that looks like a grub from a japanese beetle of someother beetle. was is good and fat and mature like the one in the pic? could be a mole just chased it up out of the ground. they don't usually come out till they pupate.
I never had much luck with eggplant. the flea beetles get mine. they make the leaves full of tiny holes so I doubt that is your problem. bunnies and gophers would take the whole plant overnight. so it must mean its some kind of insect or larvae. and if they are leaving the lettuce alone (not much goes after squash) that should narrow it down.
the following link says that both tomato hornworms and colorado potato beetles can affect eggplant. both do the kind of damage you are talking about. the hornworm would down a small plant in a number of hours though unless it was tiny

or it could be some other type of smaller caterpillar (I have some unidentified ones on my maters that eat them). or maybe its the beetle? unfortunatly you might have to catch them in the act. time for a flashlight I'd say.
http://www.geocities.com/green_cache/eggplant.html#Common_Pests_&_Pathogens