they convey; and (ii) those that are based on how searchers move through
their environment. Both types of models can be useful in facilitating our
understanding of the host-search process. The division of host search
into a hierarchical process of host habitat location, host location, host
acceptance and host suitability (Salt, 1935; Laing, 1937; Doutt, 1964) has
been a widely adopted model of the first type. This conceptual model has
proved useful for organizing information about parasitoid host-search
behaviour (Godfray, 1994), and its application to other parasite species is
apparent in this and other chapters in this book. This model does not
imply that foragers use a static hierarchical set of behaviours (Vinson,
1981), and more recent models have emphasized the more dynamic
nature of the process (Lewiset al., 1990; Vetet al., 1990; Godfray, 1994;
Vetet al., Chapter 3, this volume). These recent models have emphasized
the ranking of stimuli based on how closely they are associated with the
host; the influence of forager internal state, experience and genetics on the
parasites’ response; and the amount of directional information provided
by the stimuli.
Another conceptual model of host search is based on the way that
foragers move through the environment and when they scan for cues. In
this model, foraging strategies are classified into two broad categories,
cruise (widely foraging) and ambush (sit and wait), which represent
end-points on a continuum of strategies (Pianka, 1966; Schoener, 1971;
Eckhardt, 1979; Huey and Pianka, 1981; McLaughlin, 1989). Into which
category an organism fits results mechanistically from differences in how
foraging time is allocated to motionless scanning and moving through
the environment (Huey and Pianka, 1981; O’Brienet al., 1989). Cruise
foragers allocate more of their foraging time to scanning for resource-
associated cues when moving through the environment or during short
pauses. Ambush foragers scan during long pauses, which are interrupted
by repositioning bouts of shorter duration. These differences are sig-
nificant because the length of scanning pauses influences the types of
resources that the organism is likely to encounter. Cruise foragers have
a higher probability of finding sedentary and cryptic resources than
ambushers, and ambush foragers have a higher probability of finding
resources with high mobility than cruise foragers.
This dichotomous view of the foraging mode has been criticized
as arbitrary and an oversimplification of what is really a continuum of
strategies (Regal, 1978; Taigen and Pough, 1983). However, the dichoto-
mous view of foraging has been studied extensively over the last few
decades from field, laboratory and theoretical perspectives and has been
applied to a taxonomically diverse group of organisms, including birds
(Eckhardt, 1979), lizards (Pianka, 1966; Pietruszka, 1986), fish (O’Brien
et al., 1989), arthropods (Inoue and Matsura, 1983; Caraco and Gillespie,
1986) and, as discussed in more detail in this chapter, parasites. These
studies have indicated that foraging strategies often have a bimodal
distribution that justifies the utilization of the ambusher/cruiser
dichotomy (McLaughlin, 1989). The ambush-foraging strategy may be14 J.F. Campbell and E.E. Lewis