The origin or etymology of this turn of speech describing a group of moths is, no pun intended, obscure. Perhaps its origins can be attributed to the dull, cryptic coloration of many moth species, but eclipse is the last word that comes to mind when one encounters moths which use color to attract a mate visually, or sport eyes-spots or other startle patterns, or are toxic or distasteful and display aposematic colors to advertise their unpalatability.
Aposematic ... from Greek, apo → away, and sematic → sign or meaning.
The feathery antenna indicate this is a male Virginia Ctenucha (Ctenucha virginica) ...
The prominent purplish eyespots on the hindwings are thought to be a distraction display, startling a would-be predator for a vital fraction of a second and giving the moth a chance to escape.
So – how does all this decoration serve the ultimate goal of increasing the moths' chances of survival? Are the conspicuously spotted wings of these insects a coincidence, is one moth mimicking the other, or are both imitating a third distasteful insect species?