As Told to Andrew Taber
THE THOUGHT OF BEING categorized as a missing person was hard to accept. When there's no body to be found, the grieving has no end. For our families back in France, we had to survive.

I was lost in the Amazon with my friend Loïc Pillois. We had gone there last February to hike an approximately 78-mile section of virgin forest that was supposed to take us from our drop point on the Approuague River to Saül, an isolated former mining town at the geographical center of French Guiana.

We brought along enough food for the 11 days we had budgeted for the trek, along with a compass, a machete, a 60-square-foot tarp, and two hammocks.

Some days went smoothly, and we seemed to be on track. But on other days it would take several hours of hacking through vines to hike just one mile. On the morning of the 12th day, February 26, we knew we were in trouble. Saül sits in a small valley, but the route ahead of us kept rising. We were exhausted and out of food, and we had no idea where we were on our trajectory. Two days from Saül? Two weeks?

We knew there would be a search, so we decided to stay put. We used the tarp to create a roof, and we divided the tasks. I was in charge of food. Loïc tended the fire. We had only one lighter, so Loïc never let the fire go out. He was incredible.

When it's a necessity, it's easy to tap into the survival instinct. We became very primal. It was the rainy season, so we had water. But we started eating whatever we could. If we thought a plant was edible, one of us would try it. If he was still OK the next day, we knew we could eat it. We ate beetles, and we trapped mygale spiders [tarantulas]. They're huge and hairy. If you cook them well enough, their venom burns off and they become edible.

Still, we were starving. We chewed just to chew. Psychologically, it helped. We even ate bugs that swarmed to our excrement.

Occasionally, we'd hear helicopters. But we couldn't see them, and they couldn't see us or our fire through the thick canopy. Sometimes days would go by with nothing. That was sheer panic. Had the search been called off?

We learned later that the rescue missions did stop, on March 26, 40 days after we'd started our trek.

Around that same time, we decided that we weren't going to be found and abandoned our camp. We calculated which way was west, toward Saül, and left. A week later, we caught a turtle, probably seven pounds. It was our first meat in five weeks, and we ate absolutely everything—skin, claws, scales. We heated the blood over the fire and drank it. Honestly, it tasted amazing.

The next day, I caught another spider. I cooked it, but not enough; the venom was still active. I spit it out, but pain engulfed the entire left side of my tongue. It swelled up and my lips went numb. It was excruciating. I tried to keep going, but I was too weak. I was suffering from intestinal poisoning and couldn't go on. By this point, Loïc had lost 35 pounds and I had lost 57. One time I sat on the ground and felt a small stone jabbing me. I reached around to clear it and realized that what I felt was my own coccyx.

Loïc left me to make a final dive into the jungle in search of help. A day and a half later, on Thursday, April 5, a helicopter came and hovered above the treetops. Loïc had made it. A gendarme rappelled the 150 feet to the ground. He was crying when he took me in his arms. Of course, so was I. After 51 days in the jungle, it was finally over.

They told me I was just two and a half miles from Saül. I couldn't believe it. I had walked 75 miles only to fall in the final three. I was furious—but not at the forest. I was the one who ate that spider. I was responsible for what happened. I'll go back someday. I'll go back with my girlfriend, Emilie, who flew to French Guiana as soon as she heard I was missing. We'll find that last camp and we'll walk those last two and a half miles together.

Expert Analysis: Their desire to survive got them through, but where were the local guides? If you go into the jungle, a guide, or at least a lot of knowledge from local sources, is invaluable. Anyone with jungle time will tell you six miles a day in virgin forest is the best you can hope for. To plan for nearly seven miles a day, with no room for error, suggests a lack of experience.

One lighter between them and no communications kit? An Iridium 9505A sat phone [from $1,000; iridium.com] can be tracked online, and you can get a signal even in the jungle. We also use flares, floating fires that we tie off in the center of a creek or river so the smoke can break the canopy, and tethered location-marker balloons. The balloons weigh about four pounds and have a chemical that inflates when mixed with water. They float up to mark your location.

Not sure about the spiders; ask a venom man. [Ed's note: We did. According to Norman Platnick, of the American Museum of Natural History, it was most likely the spider's barbed urticating hairs, not the venom, that caused Guilhem's violent reaction.] Regardless, there are plenty of other things that are easy to catch, like tortoises, snails, and fish—you should always have a fishing kit in the jungle. —Ian Craddock, cofounder of Bushmasters (bushmasters.co.uk), a jungle-survival school based in Guyana