A couple of long sleeve shirts will cover most situations. Long sleeves can be rolled up, give protection from sun and from biting insects and in the cold offer an extra layer. For trips to high risk malaria zones, you can buy treated clothing. Otherwise you can simply treat your clothing yourself with Bugproof Clothing Treatment.
Cotton shirts give the most comfort in the heat, polyester based are much quicker to dry. You might find one of each a useful combination!