Vampire finch (Geospiza difficilis septentrionalis)

This villain’s haunt is worthy of its own gothic novel. Isolated by 620 miles (1,000km) of ocean on all sides, Wolf Island looms eerily over the Pacific. It is the most remote outpost of the Galapagos – a barren land of razor-sharp lava formations, tangled mangroves and brutal heat. And it is ruled by vampires.

The tiny, ordinary-looking vampire finch is a close relative of its seed-eating neighbour, the sharp-beaked ground finch. But looks belie a grisly secret.

Although they still eat seeds and grubs, the birds have adapted to island life by turning their beaks to more violent use. To feed, they simply hop aboard a larger bird, such as a blue-footed booby, and peck at the tail feathers until they are sitting in a pool of blood. Then the vampires jam in their beaks and go to town. They are particularly fond of defenceless chicks cowering in their nests.

Perhaps the most sinister aspect of the whole ordeal is that the victims barely flinch as the finches rip their skin off. One theory is that the vampires used to pick ticks off the birds – they may have discreetly taken their services to the next level. Or perhaps resisting just isn’t worth the effort.

The finches’ blood-drinking habits have allowed them to thrive even in the driest months, becoming the most numerous birds on the island. At peak feeding times, finches can be seen lining up behind a victim, patiently waiting their turn to dine.

Assassin bugs (Reduviidae family)

In woodlands and rainforests across the globe, a cold-blooded killer stalks the night. The aptly-named assassin bug has it all – stealth, strategy, and a lethal weapon.

The 7,000 or so species vary widely in their diets – some target bees while others, confusingly, suck the blood of blood-sucking vampire bats – but all are equipped with their own sinister multi-tool, the rostrum.

While other predators go to the trouble of killing their prey, the assassin bug uses its rostrum to inject live victims with cocktail of enzymes and digest them from the inside out. As the target animal turns to soup, the assassin bug’s beaklike projection then doubles as a drinking straw for slurping up the soup – whether the victim is alive or dead.

One species of assassin bug glues its victims’ shrivelled corpses to its shell as extra armour

Most assassin bugs feed on insects, which they ambush using a range of nasty tricks. The species Stenolemus bituberus hunts spiders on their own webs, luring them to their doom by gently plucking the delicate silk to mimic the vibrations of entangled prey. Then it leaps out of hiding.

But for sheer guile you can’t beat Salyavata variegata, which goes fishing for termites. First it finds some bait: it waits by the entrance to a nest, impales a worker, and sucks it dry. Then it thrusts the victim back inside. Invariably it emerges with another live termite to attack, which has clung to the body of its dead comrade – undone by a powerful instinct to pick up and remove termite corpses from the nest.

Another species, Acanthaspis petax, preys on ants. It puts its victims’ shrivelled corpses to particularly macabre use, gluing them to its shell as extra armour. Some have been seen with as many as 20 individuals piled up.

Unfortunately, humans haven’t escaped the assassin bugs’ attention. The ‘kissing bug’ has been drinking our blood for thousands of years. So-named because of their hideous habit of attaching themselves to peoples’ faces as they sleep, they even managed to annoy the world’s most famous biologist. Charles Darwin encountered them on his iconic voyage aboard the Beagle. He later wrote of the experience: “It is most disgusting to feel soft wingless insects, about an inch long, crawling over one's body.”

The bugs are the leading source of Chagas disease, caused by protozoans which live in their gut and contaminate the wound as they feed. It’s a silent killer, quietly ravaging a person’s heart for the rest of their life. Some believe it may have been responsible for Darwin’s death.

Vampire flying frog (Rhacophorus vampyrus)

The mist-swept forests of Southern Vietnam are some of the wettest places on Earth, permanently submerged in clouds which drench every surface in the canopy. It is ideal for spotting amphibians – but not obvious vampire territory.

Or so biologist Jodi Rowley thought when she visited in 2010. Before long her team had found a species of flying frog entirely new to science.

Back at the Australian Museum in Sydney, where Rowley works, she was having difficulty seeing the tadpoles’ tiny eyes. She popped one under the microscope to get a better look.

“It was to my great amazement that I saw these curved black fangs sticking out! I just assumed that they’d have the normal mouthparts for tadpoles which, you know, are quite boring beaky things,” Rowley told BBC Earth. Why would a tadpole need fangs?

The frogs live their entire lives in the treetops, where they use their webbed fingers and toes to glide among the trees.

Instead of risking predators by laying their eggs in streams or pools on the ground, females place the eggs above water-filled holes in the trees, whipping them up into a sticky foam with their back legs.

As the tadpoles hatch, they liquefy the foam and drop into the water below. But there is nothing for them to eat, so the mother returns to the hole and lays more eggs.

“They don’t suck blood or anything, they use the fangs to scoop the eggs up into their big mouths, and they suck them down whole.” Rowley explains.

Kenyan jumping spider (Evarcha culicivora)

It’s hard not to feel sorry for the Kenyan jumping spider. The arachnid, which stalks the walls of people’s homes on the shores of Lake Victoria, loves nothing more than a refreshing drink of human blood. But fate has been unkind: the spiders lack the specialised mouthparts needed to pierce people’s skin.

It turns out what the spiders prize above all else are fluffy, luxuriant antennae

Instead they must get their fix indirectly, which they do by preying on blood-filled mosquitoes. They are the only animals known to choose their prey based on what it has eaten, and the spiders are extremely fussy.

Given the choice, they only eat female Anopheles gambiae mosquitoes, the main malaria vector in Africa. But picking out a single species from the swarms of insects in the region is no mean feat.

The spiders distinguish Anopheles mosquitoes by the 45 degree angle of their bodies as they rest, and they can distinguish a mosquito that is full of human blood from one that isn’t by smell alone. What Ximena Nelson from the University of Canterbury, New Zealand, wanted to know was: how do they tell the females – which drink human blood – from the males that don’t?

To find out, Nelson launched a bizarre experiment, which involved painstakingly constructing a series of mosquito monsters worthy of Frankenstein. She cut the heads, thoraxes and abdomens off males and females from two different species, and glued them back together in different combinations. Then she mounted their bodies in life-like postures, and presented them to the spiders.

It turns out what the spiders prize above all else are fluffy, luxuriant antennae; they went for the creations with female heads every time.

Tongue-eating louse (Cymothoa exigua)

In January 2015, the internet retched in unison when a Nottingham mother opened a tin of tuna to find a pair of beady eyes peering back at her. The instigator of #tunagate turned out to be a Cymothoa exigua. It is a louse with a life so implausibly creepy, you couldn’t make it up.

The parasite starts life as a male in search of a fish. Once it has found a suitable victim, it enters through the gills, crawls into the mouth, and undergoes a transformation.

It plunges its legs into the base of the fishes’ tongue and gorges itself on their blood, growing enormously and turning female at the same time. Its eyes shrink and its legs expand.

Eventually, the fish’s shrivelled tongue falls off, and the louse replaces it with its own body. From then on, the fish uses the parasite as a prosthetic tongue. The female mates with males living in the gills, giving birth to a brood of live male parasites that swim off to start the whole grisly process again.

Vampire moth (Calyptra thalictri)

They may look harmless, but not all moths are after nectar. Calyptra moths are found across Europe where they mostly use their piercing mouths to drink from flowers and drill under the skin of fruit.

But some have evolved more bloodthirsty ambitions. Siberian Calyptra thalictri have applied their long, barbed tongues to tap into the blood of vertebrates, including humans. Male Calyptra moths from Asia will feed on gigantic prey, tackling cattle, rhinos, and even elephants.

Vampire fish (Vandellia cirrhosa)

Move over, piranhas. The tiny, translucent candiru, native to the Amazon Basin, is the stuff of a traveller’s worst nightmares.

The voracious catfish can wriggle into even the tiniest orifice and latch on, securing itself in place with backward pointing spines on its gills. Some species are just a centimetre long, although they can grow to around 40cm.

Candiru achieved legendary status in the 1990s after one reportedly swam up a man’s urine stream and lodged its body in his urethra. Thankfully, that is almost certainly an urban myth.

They mostly latch on to the gills of other catfish, although they are known to occasionally wriggle into open wounds.

The vampire bacterium

Micavibrio aeruginosavorus is the world's smallest known predator. The tadpole-shaped bacterium feasts on other bacteria, sinking its "teeth" into their cell walls and slurping up their insides.

It was first discovered over 30 years ago, but it has been difficult to study because it gets contaminated in the lab by the bacteria it feeds on. “You can give it all the nutrients it needs to survive on its own and it simply won’t grow,” explains Martin Wu, a professor of biology at the University of Virginia.

But this vampire is one of the good guys.

Its favourite meal is the pathogen Pseudomonas aeruginosa, responsible for life-threatening lung infections in cystic fibrosis patients. P. aeruginosa is hard to treat because it’s usually walled up behind a sticky glue, which is resistant to antibiotics. But M. aeruginosavorus can swim through these “biofilms”, killing the bacteria that drugs can’t reach.

“It could be the first living antibiotic,” says Wu.

The dodder plant (Cuscuta genus)

The monsters in The Day of the Triffids have nothing on this alien weed. The dodder plant sniffs out its victims, whispers them into a defenceless slumber, then eats them alive. Although the dodder was once confined to Europe, it now thrives on most continents.

Like all other plants, the dodder’s tendrils are capable of absorbing the Sun’s energy – but they don’t. Instead of growing away from the shade like most plants, the dodder uses the same light cues to grow towards it – and into the path of a potential victim.

“At the same time it also seems to be able to detect the scent given off by plants,” says Jim Westwood, a professor of plant pathology at Virginia Tech. “That might help it choose [a target].”

Once it has found a host, it gains nearly all its food by tapping into the plant’s veins using specialised suckers. But the parasite needs to know how the host is doing, so it also hacks their local “internet” and steals messages encoded in RNA.

It talks to them as it feeds, sending over its own little RNA memos. “We don’t yet know what they’re saying, but it can’t let the host put up a wall or block the feeding. It may be manipulating the host directly,” says Westwood.