many taxa are known to adversely affect the egg production of their
vectors (Hurd, 1990), studies that provide data with which to assess the
evolutionary history of this manipulation are rare.
Changes in vector behaviour must have a physiological, biochemical
and molecular basis; thus it is as important to consider these aspects of
manipulation as to try to determine whether behavioural changes actually
enhance parasite success. A few studies are beginning to unravel the
mechanisms underlying the curtailment of vector reproductive success.
However, our understanding of the biochemical and molecular bases of
the various aspects of haematophagous behaviour is negligible (apart from
the endocrine control of the responsiveness ofAedes aegyptilactic acid
receptors (Davis, 1984; Klowdenet al., 1987)).Blood-feeding Behaviour – Haematophagy
The acquisition of a blood meal by temporary ectoparasites depends upon
the realization of a series of behavioural and non-behavioural steps
leading from a state of hunger to imbibing blood. This process is not
strictly sequential, thus allowing the insect a more flexible approach to
the circumstances in which it finds itself (Lehane, 1991). Sutcliffe (1987)
described the initial stages of host location; however, the description
stops when the insect reaches the host’s general vicinity. Takken (1991)
described the host-finding process as ending when the insect alights on
the host. However, there is clearly more to the acquisition of a blood meal
than host location, and the insect must not only alight on the host but also
choose a feeding site and then commence probing and imbibing blood.
A general and more comprehensive model of blood-feeding behaviour
has four steps: (i) the appetitive search; (ii) activation and orientation;
(iii) attraction; and (iv) landing and probing. These steps include host
location, host acceptance and the initiation of blood feeding. The
initiation of any or all of these stages is dependent on endogenous factors,
such as the age, host preference, nutritional and reproductive state of the
insect, whether or not the insect is crepuscular, nocturnal or diurnal, and
their interaction. Exogenous factors, such as temperature, humidity, wind
speed, light intensity and the availability of hosts, are also likely to affect
the outcome of the blood-meal-seeking process (Fig. 13.1). All the endo-
genous factors and the insect’s response to the exogenous factors may
change because of infection by parasites. In addition, vector behaviour
may be altered by proxy, i.e. the behaviour of the vector is not altered by
direct action on the vector but by action instead on the host. For example,
a parasite might modify its host’s odour, thus making it more attractive to
the vector; this may then have a beneficial outcome for the parasite in
terms of an improved chance of transmission. We shall examine the
potential role of the parasite in manipulating the process to its advantage
and examine each of the steps outlined above for any evidence of parasite
manipulation.Parasite Manipulation of Vector Behaviour 263