that more than 800,000 species of parasitoids exist worldwide (LaSalle
and Gauld, 1991; Godfray, 1994; Quicke, 1997; Whitfield, 1998).
This suite of life-history traits has made parasitoids favoured
organisms for many studies in behavioural and evolutionary ecology.
Since parasitoid larvae derive all resources for development from one
host, a direct relationship exists between host quality and the fitness of
parasitoid offspring. The fitness of adult parasitoids is likewise directly
affected by the number and quality of hosts a female parasitizes. There is
no certainty that a foraging parasitoid will locate any hosts or that after
parasitism a parasitoid larva will successfully develop. Offspring die as
a result of immunological defence responses by the host or because the
host provides inadequate resources for successful development. Hosts are
also sometimes parasitized more than once by the same species (super-
parasitism) or a different species (multiparasitism) of parasitoid, which
results in intense competition among offspring for host resources. The
larvae of solitary parasitoids usually fight to the death for possession
of the host, while larvae of gregarious species compete for the limited
resources available in a single host. Many parasitoids are also at risk of
being parasitized by other species of parasitoids called hyperparasitoids.
While few studies focus specifically on the behaviour of parasitoid
larvae, a large literature exists on the interactions between parasitoid
larvae and their hosts. The goal of this chapter is to review these inter-
actions and their impact on parasitoid fitness. I shall first review the
morphological adaptations of parasitoid larvae for survival in different
host environments and how host quality influences parasitoid fitness.
Next, I shall examine how parasitoid larvae overcome host defence
responses, and some of the behavioural and physiological strategies
used by parasitoid larvae to exploit host resources. The last section will
focus on hyperparasitism and intra- and interspecific competition among
parasitoid larvae.Larval Adaptations to the Parasitoid Lifestyle
Parasitoid larvae vary greatly in size and morphology. This variation
is especially high during the first instar, where Clausen (1940) described
14 different larval forms. The variation in morphology of first instars
reflects in large measure adaptations for development in different types
of environments. For example, endoparasitoids possess several adapta-
tions for development in the nutrient-rich, aquatic environment of the
arthropod haemocoel. These include a very thin cuticle, which provides
no protection from desiccation but which is well suited for respiration
by passive diffusion and uptake of nutrients through structures such as
the anal vesicle (see below). In addition, most solitary endoparasitoids
produce first instars with heavily sclerotized heads, enlarged mandibles
and caudal structures, which are used in combat against competing
parasitoid larvae. These adaptations are typified by the teleaform first130 M.R. Strand