Summary
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1.
Intracellular recordings were made of receptor responses in the central region of the compound eyes of the locusts,Valanga andLocusta. The animals were maintained on their usual daily light cycle, and recordings were made at times known from previous anatomical studies to coincide with changes in ommatidial structure (Tunstall and Horridge 1967; Horridge et al. 1981; Williams 1982a). Anatomical checks were made of the areas of the retina from which recordings had been made (Fig. 1).
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2.
Angular acceptance at 50% sensitivity inValanga andLocusta respectively increased from 1.7° and 1.9° when light-adapted during the day, to 2.7° and 2.8° when dark-adapted for 10–15 min after ‘dusk’, to 4.7° and 4.9° in a fully night-adapted state. It then decreased to 2.05° and 1.9° when light-adapted for 2 h after ‘dawn’, and increased to 2.8° and 2.9° after a further 20 min dark adaptation (Table 1). Dark-adapted values were measured by exposing the cells to very dim light and counting the quantum bumps (Fig. 3), thus ensuring that there were no light-adaptive effects from the stimulus.
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3.
Sensitivity to an extended source increased in both species by at least 1 log unit during the first 1–3 h after ‘dusk’ before reaching its maximum (Fig. 6). The last 0.6 log units of this increment (3.9-fold increase) is attributed to the enlargement of the field stop and rhabdom. After 2 h light from ‘dawn’, which induces diminution of the rhabdom and field stop to their day sizes, followed by 20 min of dark adaptation, cells were 3.8 times (data fromValanga andLocusta grouped) less sensitive to the extended source than they were at night.
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4.
The increased light capture from the environment, resulting from opening up the rhabdom's acceptance at night, is obtained at the expense of spatial acuity. At night, the angular acceptance at 50% sensitivity becomes more than twice the interommatidial angle (Δϕ) (Fig. 8). As a result, discrimination of two points spaced at 2Δϕ, which is possible during the day, becomes impossible at night.
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5.
No change of spectral sensitivity (Fig. 4), bump latency (Fig. 7), or sensitivity to a point source on-axis (Fig. 5) was detected between the night and day states. The last indicates that the absorption efficiency of the rhabdom is constant, and the Airy disc is smaller than the day-state field stop. Together these findings indicate that the new photoreceptor membrane assembled at dusk does not differ fundamentally from that maintained during the day. Changes in sensitivity are therefore achieved by changing the amount of exposed receptor membrane rather than the nature of the membrane.
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Williams, D.S. Changes of photoreceptor performance associated with the daily turnover of photoreceptor membrane in locusts. J. Comp. Physiol. 150, 509–519 (1983). https://doi.org/10.1007/BF00609577
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DOI: https://doi.org/10.1007/BF00609577