Although parasites have such a bad reputation that we humans have turned this category of organism into an insult, the truth is that parasitism is a success story of evolution: unlike predation, it allows many species to get the resources they need from other, generally larger, species without killing them. But some of these organisms reach such a degree of sophistication in their survival strategies that they almost make this way of life an art of nature, in many cases even hacking the behaviour of their hosts to adapt it to their own interests.
The fungus that turns ants into zombies
Among the parasites capable of altering the behaviour of their hosts, one of the best known cases is that of the fungus Ophiocordyceps unilateralis, discovered in 1859 in Indonesia by the co-author of the theory of evolution Alfred Russell Wallace, but whose amazing survival strategy has only become understood more recently.
When a spore of this fungus infects an ant, it begins to grow inside it, devouring its tissues. The infection causes the insect to leave the treetops where it resides and descend close to the ground, where it bites forcefully on a leaf before dying. A recent study has found that the fungus colonises the muscles of the jaw, contracting them in that bite from which the ant can no longer free itself. Thus, the parasite keeps its host nailed to the place where the temperature and humidity are optimal for its own growth. Finally, the fungus grows through the insect’s head to produce more spores and spread them in the air in search of new victims.
The worm that compels grasshoppers to drown themselves
Another example of behavioural manipulators is found in certain nematomorphic worms such as Paragordius tricuspidatus or Spinochordodes tellinii. They are free-living animals that live in water and reach about 10 centimetres in length, but their microscopic larvae are parasites of crickets and grasshoppers. When one of these insects ingests the eggs, the worm develops inside it without killing it, modifying its behaviour so that it throws itself into the water. There the adult worm emerges from its host to swim freely, while the insect usually dies by drowning. Studies indicate that the parasite produces certain molecules that modify the functioning of the host’s nervous system.
The snail that serves as bait for birds
Leucochloridium paradoxum is another case worthy of an alien invasion movie. This flatworm develops in the digestive tract of birds, which excrete eggs along with their faeces. But to complete its life cycle, the parasite needs an intermediate host, a snail. The mollusc casually ingests the eggs of the worm, which hatch into a phase called a sporocyst. This grows into long, swollen sacs containing numerous individuals. The sacs then invade the tentacles at the ends of which are the eyes of the snail, so that they turn these appendages into thick, throbbing, brightly coloured tubes that look like larvae or caterpillars to a bird. At the same time, the snails’ vision is affected, so they are more easily exposed in open places. When a bird spots what looks like tasty caterpillars, it rushes to devour the snail, and thus the parasite manages to re-enter the host in whose digestive system it will reproduce once again.
The fly that is transformed into a battery of cannons
The fungus Entomophthora muscae lives up to the translation of its name, “fly destroyer”, and is also part of the legion of parasites that have learned to manipulate the minds of their hosts for their own interests. When a housefly is unlucky enough to be infected by a fungus spore, the fungus begins to grow inside its body, digesting its organs and taking over its brain. The hacked insect feels the irresistible urge to land and then crawl to the highest place it can find, where it dies while the stalks containing the fungus spores make their way through its body. These structures act like a battery of cannons powered by pressurised water. One study using high-speed cameras has calculated that the spores are ejected at about 10 metres per second (36 km/h). According to scientists, these spore discharges from certain fungi reach the highest speeds observed in nature in relation to the size of the organism.
The wasp that parasitizes another parasitic wasp
Looping the loop of parasitism, the small wasp Euderus set, discovered in 2017 and known as the “crypt-keeper”, takes advantage of wasps of another species that are themselves plant parasites. It all starts when the gall wasp Bassettia pallida, about 2 millimetres long, injects its eggs into the tissue of an oak tree, creating a gall. Normally, the larvae of this insect mature until the adult can chew its way out of the gall. But when a crypt-keeper appears, something happens: this wasp lays its eggs on the gall already occupied by the B. pallida. By an as-yet-unknown mechanism, its larvae cause the adult B. pallida to dig holes that are too small for them to get out, and they get trapped with their head stuck in the hole. The larva of E. set then feeds on the body of its prey, until, once transformed into an adult, it makes its way outside by chewing through the head that blocks the exit.
The plant that attracts the parasites of its attackers
In another curious variant, there are certain plants that are capable of influencing the behaviour of a parasite to help fight off the attack of an annoying pest. The so-called pine sawfly, actually a wasp of the species Diprion pini, lays its eggs in the needles of the Scots pine, causing damage to these trees when the larvae feed on the needles on which they are born. However, the plant has an unusual defence mechanism: a compound that coats the insect’s eggs induces the tree to secrete a volatile substance that attracts the wasp Chrysonotomyia ruforum, which parasitizes the eggs of D. pini, thereby helping the pine tree to fend off its bothersome attackers.