(chemical), galvano- (electric field), geo- (gravity), magneto- (magnetic
field), photo- (light), rheo- (fluid flow), thermo- (temperature) and thigmo-
(touch). Other environmental variables shown or proposed to influence
directional movement by nematodes include electric current, osmotic
pressure, pH, redox potential, soil moisture, surface tension and soil
texture.General characteristics of nematode movement
The function of sensory structures in plant-parasitic nematodes (Perry,
1996) has received less investigation than homologous structures in the
bacteriophagous nematodeCaenorhabditis elegans. The primary sensors
are the amphids, located on either side of the head. Through laser ablation
and mutant behavioural experiments, specific nerve endings inC. elegans
amphids have been assigned roles in perception of volatiles, aqueous
solutes and temperature (Bargmann and Mori, 1997). The sensory
structures needed to respond to volatiles, solutes, temperature and touch
are also present in parasitic nematodes. Ocelli, needed for phototaxis, are
absent in plant-parasitic nematodes (Burr and Babinszki, 1990; Robinson
et al., 1990).
From a behavioural perspective, plant-parasitic nematodes fall into
two broad groups: those that infect roots and those that infect leaves,
stems and flowers. Both must be able to move through soil and through
plant tissue during part of the life cycle. For those that infect plant parts
above ground, one or more developmental stages must also be able to
move quickly within thin and often transient films of water on foliar
surfaces. Infective stages of plant-parasitic nematodes typically exhibit
uninterrupted spontaneous movements, which are several times more
rapid in foliar than in root parasites. Infective juveniles of certain
vertebrate parasites may remain inactive until stimulated to move by heat
(Croll, 1971), vibration (Wicks and Rankin, 1997) or light (Robinsonet al.,
1990).
Most plant-parasitic nematodes are vermiform in post-egg stages.
Exceptions include the sedentary, saccate females of about a dozen genera
in the order Tylenchida (Cactodera,Globodera,Heterodera,Meloidodera,
Meloidogyne, Nacobbus, Rotylenchulus, Sphaeronema, Trophotylen-
chulus,TylenchulusandVerutus). Vermiform plant-parasitic nematodes
usually move forwards in water and on solid surfaces by sinusoidal
dorsoventral waves of the entire body, which are rhythmically propagated
backwards from the anterior end (Wallace, 1968b, 1969), a type of undula-
tory propulsion (Gray, 1953). At less frequent intervals, determined by
the species, substrate and other factors, forward waves that propel the
nematode backwards can be propagated from the posterior end. Forward
waves may result when the anterior end encounters an obstacle (Croll,
1976; Wicks and Rankin, 1997) or can occur spontaneously at more or less
regular intervals. The typical pattern seen on agar is a quick withdrawal90 A.F. Robinson