Praying mantises are renowned for their predatory prowess, celebrated as fierce hunters that dominate the insect world. The reality of their vulnerability to parasites is often dramatic, especially when considering the shocking biological events shown in viral videos. The question of whether these insects are universally infected is common. The true situation is a complex interplay of habitat, host behavior, and one of the most remarkable forms of parasitic control in nature.
Prevalence of Parasites in Mantis Populations
It is a common misconception that every praying mantis carries a parasite. Infection rates are highly dependent on the mantis species, geographic location, and proximity to water sources. In general, the percentage of wild mantids infected with the most dramatic parasite is estimated to be low, often falling in the single digits, though this can vary widely.
Surveys suggest that prevalence can range from a few percent up to nearly 30% in specific areas where the parasite’s life cycle is easily completed. Mantises living near ponds, streams, or other permanent bodies of water have a much higher likelihood of infection than those in drier, more isolated habitats.
The Horsehair Worm: A Specialized Invader
The parasite most famously associated with mantises belongs to the phylum Nematomorpha, commonly known as horsehair worms or Gordian worms. These organisms have a long, thin, and unsegmented morphology, resembling a piece of dark thread or wire. A single mature worm can grow to a length far exceeding the body size of its mantis host, sometimes reaching 30 centimeters or more coiled inside the insect’s abdomen.
Horsehair worms are obligate parasites only during their larval and juvenile stages, meaning they must live inside a host to grow. The adult worms are entirely free-living and aquatic, possessing no functioning digestive system. Their sole purpose after emerging from the host is to find a mate and reproduce in the water, completing their life cycle.
How Infection Occurs and Host Behavior Manipulation
Infection relies on the mantis consuming a previously infected intermediate host. The horsehair worm begins its life in water, where its microscopic larvae are ingested by aquatic insect larvae, such as those of midges or mosquitoes. These intermediate hosts then develop into terrestrial insects, like crickets or grasshoppers, carrying the dormant worm larvae inside them.
When a predatory mantis eats one of these infected insects, the worm larvae are released and begin to grow within the mantis’s hemocoel, or body cavity. As the worm matures, it triggers a profound change in the host’s behavior, a phenomenon known as parasitic manipulation. The worm chemically alters the mantis’s nervous system, essentially hijacking its brain function.
Research suggests this manipulation involves the worm producing proteins that mimic those found in the mantis, influencing neural pathways related to light perception and movement. The infected mantis is compelled to seek out and enter a body of water, often attracted to horizontally polarized light reflected off the surface. This impulse causes the mantis to enter the water, allowing the mature horsehair worm to emerge from the host’s abdomen to begin its reproductive phase. The mantis, often severely damaged or drowning, typically perishes after the worm’s departure.
Other Parasites Affecting Mantises
While the horsehair worm is the most famous example due to its mind-controlling behavior, mantises are susceptible to a variety of other parasitic organisms. External parasites, such as certain species of parasitic mites, are commonly found clinging to the mantis’s exoskeleton, particularly around leg joints or under the wings. These mites feed on the host’s hemolymph, or insect blood, but generally do not cause the dramatic death seen with internal parasites.
Internal parasitoids, such as the larvae of Tachinid flies, also pose a significant threat to mantis populations. The female fly lays her eggs on the mantis, and upon hatching, the larvae burrow into the host, feeding on its internal tissues. After the larvae complete their development, they emerge from the mantis’s body, which is a lethal event for the host.