Mechanisms underlying visual perception and vision-based decision-making.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The jumping spider Evarcha culicivora (Araneae, Salticidae) is unique because, although spiders are incapable of feeding directly on vertebrate blood, E. culicivora has a behavioural mechanism enabling it to feed on blood indirectly. E. culicivora is a small East African jumping spider that preys on blood-fed female mosquitoes in preference to any other prey, including male mosquitoes and female mosquitoes that have not fed on blood, as well as the far more numerous and similar looking midges in the vicinity of Lake Victoria. E. culicivora distinguishes its preferred prey of blood-fed mosquitoes using visual and olfactory cues, independently of one another. The optical cues by which E. culicivora identifies its preferred prey were investigated using lures made from dead prey and by the novel use of 3D animation software to create virtual mosquitoes that were systematically altered. It is apparent that the primary factors influencing the prey-choice decisions of E. culicivora include the mosquito's size, its behaviour, the shape of its abdomen, the appearance of its antennae, and possibly the colour of the mosquito's blood-filled abdomen. E. culicivora's preference for blood-fed female mosquitoes appears to be specifically due to the blood within the mosquito rather than the prey's size. This is the first report of a spider actively seeking out blood meals by preferentially preying on blood-fed mosquitoes. However, small juveniles of E. Culicivora have a further dietary specialization: they prefer blood-fed females of the genus Anopheles. In the absence of odour cues, juvenile E. culicivora distinguish blood-fed Anopheles mosquitoes from mosquitoes belonging to other genera, doing this with eyes of a diameter less than 200 microns. The primary optical cue by which juveniles of E. culicivora identify Anopheles is by the mosquito's characteristic resting posture. This is the first report of any predator seeking out Anopheles, the vectors of malaria, as preferred prey. Furthermore, small juveniles of E. culicivora, but not large juveniles or adults, have an effective Anopheles-specific predatory tactic for capturing Anopheles that is not used when capturing mosquitoes belonging to other genera. Evarcha culicivora has a further behavioural trait that is unique among salticids; they are attracted to the odour of a particular plant, Lantana camara. However, when in the vicinity of L. camara, E. culicivora exhibits a profound behavioural change in which it becomes indiscriminate in its choice of prey. A proximate cause of the behavioural change was found to be β-caryophyllene, a volatile produced by L. camara. This is the first report of a single environmental chemical cue affecting the prey-choice behaviour of a spider.