Praying mantises, with their distinctive posture and predatory habits, are familiar insects found globally. They are known for their ability to camouflage and patiently ambush prey. Biological interactions like parasitism are common, where one organism, the parasite, lives on or inside another, the host, benefiting at the host’s expense. Among the examples of this relationship is the interaction between a praying mantis and a hairworm.
The Hairworm’s Unique Life Cycle
Hairworms, classified under the phylum Nematomorpha, are sometimes called Gordian worms due to their tendency to coil into knots. The life cycle of a hairworm unfolds through four stages: egg, a pre-parasitic larva, a parasitic larva, and a free-living aquatic adult. Adult hairworms do not feed; instead, they sustain themselves on resources accumulated during their parasitic phase and focus on reproduction in aquatic environments.
Female hairworms deposit a large number of eggs, often in long, gelatinous strings, directly into water. These microscopic eggs hatch into pre-parasitic larvae. To develop, these larvae must infect an invertebrate host. Aquatic insect larvae, such as those of mayflies or mosquitoes, ingest the hairworm larvae, which then encyst within these intermediate hosts. When these aquatic insects mature and develop wings, they emerge from the water, transporting the encysted hairworm larvae into terrestrial environments.
Infection and Behavioral Control
A praying mantis becomes infected with a hairworm when it consumes a terrestrial insect harboring the encysted larva. Once inside the mantis’s body, the hairworm larva breaks free from its cyst and begins to grow, absorbing nutrients from the mantis’s internal tissues. This parasitic growth period can span several weeks to a few months.
As the hairworm reaches maturity, it begins to manipulate the mantis’s nervous system and behavior. This manipulation results in a change in the mantis’s actions, compelling it to enter water, despite its natural inclination to avoid it. Research indicates that hairworms can exploit the mantis’s visual system, specifically its attraction to horizontally polarized light, which is reflected off water surfaces, guiding the infected mantis towards a watery destination.
This behavioral alteration is a strategy for the hairworm to complete its life cycle by returning to its aquatic breeding grounds. Scientists have found this manipulation may involve the hairworm producing proteins that mimic or interfere with the host’s central nervous system. Some studies even suggest the parasite acquires host genes for this molecular mimicry.
The Mantis’s Inevitable End
Once the hairworm has manipulated its mantis host into entering water, the mature worm emerges from the mantis’s body, typically through its posterior end. This emergence signals the completion of the hairworm’s parasitic phase, allowing it to transition into its free-living, reproductive stage in the aquatic environment.
The mantis perishes shortly after the hairworm’s departure. The hairworm’s growth inside the mantis consumes its internal tissues and fat reserves, depleting the host’s energy and structural integrity. The hairworm’s emergence also causes internal damage to the mantis.
Even if a mantis does not immediately drown, the internal damage and depletion of resources lead to its death after the parasite leaves. The hairworm’s life cycle is linked to the mantis’s demise in water, ensuring its return to its aquatic habitat for reproduction. This makes the hairworm an obligate parasitoid, as its life cycle necessitates the death of its host. While isolated cases of host survival might occur, the typical outcome for a praying mantis infected by a mature hairworm is death, indicating that survival is an exception rather than the common result.