The phenomenon commonly referred to as “zombie spiders” is a real biological occurrence. It involves a spider’s behavior being dramatically altered and controlled by another organism, typically a parasite. These instances demonstrate complex interactions within ecosystems where one species manipulates another for its own survival and reproduction.
The Reality of Zombie Spiders
The concept of a “zombie spider” is a fascinating display of parasitic manipulation in the natural world. A parasite takes control of a host’s behavior, compelling it to act in ways that benefit the parasite, often to the host’s detriment. The parasite essentially hijacks the spider’s natural instincts and bodily functions.
This manipulation is a sophisticated evolutionary strategy, where the parasite’s genes influence the host’s “extended phenotype”—its behavior or physical characteristics. These interactions highlight the intricate relationships that have evolved between different species. The specific mechanisms vary, but the outcome consistently serves the parasite’s life cycle.
Master Manipulators
Parasitic wasps, particularly from the Ichneumonidae family like Hymenoepimecis argyraphaga, are well-documented spider manipulators. An adult female wasp temporarily paralyzes a spider, such as Plesiometa argyra, and attaches an egg to its abdomen. The spider typically resumes its normal activities while the wasp larva develops and feeds on its hemolymph, the spider’s circulatory fluid.
As the wasp larva prepares to pupate, it chemically induces the spider to construct an unusual “cocoon web.” This specialized web differs from the spider’s normal orb web, featuring stronger lines and a design optimized to protect the developing wasp pupa from predators and environmental elements. Scientists propose the wasp larva may inject substances that mimic or interfere with the spider’s molting hormone, ecdysone, which triggers web changes. Once the modified web is complete, the wasp larva kills and consumes the spider, then spins its cocoon within the silk structure.
Fungi have also been identified as spider manipulators, notably Gibellula attenboroughii, discovered infecting cave spiders in Ireland. This fungus infiltrates the spider’s body and alters its behavior, causing normally reclusive spiders to move to exposed locations. This strategic relocation benefits the fungus by allowing it to release its spores into air currents, facilitating wider dispersal to new hosts. While the precise compounds responsible are still being investigated, researchers suspect the fungus produces mind-altering chemicals that affect the spider’s nervous system, ultimately consuming the spider from the inside out.
Beyond Spiders
Parasitic manipulation extends beyond spiders, showcasing a widespread evolutionary strategy across the animal kingdom. A prominent example involves the Ophiocordyceps fungi, known for infecting ants. These fungi compel infected ants to climb vegetation and clamp onto a leaf or twig. The ant dies in this elevated position, allowing the fungus to sprout a stalk from its head and release spores from a higher vantage point, maximizing their spread to other ants below.
The protozoan parasite Toxoplasma gondii infects rodents, altering their behavior by reducing their innate fear of cat odors, and in some instances, even inducing an attraction to them. This manipulation increases the likelihood of the infected rodent being preyed upon by a cat, the definitive host where Toxoplasma gondii completes its life cycle. Similarly, horsehair worms (Nematomorpha) manipulate crickets and grasshoppers, driving them to seek water and drown themselves, enabling the adult worms to emerge and reproduce in an aquatic environment.