Geographic variation in behaviour and dim light adaptation in Cyrba algerina (Araneae, Salticidae)
Thesis DisciplineBiological Sciences
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Cyrba algerina is a salticid (Salticidae) spider that lives on the undersides of stones. Two populations were studied, Sintra and Algarve (Portugal), and shown to have similar phenology but different dominant prey. Life cycle in the laboratory was similar for the two populations, but Sintra matured at larger size than Algarve individuals, with these differences potentially having a genetic basis. Sintra individuals used prey-specific prey-capture behaviour against allopatric (Oecobius amboseli) and sympatric (O. machadoi, Trachyzelotes bardiae) spider and insect (bristletails) species. In contrast, Algarve C. algerina only adopted specialised capture behaviour against bristletails. Sintra, but not Algarve, individuals responded to the odour of O. machadoi and T. bardiae, and showed preference for T. bardiae over O. machadoi. Interpopulation variation in the use of specific prey-capture behaviour and in sensitivity to odour cues from prey is directly related to the prey available to individuals from each population, suggesting local adaptation to local prey. Preference for oecobiids seems to be controlled by an experiencetriggered developmental switch. The optics and histology of C. algerina’s principal eye suggest that living in a microhabitat with dim ambient light has favoured sensitivity at the expense of spatial acuity. Short focal length, reduced power of the eye’s diverging lens, and wide, contiguous rhabdomeres, seem to minimise the visual constraints imposed by the low light levels in C. algerina’s microhabitat. While relying solely on vision, C. algerina can detect, identify and capture prey in dim-light conditions under which other salticids perform poorly. C. algerina’s behaviour suggest use of temporal summation to improve its visual performance in dim light.