Ocellar pigmentation and phototaxis in the nematode Mermis nigrescens: Changes during development

After 1 or 2 years of dormancy in the soil, Mermis nigrescens females emerg e to lay eggs on vegetation where their grasshopper hosts are likely to fee d. Females collected at this life stage exhibit a strong positive phototaxi s and have a tubular region of pigmentation near the anterior tip consistin g of concentrated oxyhaemoglobin. A. previous investigation of the scanning motion of the 'head' and orientation of the 'neck' has implicated the shad owing of a photoreceptor inside the tube as the mechanism for identifying t he direction of light during phototaxis, Here, we describe the development of the pigment in young adult females and investigate phototaxis in early d evelopmental stages that lack the pigment. The orientation of the neck to a horizontal 420 nm stimulus (intensity 10(13) photons s(-1) cm(-2)) was mea sured for unpigmented fourth-stage larvae and immature adult females as wel l as mature females with pigmented ocelli, The orientation of the larvae an d immature adults was weakly negative, whereas that of the mature adults wa s strongly positive. Head and neck movements were otherwise the same in the three stages. Thus, the pigmentation appears to be required for positive p hototaxis, and the results provide further support for the shadowing role o f ocellar haemoglobin.