Is that based on the brown margin of the forewing? H. elegans supposedly has brown that extends further forward along the rear edge of the forewing. That seems not as pronounced in this photo as others I have looked at.

Also, in the photos I've looked at, the outer portion of the hind wing also has an extended brown area, coinciding with the extended brown of the forewing, as in this picture.

I don't see that at all on this photo.

Is there some other characteristic that is used to distinguish these species?

Another way to look at it is to look at the silvery white rather than the brown. It seems like the distal end of the forewing on H. hyalinata is roughly triangular in outline, whereas H. elegans is hook-shaped.

Note: I am by no means any sort of expert on these moths. I have simply studied them a bit after having seen one myself.

Victor,
Thanks for your input. While the referenced specimens of H. hyalinata are from Puerto Rico (hence no colection data), I suspect there may be some specimens from lower Texas and Florida that may well be H. hyalinata. I will check these out, before moving any. Thanks again for your research.
Robert