Abstract:
Purpose: Detecting the sign of retinal defocus is an important but poorly understood element in the control of eye growth. The principal eyes of jumping spiders have a layered retina, one containing green (535nm) sensitive visual pigment and a more posterior layer sensitive to red (626nm). It has been suggested that this layered structure allows computation of target distance (depth) based on the relative image defocus between layers. Due to chromatic aberration inherent in the spider eye, red light has a longer focal length, which makes the target appear to be closer. This leads to a jump error when the spiders hunt. The aim of this project was to initially demonstrate this behaviour in a New Zealand species of jumping spider, Trite planiceps, and then to observe whether the jump error improves over time while under constant red light. Methods: Spiders (n = 19) were kept in individual containers under broad spectrum white light, and were transferred to larger containers for feeding. Initial jumps were performed under white light to determine normal jump behaviour. Experiments were then repeated under either 520nm (green) or 620nm (red) light. Spiders were offered Drosophila fruit flies up to three times daily, and all jumps (n = 32) were recorded. To measure adaptation, spiders were housed and fed under 12 hours of red light per day for 7 days, and jump error over time was analysed (n = 70). Results: Under red light, spiders consistently jumped shorter (~27%) than under white (p = 0.006) or green light (p = 0.003). Jumps under green light had no difference in accuracy to those in white light (p = 0.8414). Spiders under continuous red light reduced their jump error by approximately 3.8% per day, such that by day 7 the jump error had decreased from 27% to 9% (p = 0.044). Conclusions: Our results suggest that T. planiceps utilises image defocus to compute depth information when hunting prey, and this defocus can be manipulated using different wavelengths of light. When left under continuous narrowband light, the jump error reduces within one week, however, it is unknown how this adaptation is occurring. It may occur within the eye by adjusting the position of the retinal layers or focal power of the lens (emmetropisation), or it may involve neural adaptation (learning) to the modified blur signal.