Changes in the intensity of fluorescence with pH. (A) Side view of the right half of the metathorax after the cuticular side wall and thoracic muscles were removed; the original colour images, taken at the times indicated, have been converted to black and white. The insect was initially in saline at pH 7.2 and at time 0 minutes this was replaced by saline at pH 2 and after 13.5 minutes by saline at pH 12. In the acid pH, the intensity of the fluorescence gradually declined, only to increase again in the alkaline pH. (B) Graph of the changes in intensity measured from digital images, such as those shown in (A). Red squares and lines: summed fluorescence across pixels, normalised, within an outline approximately 1 mm long drawn to cover the brightest part of the pleural arch. Black filled circles: normalised mean maximum fluorescence intensity ± 1 standard deviation recorded along 10 lines approximately 0.55 mm long and 0.1 mm apart, orthogonal to the long axis of the pleural arch. Blue circles: background fluorescence assayed as the mean minimum intensity ± 1 standard deviation from the same lines.