After leaving Tacuapí, I had planned to visit a cataract on the Río Uruguay called Moconá Falls. A geological oddity, the waterfall slices lengthways up the river, reaching the Brazilian bank two miles to the north. Paradoxically, the falls are more impressive in times of drought, when they can reach nearly 70 feet in height.

I was out of luck: heavy rain had swollen the river and rendered the cataract invisible. I took a bumpy, 15-mile trail instead, pulling up at Don Enrique Lodge, where three impeccably decorated cabins overlook a gurgling forest brook and the dense foliage of Yabotí Biosphere Reserve on the far bank.

Eliseo Lemos, the lodge’s guide, rowed me across the stream in the fading afternoon light, recounting anecdotes and tidbits on jungle survival as we walked through the forest’s pristine core.

In the 1920s, Eliseo’s great-grandfather had received 125 acres from the Argentine government in return for settling near the country’s frontier. “It was a different era back then,” he told me. “They gave him a medal for shooting 68 jaguars and built him an access road for felling 50 trees.”

Yet the early settlers also learned less destructive techniques from the Guaraní, trapping tapirs by luring them with fermented sap from the pindó palm and fishing streams with a liana whose sap paralyzes fish gills. “The liana contains a chemical so strong that you can just scoop the fish out by hand,” he said.

From Don Enrique Lodge, a little-traveled road heads north, rising and falling with the sierras’ forested gullies. Human population is scant. I spotted a few lonely pioneers eking out a living in clearings hacked from the jungle before navigating a treacherous, 35-mile mud trail that led to Yacutinga Lodge.

Sitting on a 1,400-acre tract of jungle reserve, east of Iguazú national park, Yacutinga is widely considered by serious nature buffs as Misiones’ most authoritative hotel. Its 20 rustic cabins, assembled on frames of natural-fall trunks, are half-hidden by a cascading mass of lianas and palm fronds.

Every step through the reserve’s pristine heart, too inaccessible to have ever been logged, seems to reveal some fascinating plant or creature: the gossamer of a garden spider, so resilient that two or three entwined strands provide a sturdy fishing line; giant bromeliad stems that collect rainwater; and the yacarati-á tree, whose edible pulp is rich in proteins.

Yacutinga’s great strength is the up-close contact it provides with nature: biologists working at its research station have discovered several species of butterfly new to science; even paying guests devote multi-day stays to the study of birds, butterflies, orchids or medicinal plants.

Yet the lodge’s most beguiling delight — The Insect Show — is perhaps its simplest. By day, the white fabric screen, suspended among the trees, attracts curious, uncomprehending glances from first-time guests. Spotlit by night, however, it writhes with a slithering morass of beetles, moths and bugs.

In Misiones, binoculars are indispensible; a magnifying glass, it turns out, is not.