Ha ha ha ha ha. If you make this assumption, it is almost certain not to be true.
Whilst restraining myself from openly laughing, I have to concede ammulder has a point. Who knows what the players will decide. I've previously lain on ridiculous levels of hinting that they should flee, but they seem to think their role in life is to either kill everything or die trying. As a constructive solution to the above, I suggest you model the tide of spiders as an environmental thing rather than X number of actual spiders where X is very large. If the players are able to kill spiders (as in monsters) they might just keep trying till they die. If it's an environmental effect of "take blah hp damage every round that you're within the Spider Tide", then they'll get the idea. Also, use the term "Spider Tide". I just invented that!
Also, double up on the hints. Put a small river or stream behind them near the tower. If they don't think of escaping to the tower (and they might not given spiders, you know, can climb), they might try crossing the river to escape the spiders. This not only puts them next to the tower inclining them to use it if they hadn't thought of that before, but it allows a creepy moment where they realise the spiders are guided when they start climbing overhanging branches to rain down on the other side or going a little upriver to the stepping stones.
Also, with the butterfly people, I personally would be looking for ways to save them rather than take advantage of their deaths to flee. What you have laid out is a pretty dark path. You might want to have some back up plan in case the players exhibit some decency and compassion (it has been known on occasion).
Hope this helps. Also, be prepared for the PCs trying to set fire to the entire jungle. PCs are always trying stuff like that. Might want to set the scene during a rain.
