Early in studies of the nematode, this was called the "chromatrope", because it apparently had a function in the animal's response to light.

[4] On closer examination it had the form of a hollow cylinder,[5] and the red pigment inside was determined to be hemoglobin.

[6] Haemoglobin takes the place of melanin as the shadowing pigment of the eye, and plays a role in the nematode's sensation of light, a function not observed in any other organism.

[7] This and other Mermis species are sometimes mistaken for horsehair worms, but the latter are longer and uniformly dark in colour, and their bodies do not taper.

During the following spring or summer the female emerges after a rainfall event while the landscape is still wet, often early in the morning.

The juvenile nematode pierces the gut with its stylet and enters the hemocoel, the blood-like fluid that fills the insect's body cavity, acting as a circulatory system.

There the nematode absorbs the insect's nutrients, taking glucose directly through its cuticle.

This behaviour may guide the juvenile underground, and lead the adult female to the surface, where it lays its eggs on vegetation.

The anterior tip of the body containing the eye swings horizontally and vertically while the "neck" region just behind it bends slowly.

[6] This is apparently a scanning behaviour, in which the nematode senses light and determines its direction.

In naturally infected earwig hosts (Forficula auricularia) in New Zealand a prevalence of 7.8% was observed.

Larger M. nigrescens are more likely to induce positive hydrotaxis which is probably related to maturation of the worms.