1. Place eggplant slices in a colander; sprinkle with 1 tablespoon salt. Toss well. Drain 30 minutes. Rinse thoroughly; pat dry with paper towels.
2. Combine remaining 1/2 teaspoon salt, yogurt, and next 4 ingredients (through garlic) in a small bowl.
3. Preheat grill to medium-high heat.
4. Brush eggplant and onion slices with oil. Place eggplant and onion slices on grill rack coated with cooking spray; grill 5 minutes on each side or until vegetables are tender and lightly browned.
5. Fill each pita half with 1 1/2 tablespoons yogurt mixture, one quarter of eggplant slices, one quarter of onion slices, and 1/4 cup arugula.