These, while not lethal, generally do their best to make your life as miserable as possible. In fact, it probably is one of their primary functions in the ecosystem to torture, abuse and kick the stuffing out of herpetologists in the field. Still, unlike the Horrors below, these are tolerable once you get used to them.
Leeches - Tax-Collectors of the Rainforest
Nothing characterizes the tropical rainforest experience more than leeches. There have been days where I have come back to camp with what looked to be bunches of grapes hanging off my ankles, my white socks a deep, rich crimson through and through, and the blood showing up on my jeans and even on the black canvas of my jungle boots.
Leeches are, however, perfectly harmless. Unlike mosquitoes, they carry no disease whatsoever. Unlike ants, their bites are usually completely painless. And unlike ticks, after they have drunk their fill, they drop off and crawl away on their merry way, leaving you at peace. So leeches are really quite amiable companions, and I would not even regard them as a "torment" as such if their bites were not so damned itchy. Their bites can take a week or two to completely heal, and all the while they are itchy, itchy, itchy. For days afterwards you will be itching. Sitting around a meeting table trying to get your point across is very difficult when you have to reach down underneath the table every two minutes to reach into your shoes and scratch your ankle.
You cannot avoid leeches. But you can reduce your "leech-load". Leech-socks, or, alternatively, tucking your trousers into your socks and firmly tying up your laces can bring you down to 1 or 2 leeches max. Many people go to the length of spraying their footwear and leggings with insecticide such as Baygon. I don't. I don't like it, and I don't think that it is healthy. Another common measure is to spread Tiger Balm on your legs and shoes. Haven't tried this myself, so I cannot make any regarding its effectiveness. My personal take on leeches is that these guys are just exacting "tax" - they are there whether you like it or not, and, no matter what you do you are going to pick up a leech or two. Leeches are part and parcel of the rainforest experience, to the extent that I've come to welcome the presence of leeches as indicating that I am out in some good lowland or hill forest, where the relatively intact canopy cover keep things ever-moist. As far as the torments of the rainforest go, leeches are relatively benign.
Ants - A Pain in the ... Well, Everywhere That They Bite
These guys are everywhere. Some are very small, and some are very large. Many are harmless, but many more can help you experience exquisite new levels of pain.
While in the bush, you come across the giant forest ant (sp.) very frequently. This is a large black thing, with a distinct orangish-red gaster (abdomen). Most often, you see it wandering around by itself on the forest floor. It is probably not very abundant, but it is widespread and conspicuous. I have never been bitten by this ant, and while it probably can deliver a powerful bite, it does not seem very aggressive. By the way, the juices in the belly of this ant are supposed to taste like Tom Yam - I'm told that there has been more than one jungle "Tom Yam" meal whipped up by adventurous campers.
There are other ants to be more concerned about. Stretched out under the tangled vegetation on the banks of a creek, photographing a calling frog, you do not notice the ant highway on the branches above being disturbed, resulting in a stream of the ants parading down your back. Till they bite. And bite. And bite. And bite. Hours later, after having washed most of these guys off your body, just as the excruciating pain has died into a dull throb, a scream erupts from you as a straggler still roaming about your body decides that he has had enough of you and digs into you. I am not kidding about the pain. It is hard to imagine that something so small can be so painful. Once, while climbing a rock face, I put my hand on a ledge above me to pull myself up. It felt like I had touched a fiery hot griddle. I yelled in a suitably manly way, and instantly retracted my hand. It was exactly like being burnt. I tried another place on the ledge, and seconds later I was bitten again. Again, it was like being burnt, and I reflexively jerked my hand back.
I cannot think of a good way to avoid these guys, except be careful and count on luck. Neither can I think of a good remedy except for general-purpose insect bite/sting medication and treatment (usually topical). Most of the more vicious bites remain sore for a while, and some become itchy after a couple of days. This extended reaction is probably the result of your body's immune system reacting to the venom of the bite. In these cases, some antihistamines might be useful.
Mosquitoes, Midges, Sandflies and Kin - Unwanted Dinner Guests
These guys are everywhere, but surprisingly not encountered as often as you might believe in the tropical forest at night. I must say that I do not strongly associate mosquitoes with any of my frogging experiences. There have been many nights in many forests where I haven't even been bitten once. On other nights, there have been particular stretches or points that seemed to be mosquito-infested, but the night has generally been otherwise mosquito-free. There have been nights, however, where I have been eaten alive, bit by tiny bit. Worse than the mosquitoes are the little midges or sandflies - tiny things, so small that you cannot see them, that bite your earlobes, eyes, face etc. Can drive you nuts. Insect repellent helps, but not too much. Do not use anything with DEET. Horrible for you. Horrible for the environment. And horrible for you cameras and most other equipment (DEET attacks the polycarbonate shells of the body and lenses, as well as plastics etc.).
Rattan - Caresses with Fish-hooks
Many plants in the tropical rainforest are festooned with spikes, spines and thorns. Not to the extent of desert vegetation, where it seems as if every plant is bristling with all sorts of thorns of various sizes, but, nonetheless, tropical forest vegetation can stand proud in any sado-masochistic parade. I have seen some trees that look like medireview torture devices, and others like some crazy weapon designed for fantasy evil overlords. The most commonly-encountered, and the most annoying, though, must be rattan. This spiny palm grows upwards, and, like its close affiliate, bertam, is fairly well-armed with arrays of thorns. What makes rattan particularly insidious, though, is that the ends of the branches usually have long, trailing tendrils that bear rows of small, sharp, recurved thorns. These things are vicious! It is all too easy to brush up against a dangling tendril, and the result is that dozens of the razor-sharp thorns hook onto your clothes, backpack or flesh. Freeing yourself is no easy task - almost always you will need to solicit the help of a friend, who in turn has to exercise great caution while gingerly unhooking the tendril, thorn by thorn. Brute force makes things worse, as both the tendrils and thorns are strong and resistant to breaking, and the recurved shape of the thorns means that almost always the tendrils simply get even more of a tight grip on you. I have been reduced to utter and complete immobility once, literally pinned to the ground on my hands and knees, due to an attempt at the brute-force approach. I had brushed up against a couple of rattan tendrils while bushwhacking through a thicket, and, instead of stopping to delicately and deliberately extricate myself, I decided to just push on through. In less than a minute my struggles had reduced me to a utter helplessness, trapped and pinned down by a tangled network of rattan tendrils, incapable of any movement. My friends, responding to my piteous calls for help, were quick enough in the rescue, but the embarrassment of the affair has taken somewhat longer to die down.