A long, thin worm emerging from a praying mantis, especially near water, is a startling and often unsettling sight. This natural phenomenon reveals a complex biological strategy where one organism completes its life cycle through another.
The Horsehair Worm: An Uninvited Guest
The worms seen exiting praying mantises are horsehair worms, scientifically classified as Nematomorpha. These organisms differ from common earthworms or nematodes. Horsehair worms are long, slender, and can range from yellowish-tan to dark brown or black. They typically measure 5 to 10 centimeters (2 to 4 inches) but can reach up to 2 meters (6 feet 7 inches), while maintaining a diameter of 1 to 3 millimeters.
Unlike many other worms, adult horsehair worms have a non-functional gut and do not feed during their free-living stage. Their unsegmented, cylindrical bodies lack internal systems for excretion, respiration, or circulation. These invertebrates are known for their parasitic larval stage within arthropods.
How Mantises Become Infected
Praying mantises become infected with horsehair worms through an indirect pathway involving an intermediate host. The horsehair worm’s life cycle begins when adult worms, free-living in aquatic environments, mate and lay millions of eggs in water. These eggs hatch into microscopic larvae that must find a suitable host.
Larvae are often ingested by smaller aquatic insects or encyst on nearby vegetation. When these aquatic insects are consumed by terrestrial insects like crickets or grasshoppers, the larvae transfer to this new host. The mantis then acquires the parasite by preying upon and consuming one of these infected intermediate hosts. Once inside the mantis, the larval worm bores through the gut wall and develops within the mantis’s body cavity, absorbing nutrients directly through its skin as it grows over several weeks to months.
Behavioral Manipulation and Emergence
A key aspect of the horsehair worm’s life cycle is its ability to manipulate the mantis host’s behavior. As the worm matures, it induces the mantis to seek out a water source. This behavioral change is crucial because adult horsehair worms are aquatic and require water for reproduction. The mantis, a terrestrial insect that typically avoids water, is compelled to move towards it, often jumping in.
Research suggests this manipulation involves the worm producing proteins that interfere with the mantis’s central nervous system. These chemical signals can override the host’s natural instincts, influencing behaviors like light attraction, which can draw the mantis towards the horizontally polarized light reflected off water surfaces. Once the mantis enters the water, the mature horsehair worm emerges from its body. This process usually results in the mantis’s death, either from the emergence itself or by drowning, allowing the worm to complete its life cycle in its aquatic breeding grounds.
Broader Ecological Context
Horsehair worms are not harmful to humans or pets. They exclusively parasitize insects and other invertebrates, with no evidence of infecting vertebrates. While some reports exist of horsehair worms in human or pet vomit, this occurs when an infected insect is accidentally ingested, and the worm is then expelled, posing no health threat.
These worms play a role in their ecosystems, particularly in regulating insect host populations. Their dependency on water means they are often found in puddles, streams, and other damp areas. This parasitic relationship is a natural phenomenon, illustrating the intricate biological interdependencies within a functioning ecosystem.