sophisticated mechanism called vibrational sounding (Wäckerset al.,
1998). During this type of orientation, parasitoids transmit self-produced
vibrations on to a substrate. The resonance of the reverberating substrate
provides parasitoids with information on the relative solidity of the
substrate, allowing them to scan their habitat for hidden hosts. Similar
to echo location, vibrational sounding is therefore an example of self-
communication, in which the parasitoid is both sender and receiver at the
same time. This form of sensory orientation is unique, as it is the only
known example in which parasitoids employ self-produced signals to
locate their hidden hosts, making them (at least to a certain extent)
independent of host-derived stimuli. This is particularly crucial for pupal
parasitoids, as pupae neither feed nor move and are often hidden in
well-concealed pupation sites (Vetet al., 1995). Parasitoid size is a crucial
factor in the functioning of vibrational sounding, as larger females can
produce ‘louder’ vibrations, allowing them to successfully locate hosts in
a broader range of substrates (Ottenet al., 2001).Multisensory orientation
We have seen that parasitoids possess a broad sensory arsenal, employing
olfaction, vision and mechanoreception to locate their hosts. Rather than
using the different types of information in isolation, the various sensory
parameters are often combined in multisensory orientation (Wäckers
and Lewis, 1994). The concurrent use of several sources of information
enhances a parasitoid’s flexibility in interacting with its environment in
several ways. For one thing, it greatly increases sensory differentiation.
Unlike our common laboratory approach, in which we attempt to expose
the insect to a single sensory mode, objects in the field generally present
a combination of sensory information to the foraging insect. The more
of this information that is used, the better the insect will be able to
differentiate between the range of objects encountered. When orientation
is restricted to a single sensory modality, differentiation is limited to
a single sensory dimension (e.g. odour). Each additional modality adds
a new dimension to the differentiating power. The fact that multi-
sensory information can enhance orientation could be demonstrated in
experiments with free-flyingM. croceipes(Wäckers and Lewis, 1994).
When challenged to differentiate between two alternatives, parasitoids
conditioned to a combination of olfactory and visual stimuli landed
more accurately than individuals conditioned to only the visual image
or odour, respectively.
Another reason why multisensory orientation enhances flexibility is
because the specific advantages of the individual types of information can
be combined. For instance, a parasitoid can be solely guided by odour in
situations when the object is barred from vision, while visual orientation
allows the parasitoid to stay on track when turbulence interrupts the
odour plume.48 L.E.M. Vetet al.