Panama is a paradise for people who like wildlife, although some of that wildlife is considerably more enthusiastic about surprising you than posing for photographs. Walk through a tropical forest after dark, lift a damp piece of wood, move a rock beside a trail or spend enough time around a jungle hostel and eventually you are likely to encounter one of the country's many centipedes or millipedes.
They are easy to confuse because both have long, segmented bodies and an absurd number of legs. But they are actually very different animals. One is a fast moving predator equipped with venomous legs, while the other is mostly a peaceful decomposer carrying a chemical defense system.
The easiest rule is wonderfully simple: centipedes have one pair of legs per body segment; millipedes generally have two pairs per segment. Centipedes also tend to look flatter and move much faster, while millipedes are more cylindrical, slower and often curl into a coil when disturbed.
The centipede: the one to respect
If you see something long, flat, fast and apparently determined to disappear underneath your bed before you can identify it, there is a good chance it is a centipede.
Centipedes are predators. Rather than eating leaves and decomposing vegetation like millipedes, they hunt other animals, including insects, spiders and other small creatures. Their first pair of legs has been modified into venom delivering structures called forcipules. In other words, those aren't ordinary legs. They are specialized weapons.
Panama has several kinds of centipedes, including members of the large Scolopendra group. These are the centipedes that deserve the most respect. Tropical Scolopendra can become surprisingly large, and members of the genus are capable of delivering painful venomous bites.
They are particularly interesting at night because they are active predators. A flashlight suddenly revealing a large centipede sprinting across a jungle floor is one of those moments when you immediately remember that you are not actually alone in the forest.
Giant centipedes are impressive predators
The large Scolopendra species are among the most formidable arthropods you can encounter in Panama. They are muscular, fast and extremely well adapted to hunting in dark, humid environments.
Their venom is primarily designed to subdue prey rather than kill humans. Nevertheless, a bite from a large centipede can be very painful, with localized swelling, redness and inflammation. In most cases symptoms resolve relatively quickly, although reactions can vary.
The important thing is not to panic if you see one.
A centipede isn't looking at you thinking, "Excellent, a tourist."
It is much more interested in cockroaches, beetles, spiders and other small animals.
Millipedes are a completely different story
Millipedes look frightening because they have so many legs, but they are generally much less dangerous to people.
Most millipedes are detritivores, meaning they spend their lives feeding on dead and decomposing plant material. They are part of the machinery that turns fallen leaves, wood and other organic matter back into nutrients for the forest.
And Panama is basically millipede heaven.
Warm temperatures, enormous quantities of vegetation and high humidity create exactly the sort of environment in which millipedes thrive. During particularly wet periods you may see them wandering across trails, appearing underneath logs or even turning up around buildings.
They may look like something from an alien movie, but ecologically they are little recycling machines.
The best way to tell them apart
If you only remember one thing, remember this:
Centipede = one pair of legs per body segment.
Millipede = two pairs of legs per body segment on most body segments.
There are also some much easier visual clues.
A centipede usually has a relatively flat body, long legs sticking out to the sides and an unmistakably quick, predatory movement. It tends to run rather than slowly crawl.
A millipede usually has a more rounded or cylindrical body, much shorter legs and a slow, deliberate movement. When frightened, many species curl themselves into a spiral or tight coil, protecting their vulnerable underside with their hard body segments.
So if it looks like a tiny armoured train and moves at the speed of a lazy caterpillar, you're probably looking at a millipede.
If it looks like an evil feather duster with legs and suddenly disappears under the nearest piece of furniture at 40 kilometres an hour, it's probably a centipede.
But millipedes have their own chemical weapon
This is where things get interesting.
Millipedes don't have venomous jaws and they don't bite people as a defense. Instead, many species have specialized defensive glands that release irritating chemicals when they are threatened.
Depending on the species, these secretions can contain compounds such as benzoquinones, phenols, alkaloids and, in some groups, cyanogenic compounds.
That sounds terrifying, but there is an important distinction.
A millipede is not normally trying to poison you. It is trying to convince a predator that eating it is a terrible idea.
The classic response is to curl up and release a chemical defense.
And that is why you shouldn't pick up an unfamiliar millipede with your bare hands, particularly in a tropical environment.
The "millipede burn"
The chemicals released by some millipedes can irritate human skin and can produce discoloration, redness, burning or blistering. Contact with the eyes can be considerably more serious.
This is sometimes referred to as millipede burn.
The safest approach is simple: don't rub your eyes after handling one, wash your hands if you have touched one, and keep millipedes away from your face.
If defensive chemicals get into the eye, the eye should be irrigated immediately with plenty of clean water and medical attention should be sought if irritation persists or the exposure is significant.
So the millipede isn't dangerous in the same way as a centipede.
The centipede can bite you. The millipede can chemically irritate you.
Which ones should travelers actually worry about?
For most visitors, the large Scolopendra type centipedes deserve the most respect.
If you encounter a big centipede in Panama, don't attempt to pick it up, trap it in your hand or encourage it to crawl across your arm for the perfect Instagram photograph.
Give it space.
Large centipedes can deliver painful bites, although serious complications are uncommon.
With millipedes, the concern is different. Don't handle them unnecessarily, especially brightly colored or unfamiliar ones. Their chemical defenses vary considerably between species, and you don't need to discover which chemicals a particular millipede produces by rubbing it into your eye.
Why you see so many after rain
If you visit Panama during the rainy season, you may suddenly notice enormous numbers of little creatures appearing on trails and around buildings.
Humidity changes the whole underground world.
Millipedes become more active, and other arthropods emerge from damp soil, leaf litter and rotting wood. Centipedes, meanwhile, have more opportunities to hunt because their prey is also becoming active.
This is one reason a rainy night in Panama can feel like someone has switched on an enormous hidden ecosystem.
The jungle floor that looked almost empty during the afternoon suddenly becomes busy.
Don't confuse a centipede with a harmless millipede
For travelers, the easiest practical identification system is:
Flat + fast + one pair of legs per segment = centipede.
Round + slow + two pairs of legs per segment = millipede.
Centipede = predator with venomous forcipules.
Millipede = decomposer with chemical defenses.
And if you're unsure, the best strategy is the same for both:
Don't touch it.
You don't need to know the exact species to safely admire it.
Panama's little jungle army
Centipedes and millipedes are easy to overlook compared with Panama's monkeys, sloths and spectacular birds, but they are an important part of the country's tropical ecosystems.
The centipedes are predators, helping control populations of other small creatures. The millipedes are recyclers, breaking down dead vegetation and returning nutrients to the forest floor.
Together they represent two completely different solutions to surviving in the same jungle.
One became a fast, venomous hunter.
The other became a slow-moving armoured recycler with a chemical stink bomb.
And that is one of the great things about walking around Panama. You never quite know what you are going to find underneath the next log, beside the next trail or crossing the floor after dark. In a country this biologically rich, even the creatures with far too many legs have a fascinating story to tell.