What is the resinous secretion of a coccid insect used primarily in varnishes?

The resinous secretion of a coccid insect, specifically the female lac insect, is known as lac. This substance is harvested from the bark of certain trees where the insects reside and is processed to produce shellac, which is commonly used in varnishes, coatings, and as a food glaze. Lac is particularly valued for its ability to dry quickly and provide a durable finish.

In contrast, copal, rosin, and shellac, although all used in varnishes, are derived from different sources. Copal comes from tree resins and is used in varnishes for its hardness and moisture resistance. Rosin is obtained from pine trees and is primarily used in music and certain industrial applications, not directly as a varnish itself. Shellac, while made from lac, refers specifically to the processed form that is dissolved in alcohol to create a liquid finish, rather than the raw resinous secretion. Thus, lac accurately identifies the resinous secretion of the coccid insect in the context of the question.