, and powdered milk. Foods infested with these insects will have silk webbing present, especially near the food surface.

Adult moths are nearly 1/2 inch long and have distinctive wing markings. The base of the forewing is pale grey and the outer two-thirds is reddish-brown with a coppery luster The larvae are generally dirty-white in color with shades of yellow, pink, brown, or green, depending on its food. Mature larvae, which are about 1/2 inch long, usually move fairly long distances from the feeding site before pupating within silken cocoons. There may be 4 to 8 generations of Indian meal moths per year.
The larvae produce silken-type webbing throughout the material they're feeding on. Mature larvae move away from infested materials to pupate in neighboring cracks and crevices. They can easily have six generations per year. Homeowners often discover these infestations when great numbers of larvae are seen moving away from infested materials, sometimes dispersing over the entire room. 
Because of their habit of moving some distance from infested products, an intensive cleaning routine is necessary to find and eliminate Indian meal moths, especially in a commercial setting. Fumigations can then be necessary.