A recent article on the Science of Us website (http://nymag.com/scienceofus/2016/10/why-are-so-many-people-scared-of-bugs.html?) explores this phenomenon. There are several reasons for an elevated fear factor.
First, insects can be really dangerous. The ones that bite or sting can cause real trouble, especially if an allergy is involved. Plus some carry serious diseases, like the recent outbreaks caused by the Zika virus. Perhaps we automatically transfer fear of the dangerous insects over to anything that is small and crawls. Or perhaps we’ve become attuned through evolution to avoid anything potentially dangerous, no matter how tiny.
Second, we find them disgusting. There are many animals that can be dangerous, like bears for example, yet we make bears into plush toys (teddy bears) and cute animated characters. Yet we rarely do the same for bugs. Psychologists studying disgust have identified a “rejection response” — the feeling that you need to get away from something as soon as possible. This has been partly shaped by culture, but it also has a biological origin as a mechanism designed to keep us safe. We avoid rotting food because it can make us sick, and the presence of insects can be a sign that something is unsafe. And over time, the messenger and the threat tend to merge in our minds.
Third, they are scary because they’re so different. They have too many legs and eyes, their skeletons are on the outside, they can move in huge swarms, etc.
In a 2013 interview with Popular Science, ecologist Jeffrey Lockwood, author of The Infested Mind, said most of us also see insects as uninvited guests. “Now that we’ve moved into urban environments where close quarters and hygiene are at a premium, we find that the vast majority of our interactions with insects are negative in that they are the things that are invading our homes and our private spaces,” rather than things we see in nature. So an encounter with a bug seems more like an encroachment on our home territory. There’s some symbolism at work here, even though insects are very small compared to us.