The Lens

What kind of lens you need largely depends on what size critters you plan on photographing. For butterflies, most larger moths, large dragonflies, praying mantises, grasshoppers, scorpions etc., a 1:1 macro lens is a bit of overkill (though it doesn't hurt to have one). You simply can't get the full body of the insect in the frame at 1:1 magnification, though depending on your inclination, you might be able to get very interesting shots of parts of the animal.

So if you want mostly whole-body shots of larger insects, I'd get a lens with 1:2 or 1:3 maximum magnification, and a relatively large working distance. One of those Sigma/Tamron 70-300mm "macro" lenses might suffice in this case. I have an old (and quite horrible) Tamron 70-300 lens that is awful for most things, but really does take quite nice faux-macro photos.

These lenses have a few advantages, such as relatively light weight, versatility (you can shoot birds as well as insects) and often a very long working distance, allowing you to use the built-in flash without shadowing the subject. The picture below is taken with one of these lenses.

For smaller critters, such as most bugs, beetles, smallish spiders, you really need a true 1:1 macro lens. In general, for this kind of application, I'd default to a 100mm macro lens, preferably with image stabilization. However, if you're really serious about getting photos of shy insects, a 150mm or a 180mm lens might be better because of the increased working distance, though at these focal lengths, image stabilization becomes non-optional while hand-holding. The photo below is taken with a 100mm macro lens.

If the insects are even smaller (think mosquitoes or gnats), you will really need to fit your macro lens with extension tubes (or get the Canon MP-E 65mm lens).

The flash

In a jungle, you'll find that a flash is indespensable, especially when shooting insects. However, an external flash isn't always necessary (though again, it's nice to have one). In general, the longer the lens, the crappier the flash you can get away with. With my 70-300mm lens, I can get away with just using the built-in flash.

With a 100mm macro at its minimum focusing distance, I really need an external flash so that the lens doesn't shadow the subject. With a 50mm macro at 1:1, I either need to bounce the external flash off something, or use a ring flash (or some other sort of lens-mounted flash).

In the day, the flash can often look harsh, especially when you almost have enough light, so a diffuser helps (or a piece of tissue paper folded in 3 and taped over the flash).

Off-camera flashes can also help, but only with insects that don't move a lot. Otherwise, they'll be gone before you can even begin setting up the flash(es).

Tripod

Tripods help with relatively slow-moving insects, but with the faster-moving or shy ones, it's often hard to set up the tripod in time to catch them. Personally, I think image stabilization+flash is the way to go most times. A monopod might be an option if the lens you choose comes with or can be fitted with a tripod collar.

Other stuff

For humidity, don't expose the camera to sudden changes in temperature. For rain, carry a plastic bag and an umbrella around. If you want to take photographs when it's raining heavily, drape the camera and lens with a plastic sheet, take your photographs quickly and put the camera away. Or even better, get someone to hold the umbrella for you.

If I go to a particularly humid place, I tend to send my camera in for a routine service and clean after the trip. It'll cost you a small amount, but it's certainly cheaper than getting fungus off the sensor!

Sorry for the mega-post, hope this helps.