As the Stung Are of the Adder

Every now and then, as I'm pushing through these texts for the Shakespeare-in-a-year project, I slow down and savor the rhythm, counting the beats of each line.  That sometimes brings out some interesting qualities, particularly when you think about how it needs to be spoken.

So in King Lear 5.1, I noticed this speech of Edmund:

To both these sisters have I sworn my love,
Each jealous of the other as the stung
Are of the adder.

The syllables in bold are the stresses of iambic pentameter; but for the meaning to shine through clearly, I think there's also a slight emphasis on the underlined words. 

And what that does is highlight a certain resonance between the word "other" and "adder" (in my American English, using the IPA, I might render them as  äðɘr / ædɘr).  

These lines initially work on an intuitive level, because the natural impression of the image is that Edmund is the adder who has stung both sisters (true enough), and they are both "jealous" of each other in OED's sense 4: Troubled by the belief, suspicion, or fear that the good which one desires to gain or keep for oneself has been or may be diverted to another; resentful towards another on account of known or suspected rivalry.  

But when we look more closely at the simile, it's a bit perplexing.  Are the stung "jealous" of the adder?  In what sense might that be true?  It must mean mistrustful.  (And indeed that is meaning 5b in the OED: †b. Doubtful, mistrustful. Obsolete.)

So the word "jealous" is working in at least two senses here (within and without the simile).

While the literal and figurative meanings of "adder" seem more obvious, here are the most plausible relevant senses the OED gives us in the relevant time-frame:

 1. b. figurative. A treacherous, deceitful, malicious, or pernicious person or thing (also as a term of abuse); the type of envy or treachery. (OE—2006)
 
 2. a. The common or northern viper, Vipera berus, a small, venomous Eurasian snake found widely in northern and central Europe, having a characteristic dark zigzag line down the back. More fully European adder.  Adder is the historical and popular name, originally carrying connotations (as the ideas of darting and stinging) not associated with the name viper.  (OE—1995)

So I'd say the potential "adders" are three-fold: 

• Within the simile: Each sister is mistrustful of the other, just as those who have been stung/bitten (by an adder) are mistrustful of the adder (literal sense 2a).  For purposes of the simile, the adder does not map onto any of the characters; it has to be a generic, literal adder for the simile to function.
• Each sister sees the other as an adder (figurative sense 1b; poisonous/malicious and potentially fatal to the hoped-for union with Edmund);
• Edmund is the adder (figurative sense 1b) who has "stung" (cold-bloodedly poisoned/betrayed) each sister with false promises of love.