Ok, observe this picture:
Now Im probably wrong, but I know I had seen something that looked like eyes on them before. It may just be a discolorisation on their skin but those green slits look distinctly like eyes. Now if theyre not eyes then we could open up the idea of sonar, or an incredble sense of smell. Maybe they are like caterpillars and flies and have tiny hairs on their bodies which can sense the slightest movement in the air surrounding them.