their incredible diversity of associations, and the separation of research
into distinct disciplines that rarely intercommunicate. Thus, conven-
tional parasitology has been focused on viral, bacterial, protozoal,
helminth and arthropod parasites of vertebrate animals, but the parasites
of most invertebrate animals and plants have largely been excluded.
Clearly, an integration and synthesis of disciplinary perspectives and
experiences are requisite for continued progress in the understanding of
parasite behaviour, but the means to achieve this goal are uncertain.
Nevertheless, a good first step is to acquaint ourselves with the ideas of
our colleagues outside our specific disciplines.
The definition of parasites remains thorny and controversial (Cheng,
1991). The group is not monophyletic, making it difficult to say just what
makes a parasite a parasite (Brooks and McLennan, 1993), and the discrete
limits set to delineate parasitism are seldom biologically meaningful
(Askew, 1971; Kennedy, 1975). We have adopted the broad and inclusive
definition proposed by Toft (1991). Parasitism is a two-trophic level
interaction where the parasite is the receiver of some benefit at a cost
to the host donor and the parasite exhibits some degree of symbiosis.
Symbiosis literally means ‘living together’ and refers to a physically
intimate association (Cheng, 1991). The host could be a single organism,
a host in the traditional sense or a defined group of organisms (Stuart,
Chapter 15). In terms of behaviour, parasites share a common need to find
new hosts, to infect their hosts and to find the appropriate locations
within their hosts.Host Finding
Searching for resources is a critical aspect in every organism’s biology,
because of the close link between foraging success and fitness. Parasites
are adapted to exploiting the free-living environments associated with
their hosts, but hosts are patchy and ephemeral resources. In addition,
adaptations to exploit a living host can be negatively correlated with a
parasite’s ability to persist in the external environment. Thus, a central
problem faced by all parasites is how to bridge the gap between hosts.
Parasites use one of four basic strategies to bridge the gap between
hosts: passive transmission using infective stages; active transmission
using infective stages; active transmission by female oviposition; or trans-
mission facilitated by another organism (host manipulation, intermediate
hosts or vectors).
Species with passive transmission between hosts typically use a
strategy of contaminating the hosts’ environment with large numbers of
persistent infective stages to maximize the probability that they will be
encountered and consumed by hosts (Viney, Chapter 6). In active trans-
mission, infective stages generally exhibit behaviours that affect their
distribution and probability of encountering a host (Combeset al., Chapter
1; Campbell and Lewis, Chapter 2). In parasitoid wasps (Vetet al., Chapter338 E.E. Lewiset al.