The relationship between the praying mantis and the hairworm is a striking example of parasitic control. The hairworm is a long, thread-like creature that develops inside its insect host, ultimately hijacking the mantis’s central functions. This process transforms the terrestrial insect into a vehicle for the parasite’s return to water, which is necessary for its survival and reproduction.
Defining the Hairworm (Nematomorpha)
Hairworms belong to the phylum Nematomorpha and are often called horsehair worms or Gordian worms. Adult worms are incredibly long and thin, resembling a piece of horsehair. They typically measure between 5 to 40 centimeters, though some species can reach up to two meters in length, with a diameter of only one to three millimeters.
As free-living adults, hairworms lack functional digestive, circulatory, and respiratory systems. They are non-feeding organisms, relying entirely on nutrients absorbed during their parasitic stage inside the host. Adult hairworms spend their brief aquatic lives focused solely on reproduction.
The Complete Parasitic Life Cycle
The hairworm’s life cycle begins in an aquatic environment where adult males and females mate. The female deposits millions of microscopic eggs in long, gelatinous strings, which hatch into tiny, pre-parasitic larvae. These larvae are ingested by an intermediate host, often the larva of an aquatic insect like a midge or mayfly.
Once ingested, the hairworm forms a protective, dormant cyst inside the intermediate host’s body cavity. When the aquatic insect metamorphoses into a winged adult, the parasite is transported to the terrestrial environment. The praying mantis becomes infected when it preys upon and consumes this infected adult. The cyst dissolves in the mantis’s gut, allowing the larva to bore through the intestinal wall and enter the mantis’s body cavity (haemocoel).
Inside the mantis, the hairworm absorbs nutrients directly through its skin, growing dramatically to a length that can exceed the host’s body multiple times over. This growth phase lasts several weeks or months, during which the mantis continues to feed and move normally while the parasite matures. The hairworm stores enough energy during this period to fuel its entire adult, free-living, and reproductive phase.
Behavioral Manipulation of the Mantis Host
When the hairworm reaches full maturity, it must return to water to reproduce. Since the praying mantis is a terrestrial insect, the parasite initiates behavioral manipulation, compelling the mantis to seek out and enter a body of water. This manipulation involves the hairworm producing specific molecules that interfere with the host’s central nervous system.
Recent research suggests the hairworm achieves this control by deploying proteins that mimic those involved in the mantis’s own neural pathways. The altered mantis behavior is highly specific, often involving an attraction to horizontally polarized light. This light is strongly reflected off the surface of water, driving the insect toward pools and streams.
Once driven to the water’s edge, the mantis jumps in, completing the parasite’s life cycle. The mature hairworm then physically emerges from the mantis’s body cavity, usually near the anus. This exit allows the worm to begin its search for a mate in the aquatic environment.
Human Safety and Ecological Context
Hairworms are highly specialized parasites for invertebrate hosts like praying mantises, crickets, and beetles. They pose no threat to mammalian health, as they are not known to parasitize humans, pets, or livestock. Instances of hairworms found in human waste or water are considered “pseudoparasitism,” meaning the worm was accidentally ingested or released from an infected insect nearby.
The presence of hairworms serves a unique ecological function within the food web. By driving terrestrial hosts into the water, they provide a sudden influx of nutrients from the drowned insect into the aquatic ecosystem. Furthermore, the adult worms, now free, become a food source for fish and other aquatic life. Their main ecological contribution is bridging the flow of energy between land and water.