Hmm, I like you're idea alex, but it's more like standard mind control, not at all like a chryssalid.
I guess Winter will find himself to be in a bit of a pickle here: chryssalids + realism = difficult to explain
![Wink ;)](http://ufoai.ninex.info/forum/Smileys/default/wink.gif)
Personally I think you should be a bit more lenient this one time, and create a worthy successor for the chryssalids, even if you can't explain it 100%
I suggest the following stages:
1. melee attack by hydra, soldier gets infected
2. Soldier can still be controlled, but is infected with a parasite or some other nasty thing growing in him. Can be stunned and treated in hospital.
3. Soldier loses consciesness. Cannot be rescued anymore.
4. Soldier gets up again, but is some zombie type creature (slow, but strong melee attack)
5. If shot, cracks open and another hydra comes out
6. Hydra maybe needs another round or two to gain its full strength.
At stage 3, your soldier already counts as enemy, so has to be neutralised like all the other aliens if you want to win the mission.
If this is too easy, cut out stages 2 and 6 (and maybe 3)