Correspondingly, ectoparasitoid larvae usually lack specialized adapta-
tions for attaching to hosts or fighting. In contrast, ectoparasitoids that do
not paralyse their hosts often produce larvae that have setae or suctorial
discs to facilitate attachment to the host’s cuticle. Ectoparasitoid larvae
also have exoskeletons, which provide some protection from desiccation,
and often possess a tracheal system for respiration.
As the majority of adult parasitoids oviposit into hosts directly,
parasitoid larvae usually lack adaptations for moving or surviving outside
the host. Yet, as noted above, a few parasitoids oviposit away from
the host and rely on a mobile first instar for host location. In the
Hymenoptera, the families Perilampidae and Eucharitidae are comprised
exclusively of species that produce mobile larvae. Many parasitic Diptera
in the family Tachinidae and all parasitoids in the Coleoptera, Neuroptera
and Lepidoptera also produce mobile first instars. Two main types of
searching larvae are distinguished (Askew, 1971). Triungulin larvae
produced by parasitoid beetles, moths and neuropterans move using
conventional legs, while planidium-type larvae produced by the afore-
mentioned taxa of Hymenoptera and Diptera move via elongated setae
(Fig. 7.1d, e). Many tachinids lay eggs that hatch into planidial larvae
or directly lay planidial larvae, which hatch from eggs brooded in the
female’s reproductive tract (i.e. they larviposit). The flies ovi- or larviposit
in only the general proximity of hosts, which are then located by the
planidium (Clausen, 1940). Other parasitoids that produce planidial
larvae do not locate hosts directly but instead are transported phoretically
by the adult host. This is especially common among species that
parasitize larvae of social Hymenoptera. For example, the Eurcharitidae
exclusively parasitize ant larvae. Adult females deposit their eggs
in locations that are frequented by foraging ants. After hatching, the
planidium remains motionless until jumping on to a passing ant. The ant
then carries the parasitoid back to the colony, where it parasitizes an
ant larva (Heraty and Barber, 1990).
Unlike the variation in morphology and behaviour of first instars,
subsequent larval stages are much more uniform among parasitoids. The
vast majority of parasitic Hymenoptera are sacciform or hymenopteriform
during the second and later instars (Fig. 7.1c, f). These larvae have
reduced mandibles and very limited mobility and are adapted primarily
for consumption of host haemolymph or tissues. The middle and late
instars of endoparasitic braconids and some banchine ichneumonids are
similar to this but are classified as vesiculate because of the presence of a
structure called the anal vesicle (Fig. 7.1g). The anal vesicle is in essence
an eversion of the hindgut, which during the larval stage is unconnected
to the midgut. Everted, the anal vesicle is involved in both respiration and
uptake of nutrients (Edson and Vinson, 1976). The parasitoid larva then
retracts the anal vesicle after moulting to its final instar in preparation for
metamorphosis.132 M.R. Strand