Unique Locomotory Mechanism of Mermis nigrescens, a Large Nematode That Crawls Over Soil and Climbs Through Vegetation

Females of Mermis nigrescens, a nematode parasitic on grasshoppers, climb through terrestrial vegetation where they lay their eggs. The 100-mm-long body of these nematodes bridges gaps in this three-dimensional substratum, and crawls efficiently over planar surfaces. The nematodes do not use the classical undulant pattern of nematode locomotion as one coordinated unit; instead they propel themselves in several independent, locally controlled zones that propagate posteriorly. A repeated motion of their anterior end laces the body around fixed objects at which force may be applied. Propulsive force is applied to objects as the body glides past the contact site. Intermediate loops are elevated above the surface where they cannot contribute to propulsion. These loops rise and fall with time due to varying differences in propulsive forces between the contact sites. Forces are applied to the objects by internally generated bending couples that are propagated along the trunk, propelling the body in a cam-follower mechanism. Bending couples are generated by the contraction of ventral or dorsal longitudinal muscle bands that apply compressive force to the cuticle. The muscle bands, consisting of a single layer of obliquely striated muscle cells, are closely applied to the cuticle and are separated from it only by a fibrous basal lamina and a thin extension of a hypodermal cell. The myofilaments of each sarcomere are parallel to the body axis and attached perpendicularly via dense bodies (z-line equivalents) to the basal lamina, which in turn is fixed to the cuticle via filaments passing through the hypodermal cytoplasm. Consequently, forces are transmitted laterally to the cuticle over the entire length of the muscle, compressing it parallel to the surface without need for attachment to the terminal ends of the muscle cells. Thus the muscles are engineered for local control of bending and avoidance of buckling. There is evidence that the motor nervous system of Mermis may not be as simple as in classical nematode examples, which may explain why Mermis is capable of a much more localized control of ~ocomotory motion. Q 1994 Wiley-Liss, Inc. After mating, the nonfeeding female of the parasitic nematode Mermis nigrescens lies dormant in the soil for almost 2 years while her eggs develop. Then, during wet weather, she migrates to terrestrial vegetation on which she lays her eggs (Cobb, '26, '29; Christie, '37). Unlike the well-studied nematodes Caenorhabditis elegans and Ascaris suum, adult female Mermis do not move by the classical undulant pattern (Gray and Lissmann, '64). Instead, they locomote with a variable, puzzling wave motion that has never been analyzed or described, although other nonclassical locomotory patterns of nematodes have been noticed (Stauffer, '24; Streu et al., '61; Reed and Wallace, '65; Croll, '70; Crofton, '71; Lee and Atkinson, '77). The movements involved may be compared not only to those of typical nematodes, but also to those of elongate aquatic and terresAddress reprint requests to Carl Gans, Dept. of Biology, University of Michigan, Ann Arbor, MI 48109.