You're going to Central America. You'll be in rain forests. Expect insects, especially mosquitoes. We prepared for bugs by spraying all of our clothes with a mosquito repellent that lasts for two weeks, just before we left (this repellent is meant for clothing, not skin; it is available at some outdoor recreation stores). We also packed a number of long sleeve shirts and pants instead of the T-shirts and shorts favored by most tourists. We even took along spray-on repellent and mosquito netting. In the end, though, I rarely saw a mosquito. The key to this was the time of year that we went. Our trip was in April at the very end of the dry season. In May and June the wet season begins, and from then until the next dry season you will get a lot more bugs. Our timing was perfect, because we went at the time of the year with the fewest mosquitoes. The only downside was that most of the grass at the ruins were brown rather than the lush green that you see in the tourist postcards. We also encountered a couple of large spiders and a millipede in our hotel rooms, but they were not a problem.
Travel tip: Avon's "Skin So Soft" sunscreen is what we used, and I highly recommend it. It has an SPF of 30, contains a mosquito repellent, and is gentle to the skin. I think it was what kept away the few bugs that we did encounter.