When a male sheep crab (Loxorhynchus grandis) is infected by Heterosaccus californicus, a parasitic barnacle, it stops developing fighting claws, and its abdomen widens, providing a womb for the barnacle to fill with its brood pouch. Nurtured by the crab, the eggs hatch. Thousands of baby barnacles disperse to infect anew.

Many parasites not only feed off their hosts, but appear to manipulate the host’s behavior in a way that is advantageous to the parasite’s life cycle. Recent research indicates that this influence occurs at a genetic level—certain parasite genes seem to be able to take control of the host’s brain. Research has shown that in some cases a single parasite gene is responsible for altering the host’s behavior, though in most instances it is thought that the phenomenon is brought about by a combination of genes.

Larvae of horsehair worms infiltrate house crickets when they scavenge dead insects, and then grow inside them. The cricket is terrestrial, but the adult stage of the worm’s lifecycle is aquatic. When the worm is mature, it alters the cricket’s brain, causing it to leap into the nearest body of water. As the cricket drowns, the mature worm emerges.

When a female Dinocampus coccinnellae wasp stings a ladybug, it leaves behind a single egg. After the egg hatches, the larva begins to eat its host from the inside out, carefully avoiding the vital organs. When ready, the parasite emerges and spins a cocoon between the ladybug’s legs. Though its body is now free of the parasite, the bug remains enslaved, standing over the cocoon and protecting it from potential predators. Some ladybugs actually survive this ordeal, re-grow their organs, and make a full recovery.

Hyalella azteca, a tiny amphipod, lives at the bottom of lakes and ponds. Amphipods that have been invaded by the larva of a thorny-headed worm abandon the dark safety of the depths and swim to the surface. For the amphipod, this journey proves fatal, as it is a favored food of waterfowl, but for the larva (turned orange by pigments taken in from the host’s tissue) it is essential. Thorny-headed worms can grow to maturity only in the gut of waterfowl.

After the flatworm (Ribeiroia ondatrae) reproduces asexually inside a snail, its larvae find a bullfrog tadpole and burrow through its skin, forming cysts around the frog’s developing limbs. With legs added, subtracted, or compromised, the ungainly victim is easy prey for frog-eating birds like herons. Inside the heron, the parasite reproduces sexually. Its eggs re-enter the water when the bird defecates, infecting new snails to start another round.

When spores of an Ophiocordyceps fungus land on an Amazonian ant, they penetrate its exoskeleton and enter its brain, compelling the host to leave its normal habitat on the forest floor and scale a nearby tree. Filled with the fungus, the dying ant fastens itself to a leaf or another surface. Fungal stalks burst from the ant’s husk and scatter spores onto ants below, to begin the process again.

Bird droppings gathered by the arboreal ant (Cephalotes atratus) to feed its larvae, sometimes contain a hidden menace: a parasitic nematode, Myrmeconema neotropicum. Once inside the ant, the nematode turns its host’s abdomen maroon, so that it mimics a ripe berry attractive to birds. To make its unwitting host even more enticing, the nematode induces the ant to walk with its abdomen raised.

The parasitic Hymenoepimecis argyraphaga wasp lays its egg on the abdomen of a Leucauge argyra spider. The larva feeds on the spider’s bodily fluids. As the larva pupates, the spider begins to rip down its web and reconstruct one that is perfect for protecting the larva’s cocoon from predators. The larva forms its cocoon after sucking the spider dry.

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