The head isn't always protected by the helm though.
The face should be wide open... but I guess attacks to the face aren't lethal so the spider keeps trying to gnaw its way into the brain?
brb arena.
Test 1. 14 Dwarves fight 1on1 with 14 GCS and wearing only a iron helmet.
Result: All 14 dwarves were bitten in the lower and upper body. Causing paralysis. The spiders continue to attack the body even after the dwarves were both paralyzed and passed out. Not a single blow was aimed at their heads. Death was very violent and painful.
8 dwarves suffocated due to organ failure while 6 dwarves died from internal bleeding before they had the chance.
Test 2. 14 dwarves fight 1on1 with 14 GCS and wearing armor, high boots and robes from dog leather .. and a iron helmet.
Result: The dwarves were attacked all over. The spiders seemed to use different approaches and some would focus on the legs or the upper body/head while others still the lower body. The strategy changed when dwarves passed out, rather than when they became paralyzed.. and the usual strategy was to bite them in the lower body, which I suppose was the least protected of the vital areas. Though some spiders kept trying to attack the head they strongly prioritised the lower body.
One spider was killed and another badly wounded in the battle. 2 dwarves died due to suffocation but 12 slowly bled to death. Either the spiders were low on venom at this point or the leather armor made it more difficult for them to inject it ... or both.
Test 3. Several dwarves are dropped with full iron and adamantine gear from a height of 4 Z levels where the survivors will be attacked by fresh GCS. The armor: iron helm, breastplate, mail shirt, gauntlets, high boots and chain leggings. also troll fur robe and cape. (because I cant find candy robe and cape).
6 dwarves survived the fall. 4 were passed out from the fall and 2 were unhurt.
Result:
passed out ones: - bitten exclusively in the head until regained consciousness, fought their way out of the web and stood up. then the legs and arms and general body was attacked. once caught in the web again the head was singled out.
unhurt 1: attacked all over but occassionally caught in the web. then his head is exclusively attacked
unhurt 2: attacked all over like the others .. however the spider tried to bite him in the UPPER LIP which was deflected by the IRON MAIL SHIRT
eventually all the spiders were slain and not a single iron-clad dwarf was even WOUNDED by them. the only injuries are the ones sustained in the fall... and they have leveled up pretty well!
Made a few tests.
Conclusion: GCS seem to prefer to attack unarmored vital organs, if none are available, however, they will try in vain to attack the most vital organ: the brain.
While reading on the wiki I think I understand why the face isn't attacked. Because the facial features are a "child" feature of the face, which is why helmets don't cover them. However the iron mail shirt has the ability to shield both throat and facial features, even if that doesn't make any sense... it's the way cloth "abilities" are programmed but anyway:
The face isn't actually part of the head. So if a spider tries to attack the HEAD then it cannot hit the face. Even though the helmet doesn't cover it. How odd is that?
Anyway the spiders appear completely harmless if you have iron armor around.
edit: i did a few more tests with dwarves attacking eachother while wearing massive armor .. except mail shirts. apparently they are able to punch out eachothers teeth, bite eachothers ears and attack the eyes.. through the helmets.
i miss the days when a dwarf could be stabbed in the eye AND the brain in one blow.
final edit: i checked to see if it's possible to inject poison in a dwarfs' lip, eye, nose, ears and teeth. the nose, teeth and ears were torn off but the rest served to inject him with deadly amounts of venom. So cloaks, hoods, mail shirts, robes, capes ... anything that can get between a dwarven face (located on his throat) and the spider is good to have on hand if you must deal with them.
I am going to put the mutilated shell of a dwarf out of his misery now. Hope my semi-on-topic rambling helped you in some way.