File:Elementary entomology (1912) (21030584920).jpg

Title: Elementary entomology
Identifier: elementaryentom00sand (find matches)
Year: 1912 (1910s)
Authors: Sanderson, Dwight, 1878-1944; Jackson, C. F. (Cicero Floyd), b. 1882; Metcalf Collection (North Carolina State University). NCRS
Subjects: Entomology
Publisher: Boston, Ginn
Contributing Library: NCSU Libraries
Digitizing Sponsor: NCSU Libraries

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The surface of the compound eye is composed of numerous hexagonal facets, each of which is the end of a single eye element called an ommatidium, which is prac- tically a separate and distinct eye. Each ommatidium is composed of the various optical elements necessary for vision, but it receives impressions only in a straight line, which form only a very small part of the total field of the insect's vision. This is due to the fact that each ommatidium is surrounded by black pigmented cells, which ab- sorb or reflect the light, as shown in Fig, 46, so that only those rays which come in a straight line impress the retina. Thus the whole view formed by the images from all the ommatidia as they reach the optic nerve must be like that of a mosaic. Insects are able to distinguish forms at but relatively short distances, vary- ing from two to five feet, and to see distinctly only near-by objects. Large eyes, as those of the dragon- fly, give a wide field of vision, and numerous facets would give a greater distinctness of vision. In- sects' eyes are well adapted to detect motion, as a moving object affects the facets in succession, and motion is thus observed without moving the eyes. They are able to distinguish colors and often respond quite definitely to them, but their color sense seems to have a different range from that of man, as ants are sensible to the ultra-violet rays. Fig. 45. Portion of compound eye of fly (Calliphora vomitoria), radial section c, cornea; /', iris pigment; n, nerve fibers ; >ic, nerve cells ; r, retinal pig- ment; /, trachea. (After Hickson, from Folsom)
Text Appearing After Image:
Y\G. 46. rilustrating mode of vision in compound eye " The light enters through the cornea. The rays which strike the sides of each tube or cone are absorbed by the black pigment which surrounds the tube. Ac- cordingly those rays of light only which pass through the crystalline cones directly (or are reflected from their sides), such as a-a', b-b', c-c', d-d', e-e', will ever affect the nerves at a', b', c', d\ c'." (After Lubbock, from S. J. Hunter)

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