The best answer is that it depends on the weather. It's never a bad idea to pack extra layers in your hiking backpack, but your goal with your clothing choice should be layers that breathe well enough to keep you from overheating while still providing protection from UV rays and flying insects.If you're hiking in a particularly buggy location, you may opt for long sleeves and long pants. But if it's fall in a place like the high Sierras where bugs are few and far between, you might be more comfortable in shorts and a moisture-wicking shirt, provided you apply and reapply sunscreen throughout the day.