Contrast sensitivity and visual acuity of Queensland fruit flies (Bactrocera tryoni)

Abstract

This study examines the visual acuity of Queensland fruit flies (Bactrocera tryoni) by analysing their turning responses to an immersive visual stimulus consisting of a pattern of vertical stripes presented at various angular periods and rotational rates. The results infer that these flies possess an interommatidial angle of approximately ${2}^{\circ }$, and an ommatidial acceptance angle of approximately ${1.72}^{\circ }$. This suggests that the visual acuity of Queensland fruit flies is substantially better than that of the classical vinegar fly (Drosophila melanogaster), and is comparable to those of the housefly (Musca domestica) and the honeybee (Apis mellifera). The contrast sensitivity of Queensland fruit flies is comparable to that of the housefly.

Appendix: Temporal aliasing of the contrast-frequency response

Here, we provide an additional analysis of the response-versus-contrast frequency data presented in Fig. 4, in which we include the data from the responses measured at commanded contrast frequencies greater than 30 Hz.

As a result of temporal aliasing caused by the 60 Hz refresh rate of the monitors, a grating that is commanded to move at a contrast frequency greater than 30 Hz will appear to move in the opposite direction. If the commanded contrast frequency is $x\,\hbox {Hz}$, where $x>{30}\hbox { Hz}$, then the apparent contrast frequency y of the grating will be $y=(60-x)\,{\hbox {Hz}}$ and the grating will appear to move at an angular velocity of (y / s) deg/s, where s is the spatial frequency of the grating. Ignoring the negative polarity of the response and considering just the magnitude, we can include the data from the responses measured for commanded contrast frequencies $x>{30}\hbox { Hz}$ by treating them as responses to contrast frequencies of $y=(60-x)\,{\hbox {Hz}}$.

Fig. 6

An augmented version of Fig. 4, which includes the data from the responses to the temporally aliased stimuli, as described in the text

The results of performing the above operation to include the responses to the temporally aliased stimuli are shown in Fig. 6, which shows freshly fitted spline curves to accommodate the additional data. It is evident from this figure that inclusion of the additional data does not change the shapes of the spline-fitted curves or the locations of their peaks in any major way.

It is important to note that, although the refresh rate of the monitors is 60 Hz, there is no 60 Hz flicker in the visual display because the monitors (Dell 2209WA) have LCD screens (Lawson and Srinivasan 2018).