Praying mantises, known for their distinctive appearance and predatory behavior, can host internal parasites. This phenomenon, while unsettling to observe, is a well-documented aspect of their biology and a remarkable example of parasitic manipulation in the natural world.
The Horsehair Worm Connection
The primary internal parasite associated with praying mantises is the horsehair worm, belonging to the phylum Nematomorpha. These long, slender worms are obligate parasitoids, meaning they require a host to complete their life cycle, often resulting in the host’s death. Adult horsehair worms typically live freely in aquatic environments like ponds, lakes, or streams, where they reproduce by laying eggs in long, gelatinous strings.
The life cycle begins when the worm’s microscopic larvae are ingested by an intermediate host, often aquatic insect larvae like mosquitoes or midges. These larvae then encyst within the host’s tissues. A praying mantis becomes infected when it preys upon an infected insect, such as a cricket or grasshopper, that has ingested the encysted horsehair worm larvae. Once inside the mantis, the larvae excyst and grow, absorbing nutrients from the mantis’s hemocoel. This developmental phase can take weeks to months, during which the worm significantly increases in size, sometimes reaching lengths up to 44 centimeters (17 inches).
As the horsehair worm matures, it needs to return to an aquatic environment to reproduce. The worm secretes proteins and chemicals that manipulate the mantis’s central nervous system, altering its behavior. This manipulation causes the terrestrial mantis, which typically avoids water, to seek out a water source and jump in, effectively drowning itself. The mature horsehair worm then emerges from the mantis’s body to complete its life cycle by mating and laying eggs. Research suggests this behavioral manipulation involves changes in the host’s nervous system, potentially by influencing neurotransmitters.
Spotting an Infected Mantis and Public Safety
Observing a praying mantis infected with a horsehair worm can be a striking experience, particularly when the worm emerges. Behavioral changes might include unusual lethargy, erratic movements, or an attraction to water sources. The most definitive visual cue is the emergence of the long, dark, thread-like worm from the mantis’s abdomen, which often occurs when the mantis is near or in water. The worm can appear suddenly from the insect.
Despite their unsettling appearance and effect on their insect hosts, horsehair worms pose no threat to humans, pets, or other mammals and birds. Their parasitic life cycle is highly host-specific, exclusively targeting invertebrates like insects and crustaceans. Even if accidentally ingested, these worms are not parasites of vertebrates and will typically pass through the digestive system without causing harm. Horsehair worms are considered harmless to people, domestic animals, and plants.
If one encounters a mantis exhibiting signs of infection or witnesses a worm emerging, it is best to observe from a distance. There is no need to interfere or attempt to remove the worm, as this is a natural biological process. The mantis typically does not survive the emergence of the worm, as the parasite has consumed significant internal tissues and manipulated its host to its demise. While other types of worms can exist in insects, horsehair worms are overwhelmingly the most common and notable internal parasites in praying mantises that attract public attention.