For some people the measure of a monster is how many it requires of them to be scary. Opinions vary from the necessity of one being more than enough danger all the way up to requiring hundreds of them. Some people don't even require there to be danger. Certainly this is true for a lot of people when it comes to creepy crawlies. Things with six or more legs--the more the scarier--tentacles, leathery wings, partially furry bodies, rubbery skins and faceless ends, chitinous maws, wings and stings--these are a few of the creepiest things...
More than anything it isn't the physical aspect that scares the worst, but the intent. A bunch of spiders drop on you and run around. It chills the blood of some. When they drop down and crawl into your clothes and burrow into your skin.... there's the stuff of nightmares. When wasps sting you so that you scream and the rest of the hive can crawl into your lungs and nest there... can you help but cough reading that? When the old seer leans forward to tell you your fortune and he opens his mouth to reveal his tongue has been replaced with a six-pack of long worms... you have to wonder what the...
Science usually takes the fear away from things, but when it comes to the creepy and the crawly it sometimes only makes it worse. You think your the height of life on the planet and you learn that you're outnumbered by insects by more than you want to seriously think about. You think that you can get away from them, but you're never any more than six feet away from a spider--imagine if they stowed away in your mountain climbing gear to go to the frozen heights, or worse one materialised in your diver's mask, or your space suit, just to make this rule absolute.
Music: Riad N' the Bedouins by Guns N' Roses. Or get MP3s.