In reply to:

I think if you check a Latin dictionary (and do check; I can't vouch for my memory), you'll find 'opera' is a feminine singular as well as the more familiar neuter plural. Meaning much the same, IIRC.

How right you are, Nicholas. Thanks for that.

opera, ae, f. [opus], service, pains, exertion, work, labor (opus is used mostly of the mechanical activity of work, as that of animals, slaves, and soldiers; opera supposes a free will and desire to serve).

opus, eris, n. [Sanscr. ap-as, work; whence apuas, gain; v. ops; cf. also Germ. üben]. A. In gen., work, labor (cf.: labor, ars, opera) B...