between studies conducted on natural and non-natural combinations of
vectors and parasites. For the purposes of this review, we would consider
vectors infected by a parasite with which they may become naturally
infected in the wild as being a natural combination and vectors infected
with parasites that they are unlikely to become infected with in the wild
as being a non-natural combination. Of course, in the natural situation,
there are likely to be degrees of association between vector and parasite. In
those with a long evolutionary association, the effect of the parasite may
be significantly different from that in a recently developed association.
Further problems may occur with laboratory infections, which may be
excessively heavy or light and thus not reflect the true situation encoun-
tered by either parasite or vector. Thus the loss of flight activity inA.
aegyptiinfected withB. pahangioccurs when the filarial worm invades
flight muscle, causing a loss of flight activity associated with a specific tis-
sue pathology (Townson, 1970; Paige and Craig, 1975). Although cells of
the Malpighian tubules ofA. trivittatusinfected withD. immitisare rup-
tured as L3 larvae migrate to the mouth-parts, there have been no other
pathological effects reported to account for the increased flight activity.Activation and orientation
Activation occurs when the insect comes into contact with a suitable
signal from a potential host and switches from appetitive non-directional
flight to directed host-location behaviour. Orientation is the tendency for
the insect to fly upwind after activation. Activation itself may not result
in a behavioural change but, rather, may prime the insect for acceptance of
a subsequent orientational cue. These cues are generally olfactory, but
visual cues are also important, especially in tsetse- and blackflies. The
role of CO 2 in activating and inducing upwind flight is well known
in mosquitoes (Laarman, 1958; Clements, 1963), and Takken (1991) con-
sidered that it might possibly trigger the flight response in hungry mosqui-
toes. In tsetse-flies, activation and orientation are also caused by CO 2.
No experiments have been specifically designed or conducted to
investigate the effect of parasitic infection on vector behaviour in this
aspect of host blood-feeding behaviour.Attraction
Attraction is the process by which the vector is drawn from a distance to
the host. Host odours (other than CO 2 ), visual cues, heat and water vapour
all play a role in attraction to the host. Odours may play a more prominent
role as long-range attractants, with visual, heat and water-vapour cues
becoming more important factors at shorter range. There may also be sig-
nificant additive and synergistic interactions between these cues. These
interactions have been largely unexplored in blood-feeding arthropods.266 J.G.C. Hamilton and H. Hurd