I would have thought that even in the examples given, the word 'viscera' is still being used as a plural. As a medico I use the singular 'viscus' not infrequently - e.g.. 'gas under the diaphragm is a sign of perforation of a hollow viscus'!

Furthermore, there is a difference between truce and insignia on the one hand, and graffiti and viscera on the other, in that the first two are the only ones that are truly plurals that have come to represent a singular item. (You can have 'a truce' and 'an insignia' but not 'a graffiti' or 'a viscera'.)

'Ephemera' would be another in the same category as viscera I would think....

I just came to write the same thing. I don't remember ever seeing "viscera is" or "viscera was". Google gives 57,600 hits for "viscera are" and 46,900 for "viscera were". Out of the first ten hits for the former, nine are direct, ordinary uses of the word (the tenth is "CINEMA VISCERA are proud to present"), and it's ten out of ten for the latter.

For "viscera is", Google gives 45,300 hits--but out of the first several dozen I looked at, only one was a direct, ordinary use ("By this way, the viscera is not easy to be demaged [sic]"). Other uses include "Viscera is a band", "Perotin, c.1200: Beata viscera is part of these releases", "Situs inversus of the abdominal viscera is not a rare condition", "'Viscera' is choreographed", "Disposal of viscera is a problem", "Viscera is a non profit organization", "Viscera is a great word to know", etc. "Viscera was" gives 18,200 hits, which are of the same sort as the hits for "viscera is", such as "Viscera was released in the summer of 2000".

I agree with you mollyflea
a viscus as in.. internal organ, viscera as the plural form or also called GI tract and informally the 'gut'

And WOW you did a lot research talesoftrivia...welcome both of you to the board.

I've seen graffiti described as mass noun or plural noun sometimes used with singular verb.

graf·fi·ti (AHD of E)
NOUN:
(used with a sing. or pl. verb) Plural of graffito.

I would say: 'This is a genuine Keith Haring graffiti.' But...