What these animals lack in size, they make up for in their ability to make you wish you were dead, thanks to their painful bites and stings. Sometimes, these tiny creatures can even kill.

1. Irukandji Box Jellyfish

Jamie Seymour

Believed to be the most venomous creature in the world, the Irukandji is roughly the size of a peanut. It thrives in warm water, making Australia’s Great Barrier Reef its preferred home. This hard-to-see creature is outfitted with tentacles and has the ability to fire its stingers into its victim. If you are hit, you will experience what is described by victims as “the worst pain of your life—pain so intense that the maximum dose of morphine barely takes the edge off.” According to the documentary Killer Jellyfish, whether you live or die from an Irukandji sting depends on your current state of health as well as “ your blood vessels' ability to handle the pressure.” There is no antidote for an Irukandji sting. Hospitals help to manage symptoms, but if you are stung, you simply must wait it out. “No single pain killer works for everyone, except for time.”

Where it Lives: While Irukandjis prefer the warm waters of Australia, they have been spotted in the British Isles, as well as in Japan and in waters of the Atlantic Ocean.

The Sting: At first, the sting from an Irukandji is not too bad. You may experience some swelling along the site of contact and some minor discomfort. Five to 45 minutes later, though, the real pain sets in. Victims will start to experience a severe backache, headache, or shooting pains in their muscles, chest, and abdomen. Additional symptoms may include nausea, restlessness, and vomiting. If left untreated, “irukandji syndrome” can lead to pulmonary edema, a build-up of fluid in the lungs that can be fatal if not properly treated.

Scary Fact: The Irukandji is only 2.5 centimeters in diameter and weighs in at less than 1 ounce. The jellyfish’s four stinging tentacles are nearly as fine as a hair and trail up to 30 centimeters behind it. According to Jamie Seymour, a professor of biology at James Cook University who has made a career out of tracking these tiny predators, the Irukandji are quick and agile swimmers. Certain species of box jellyfish can swim almost as fast as an Olympic swimmer, giving them the ability to navigate through the water at speeds of up to four and a half knots!

Do Stinger Suits Work? Australian Medical Association of Queensland president Dr. Alex Markwell points out that the Irukandij are small and transparent, which makes them very difficult to see and avoid. “Barrier protection such as Lycra sting suits or wetsuits help, but often hands, feet and face are still uncovered leaving people vulnerable to stings,” she explains.

Video: In this clip from Killer Jellyfish, Seymour and Teresa Caratte get stung and deal with the repercussions in a nearby hospital.

Seymour’s symptoms last for a reported 2 days while Caratte battles Irukandji syndrome for a whopping 2 weeks!

2. Asian Giant Hornet (Vespa mandarinia)

Takehiko Kusama

In the States, we make fun of our friends for overreacting when a winged insect flutters by, but Japanese residents have a valid excuse for panicking. The Asian giant hornet is two inches long with a wingspan of around three inches and an oversized mandible with a strong black tooth. These giants of their species have the ability to dispel venom with an enzyme so strong that it’s said to be able to dissolve human tissue. The Asian giant hornet kills more people each year than any other animal in the country of Japan. Fortunately for humans, the hornet’s preferred victim is a honeybee. Each hornet has the ability to take out around 40 European honeybees in a mere 60 seconds, and it’s been reported that just a handful of these insects can kill over 30,000 bees within hours. While they don’t tend to go after humans, a hornet will sting you if you aggravate it. In this scenario, your normal escape plan of running away will not save you as these creatures can fly at speeds of 25mph!

Where it Lives: According to Ross Piper, author of Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, these massive insects prefer to live in the mountainous region of Japan. They have also been spotted in areas of Russia, Korea, China, Taiwan, Nepal, India, and Sri Lanka.

The Sting: Dr. Masato Ono, an entomologist at Tamagawa University has stated that the sting of a Japanese hornet is exceptionally painful, describing it as “a hot nail through my leg!”

Crazy Fact: In lieu of steroids, some professional athletes are now ingesting vials of Asian hornet vomit. The larvae of these hornets are said to regurgitate a liquid which, if ingested by their parents, gives them the ability to fly over 60 miles at a speed of 25mph. A synthetic form of this hornet juice is being sold on the market and has been used by professional athletes in competition. Long distance swimmer Naoko Takahashi claimed to have had some of the juice before her record-setting gold medal swim at the 2000 Olympics in Sydney.

How To Avoid Being Stung: Piper says the best way to avoid a run-in with an Asian giant hornet is to steer clear of their homes, which are commonly found in forested areas—in abandoned animal burrows, the hollow beneath a tree, tree holes, etc. “As with all stinging insects, the key to not being stung is not to annoy or threaten them. All wasps are very protective of their nests and this is when stings are most likely, i.e. if someone gets too close to the nest or disturbs it."

Video: In this video clip from Hornets From Hell, you’ll see these creatures in action and will hear a first-hand account of a scientist who was stung in the field.

Additional Source: Hornet Venoms [PDF]

3. Candiru

Dr. Peter Henderson

Among the strange creatures that live in the Amazon River is a parasitic eel-like fish known as the Candiru. These tiny creatures feed mainly on blood, can often be found inside the gills of other fish, and have been known to swim up a human urethra. Once they wiggle their way inside, they expand, nestle in, and send you to the emergency room in need of an invasive medical extraction. Though sometimes considered an urban legend, the Candiru is a real threat to unsuspecting swimmers. In 1877, an Amazon physician named Dr. Castro performed one of the first known extractions of a Candiru from a urethra, stating: “I have myself extracted from the urethra of a negress a little candiru which had penetrated during micturition while bathing in the river. The patient experienced cruel suffering for since I had to drag the animal out. The extraction was difficult and the mucus membrane was lacerated.”

What Makes It Even More Terrifying: Once the Candiru travels through a urethra, it spreads its gills, which are outfitted with spikes, as it gasps for oxygen. As you can imagine, the process of being “invaded” by a Candiru is quite painful. Housing a Candiru in your urethra can lead to inflammation, hemorrhage, and, in the worst-case scenario, amputation or death.

Urban Legends: Rumor has it that this fish is attracted to the smell of urine—so attracted, in fact, that it’s said to be able to jump up out of the water and into the urethra of a man peeing at the water’s edge. In actuality, Candirus are said to track down their fish victims by sight and are incapable of physically swimming up a stream of urine. They can, however, easily slide into your genitals if you are already in the water.

Where it Lives: The Candiru favors the water of the Amazon River and lives in the countries of Brazil, Peru, Ecuador, Colombia, and Bolivia.

How To Avoid a Run-in with This Creature: The obvious solution is not to swim in the Amazon. If you must do so, wear briefs or some kind of protection under your swim trunks and by all means, do not pee in the river.

Video: In this video clip from River Monsters, you’ll meet a gentleman who had a Candiru in his urethra for four days before making it into the ER in Manaus, Brazil for surgery. Watch at your own risk!

Additional Source: Candiru: Amazonian Parasitic Catfish [PDF]

4. Bullet Ant (Paraponera)

Gerald Urquhart

Known by the rainforest locals as “hormiga veinticuatro” or “24 hour ant,” the bullet ant is said to possess the most painful insect bite in the world. Their venom is a neurotoxic peptide, which is used to kill caterpillars and other small critters, but to people, a bullet ant sting is one of the most painful experiences in the world. One sting from a bullet ant will cause swelling resulting in pain for an entire day. Aggravating one of these creatures—actually the biggest ant in the world—is something you’ll regret for quite some time.

Where They Live: Bullet ants are found in tropical rainforest areas across Central and South America.

The Sting: According to the Schmidt Pain Index, a bullet ant sting ranks among the worst pain you will ever feel. If an ant feels threatened, it will use its mouth to grab onto you before it injects you with its venomous sting. Terry McGlynn, Associate Professor of Biology at California State University Dominguez Hills, was stung while working in a lab with bullet ants. He describes his experience as follows:

Imagine that you put your finger flat on the countertop and hand someone a hammer and ask them to hit your finger as hard as they can. That's what the sting itself feels like. Then, it ached just like you would expect for a few hours, throbbing and swelling enough that it was difficult to concentrate on work. Then, for several hours, I lost the muscular strength to hold on to things with my hand, so I couldn't hold my coffee mug alone in the hand that was stung. Then, it was numb for several hours, and then the next morning it felt like nothing had ever happened.

How Badly Do You Want to be a Man? In the Satere-Mawe tribe of the Amazon rain forest of Brazil, bullet ants are a part of a traditional coming-of-age ritual. A boy as young as twelve must stick his hand into a pair of hand woven gloves, which is filled with bullet ants. The boy must do this 20 times in total, ten minutes at a time. This ritual is said to test “one’s worthiness to take on adult roles.”

Tip for Avoiding Being Stung: McGlynn says it comes down to being careful as to where you put your hands. “[Bullet ants] typically nest at the base of large rainforest canopy trees and walk up the trees and other routes to forage for sugar water and prey in the canopy,” he says, adding that the way you’d typically come into contact with one is to accidentally rest your hand on a tree trunk. If you see an ant, McGlynn advises that you refrain from swatting at it, which "will only make it mad.” He also says you should keep your composure in the event one walks on you: “If you stay calm, you'll be just fine. But if it's disturbed, then it might sting you.”

Video: In this clip from National Geographic, you’ll follow the journey of Ted from the Satere-Mawe tribe as he sticks his hand into the ceremonial gloves that will transform him from a boy to a man.

5. Golden Poison Dart Frog (Phyllobates terribilis)

Thinkstock

The Golden poison dart frog is not an animal that you would ever want to come in contact with: Phyllobates terribilis is the most poisonous vertebrate on the planet. The frog is currently on the endangered species list and organizations like the World Land Trust are fighting to keep it alive. The frogs range in size from half an inch to 2 inches long. One frog is said to house enough deadly poison to kill ten grown men or 10,000 mice. In addition, the tiny frog is so toxic that it’s said that merely touching a paper towel that has been in contact the frog can be fatal. Tribes of the rainforest have traditionally used its poison on the end of their blowgun darts during hunting ceremonies.

Where They Live: The frog makes its home in a small coastal region in southwestern Colombia, but scientists aren't sure of the actual extent of its range.

The Poison: The golden poison dart frog's skin is saturated with an alkaloid poison that contains batrachotoxins, which prevent nerves from transmitting impulses and lead to muscle paralysis. The poison is about 20 times more toxic than a typical poison dart frog; a single frog's skin contains enough poison to take down two full-grown bull elephants!

Strange Fact: According to Oswaldo Cortes, who led a study on the frogs in conjunction with the Conservation Leadership Program, The golden poison dart frog gets its toxicity from the mites that it ingests. “Traditionally scientists thought the frogs obtained their venom from ants, however, a new study in the Proceedings of the National Academy of Sciences has revealed it is in fact mites that causes them to be so dangerous.” The frog then secretes the poison through its skin glands. The toxins are used to defend the frog against a predator; it does not use its toxins to hunt for food. Since the frog gets its toxic nature from its meals, golden poison dart frogs held in captivity will not be nearly as toxic.

How to Keep Away: It’s plain and simple—don’t touch it! Avoid areas where golden poison dart frogs thrive, such as rainforests of Colombia. If you’re in their habitat, wear protective boots and long pants. In the rare occasion that you brush up against one, you’ll probably want to throw that outfit away.

Videos: You can see a golden poison dart frog in a terrarium in the video below.