Learn English – the plural of ‘the last will and testament’

The -ness suffix is an extremely productive one in English, with thousands of resulting words being attested by the dictionary and more being coined daily as need or whim arises.

Although it is typically used to turn adjectives and participles into abstract nouns expressing a state or condition, like hardness or willingness, it can even be roped into service by pronouns and adverbs, as in me-ness or everydayness.

In general, the resulting state or condition is considered a mass noun not a count noun. Words like kindness, happiness, openness, darkness, and madness do not normally lend themselves to inflected plurals like kindnesses, happinesses, opennesses, darknesses, and madnesses.

However, like other mass nouns, these can become plurals when you are discussing separate instances of that thing, such as the darkness of a moonless night being distinct from the darkness of a shuttered room, and those therefore being two different darknesses, or even simply speaking of small acts of kindness as kindnesses.

Moreover, some common words ending in -ness have more readily passed into count-noun territory, including such words as illnesses and businesses. This is more easily done once the word is no longer apprehended as having been a base adjective to which a substantive-producing suffix has been applied.

So while you would be in unassailable territory using a construction like “different degrees of stiffness”, writing “different stiffnesses” can certainly make sense in the right context.

PS: Words like witness and wilderness, and of course lioness and villainness, have their own stories. The first pair gained their -ness long ago and have no base words to which we can now reduce them, while the second pair are actually the feminizing -ess suffix applied to a word that already ends in -n.

Mantids is not plural for mantis, but for mantid, which is another term for mantis:

mantis

man·tis (mănʹtĭs)

n. pl. man·tis·es or man·tes (-tēz) Any of various predatory insects of the family Mantidae, primarily tropical but including a few Temperate Zone species, usually pale green and having two pairs of walking legs and powerful grasping forelimbs. The mantis feeds on live insects, including others of its own kind. Also called mantid.

(American Heritage Dictionary)

Evidently, mantid can also have another, more specific meaning:

1. See mantis.
2. A mantis in the family Mantidae.

In summation: the correct plural for mantis is mantes or mantises, which is what you use. The correct plural for mantid is mantids.