Because it is necessary to keep its back very clean, that the light which its body emits may not be dimmed.
1128. Why does the glow-worm emit a light?
Because the female glow-worm is without wings, but the male is a winged insect. The female, therefore, is endowed with the power of displaying a phosphorescent light. The light is only visible by night, but it is, nevertheless, beautifully adapted for the purpose stated, because the male is a night-flying insect, and never ventures abroad by day.
"They that go down to the sea in great ships, that do business in great waters these see the works of the Lord, and his wonders in the deep."—Psalm cvii.
There exists some difference of opinion between naturalists upon the uses of the light of a glow-worm; there are some who doubt that it is exhibited to attract the flying insect. The objectors, however, offer no explanation of the luminous properties of the worm. Sir Charles Bell says the preponderance of the argument is decidedly in favour of the explanation we have given.
1129. Why does not the iris of the fish's eye contract?
Because the diminished light in water is never too strong for the retina.
1130. Why is the eye of the eel covered with a transparent horny covering?