I'm not really sure, but we could be scared of insects because of what we associate them with. If we see Flies and cockroaches for example, we think of germs, disease and dead things because both of these insects have been related to decaying (maggots eating carcasses etc) and unclean things (cockroaches at disgusting hotels etc). Also, insect phobias maybe because insects are related to germs and disease spreading (i.e mosquitos and malaria etc) so we naturally dislike and fear them. Bad experiences with insects, bugs and creepy crawlies may also cause a fear of that happening again. I'm not scared of most insects, but I don't like earwigs, millipedes or woodlice, and I think thats partly due to the creepy way they move, and the strange way of having their skeletons on the outside!!!! I think we dislike bugs because they are so incredibly different to us, such as their exo- skeletons, 6-8 legs, and tiny size, so we find it hard to see them as not harmful. Also, during evolution, we may have developed a natural fear of bugs incase they are poisonous or dangerous. Insects are pretty useful and clever though; without spiders we would have way more flies than we do now, without flies, maggots, cockroaches etc. there would be a multitude of dead bodies around the place, and without dung beetles there would be a lot of dung left lying around! So, for a lot of reasons, many people are frightened insects.