By far the scariest aspect of this film...
is the fact that, in real life as well, spiders are everywhere and they are virtually undetectible.
In the movie when Jennings is looking for spiders under his daughters bed before bedtime and says "All clear.", there really is no possible way to know with 100% certainty that the room is clear of creatures that can fit, squeeze and crawl in from any possible crack and hide in places that he couldn't possibly see.
Couple that with the fact that the spiders in this film are deadly and there is a REALLY good chance that his daughter could be bitten in her sleep and you've got a really good reason to simply leave the area rather than stay.
Go into your kitchen and look around really well for spiders, cover every possible crack and space you can think of...you still won't see spiders or insects that are hiding in there even if you spent hours looking.
That's a truly scary situation, that you can EASILY be killed at any moment from creatures that you cannot hear, see or detect with any degree of accuracy or certainty.
If you expect the unexpected doesn't the unexpected then become the expected?