I have already asked a question about the difficulty progression in my endless runner game: How to make difficulty progression in my endless-runner game?
Now I have to decided that it is a good approach to make the following:
Camera is always drifting up, slowly, but preventing the player from idling.
If squirrels jump, the camera follows them and the spiders also update their position to be slightly below visible area. Their speed is a little bit faster than the camera's drifting speed, so the player will panic out of the fear of the spiders catching them.
I want to make feel of constant chase by spiders and force player to run, make quick decisions and make mistakes. Player need to know exactly how close the spiders to squirrels are.
P.S: You can see earlier version of this game here: http://ludumdare.com/compo/ludum-dare-34/?action=preview&uid=66055
EDIT: The problem is that spiders are always relatively close to player and distance between them and the spiders matters. Once spiders have left the visible area, player feels safe. Firstly, spiders run with constant speed and if the player runs a little bit faster, he is always in the safe area and doesn't see any spiders.
The idea is to have player under pressure all time. The difficulty is that the visible area between the spiders and squirrels is small
FINAL DECISION: The final decision was made because of majority votes, and I suppose this answer suits for many games. But there is more than one good answer, and most of the answers are worth reading and implementing. That is what I am going to do.
Thanks everyone for your great answers!
FINAL NOTE: you can check final implementation finding released game on market - https://play.google.com/store/apps/details?id=by.Ludum.Vaverki