Chryssalid
Nasty. Toothy. Very very fast. Able to implant an egg (that turns you into a Zombie) with little regard for body armor. The ability of single Chryssalids to multiply and wipe out entire squads makes them more feared than even the tougher Ethereals.
Note though that their lack of ranged attack or flying ability makes them easy meat once you have flying armor. They accompany the Snakeman race on Terror and Base missions.
Weak against explosives, says the USG. Nu-uh, they're weak against AP, says NKF. Chryssalid Damage Modifiers: Fire-80%, Stun-90%, says the OSG.
Jasonred: Actually, Kasey Chang's USG says they're weak against Fire and Stun. It's the ingame UFOPEDIA that claims that they're weak against Explosives. Something about their brittle shells. As for AP... well, go ahead and try using rifles and pistols on those terror missions. Have fun...
A HORRIBLE way to meet one is shooting a relatively harmless zombie with reaction fire as it shambles in for melee... then a Chryssalid pops out and SPRINTS at you, turning your squad into cheese. Always bring Incendiary rounds to Snakemen sites to take out the Zombie, as this will prevent the Chrys hatching.
Jasonred: Only works if the KILLING BLOW was Incendiary. Good luck hoping your reaction shots used Incindary ammo. ... In other words, while it's nice knowing about the Incendiarys not birthing Snakemen, I don't think it's actually all that useful. Chrys are in fact LESS dangerous than most aliens during X-com's turn, since they only have melee and can't reaction fire. If you have enough TU to kill a zombie with Incendiary, it's more practical to auto-shot him with Lasers or Plasma. ... Just don't kill Zombies with Grenades. During Aliens turn, reaction fired Incendiary's aren't a safe technique.
Chryssalids have 110TU, and each melee attack costs 14TU. This means they can be out of visual range at the beginning of the turn and cross the 21 intervening tiles and still attack you in the same turn!
They can perform 7 attacks per turn max at Beginner difficulty and 9 at Superhuman difficulty.