2 questions:
1) Are artifact bows especially good, or do they just help the bashing heads in part?
Here's what the wiki says: As ranged weapons, "The quality of the crossbow affects the accuracy of the shot, while the material of the crossbow is unimportant. "
As melee weapons: "Choosing a better material will, of course, result in a more damaging butt-stroke - but crossbows make a poor melee weapon anyway. "
2) How can I find a cave spider, fast? I have no silk and he demands it
Stupid elfs only bringing their rope reed stuff as usual. Sigh.
You may need to send the military to explore the cavern and reveal all of it, lest you get spammed by pathing failures for webs that the weavers want to collect (they have X-ray web vision) but which aren't pathable (they can only see un-revealed webs, not un-revealed cavern tiles).
Two adendums: if you have a skilled enough bookkeeper, you can 'magically see' at least some of these webs too. Forbid them all, then specifically unforbid those that you know your dwarves can reach - that's likely faster than getting a militia down there and the whole caverns searched out (least the way I play). If you mean to save this dwarf, and if the elves just came - dig down and down and down. Fast and fast and fast - find those caverns or he is toast.
Also, for your future embarks, you might want to give some thought to what items dwarves tend to ask for when they have moods that are hard for you to supply, and bring maybe 5 of each with you straight as you embark. You can count on at least a full year and a half from the point of embark before you can reliably get whatever you ordered from the first dwarven caravan - so bring enough for a couple moods at least. Many materials are very common, but silk/glass/yarn/shell can be difficult to produce without sustained effort to get them.