File:The biology of dragonflies (Odonata or Paraneuroptera) (1917) (20195717629).jpg

Title: The biology of dragonflies (Odonata or Paraneuroptera)
Identifier: biologyofdragonf00till (find matches)
Year: 1917 (1910s)
Authors: Tillyard, Robin John, 1881-1937
Subjects: Dragon-flies
Publisher: Cambridge (Eng. ) : University Press
Contributing Library: University of California Libraries
Digitizing Sponsor: Internet Archive

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148 THE SENSE ORGANS (CH. If a number of transparent tubes be arranged front of a retinal
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Fig. 64. Diagram to explain method of vision of the com- poimd eye. After Lubbock. diagram (fig. 64). side by side in screen, and separated from one another by a coating of black, the only light which will traverse each will be those rays parallel to the long axis of the tube (e.g. aa', bb', cc'). All oblique rays (e.g. aw, ay, bx, bz) fall on the sides of the tube, and are absorbed by the black pigment. Now if the tubes be made very numerous, long and narrow, like the ommatidia of the compound eye, each separate unit would only perceive a very small portion of the total field of vision. The impression received through such an eye with 20,000 ommatidia would be in the form of a landscape made up of 20,000 little pieces, each accurate as to colour, distance and intensity. Anyone who has seen pictures constructed of mosaics will readily admit that such an impression would be an excellent representation of the scene, though not so perfect as the complete picture formed by the vertebrate eye. Now, let us suppose that a very small object, so small as only to be represented on owe of the 20,000 mosaic pieces, were svddenly to move. The nerve-ending receiving the impression through the corresponding ommatidium would at once respond to the move- ment; the mosaic would be altered by a sudden alteration in one of its pieces. Such a small movement might well pass unnoticed by the vertebrate eye. But the compound eye would be quick to register it, and the brain would act accordingly. If however the change were made very slowly and steadily, it would probably pass unnoticed. What we know of the power and method of vision of the Dragonfly exactly agrees with this. The quiver of an eyelid on the part of the collector is enough to frighten the coveted prize away, though the stealthy approach of the collector's whole bulk may not even be noticed. Huxley accepted the Mosaic Theory, with the important addition that he considered the rhabdomes to be not merely light-transmitters, but light-transmuters. That is to say, they

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