My Life As a Tarantula Toe Tickler by Bill Myers

Boy-genius Wally McDoogle makes one very minor mistake…and decides to hide the truth from his parents. What harm could come from hiding one tiny mistake? Much more than Wally bargained for! Junior Genius (the spoiled, super-inventor from My Life As a Sky Surfing Skateboarder) turns Wally into a human guinea pig for his latest creation-the Mind Magnifier. And something goes terribly wrong. Instead of a dramatically increased brain size, Wally ends up with ears big enough to use for hang-gliding!

When you mix in giant flying snails and Tina, a giggling tarantula the size of a small house, you've got problems of major proportions. Now, our boy blunder must save Tina, his life, and the entire city! In this life-and-death struggle, Wally learns the importance of admitting mistakes and not hiding the truth from his parents.