When an American bushwalker falls into an unexplored gorge, he must help his three companions escape a ravenous arachnid humanoid to survive.

"american" has no place here (read my review of your australian theme park theme).

I understand you are trying to explore the M.I.T.H. genre but I recommend you to read again the blake snyder chapter about genres. You have to find a new way to combine the formula. Bushwalker, already seen too. Giant insect, already seen. Wild natural setting, already seen. You should struggle to find at least 1 element that bring something 'new' to the formula. Or a new combinationo f some old elements. But the most difficult is to build a logic in the whole concept. It's not about " a libanese cook must escape from the AmazonTM warehouse before robot Hitler kills everybody": I've never seen this before but it's plain s**t.

Come on! I'm sure you can write a killer logline.

I'm pretty familiar with Deliverance and John Boorman but that's not the tone I'm after, although it is very similar. This is a straight up monster movie, the MAIN action is to survive the monster, to not get eaten. How would you write the logline for Deliverance in 25 words or less? Their Main action is survival.

Then the "action" is resolving the internecine conflict and should take pride of place in the logline. The Spider is just the catalyst. I repeat my suggestion about Deliverance.