File:Elementary textbook of economic zoology and entomology ((c1915)) (21236511991).jpg

Title: Elementary textbook of economic zoology and entomology
Identifier: elementarytextbo00kell (find matches)
Year: (c1915) ((c10s)
Authors: Kellogg, Vernon L. (Vernon Lyman), 1867-1937; Doane, Rennie Wilbur, 1871-
Subjects: Zoology; Insect pests
Publisher: New York, H. Holt and company
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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132 ECONOMIC ZOOLOGY AND ENTOMOLOGY However, when the pigment surrounding the ommatidia is drawn back from their anterior ends, light rays can pass through the lateral walls of the ommatidia from the lenses of adjoining ommatidia, and thus the reflected rays from a single point in an object may reach and stimulate several adjacent rhabdomes, forming a picture in a somewhat different way from that by the strict mosaic method. This picture is called a superposition image as contrasted with the apposition image of the true mosaic vision. The focal distance of the lenses in the compound eyes is usually about two yards, so that these eyes see objects best at that distance from the insect. The sharpness or clearness of the image formed depends, too, on the number and size of the sepa- rate ommatidia. The smaller and the more numerous they are the more perfect will be the mosaic; that is, the more complete and clear will be the picture seen. The number of facets in the com- pound eyes of insects varies from three or four to twenty thousand or more. The simple eyes, or ocelli, are very different from the com-
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FIG. 54.—Part of corneal cuticle, showing facets, of the compound eye of a horse-fly, Therioplectes sp. (Greatly magnified.) pound eyes in make-up. Each ocellus has but one lens, but behind it is a varying number of sensitive or optic cells each with anterior crystalline part and posterior retinal or percip- ient part. But the very short focus of the lens, usually but a few inches, and the primitive character of the structure of the part behind the lens, limit the vision probably to little more than a perception of shadows in imperfect outline. The ocelli can only perceive objects very close to the insect, and then with but little clearness. In fact the vision of insects, either by means of compound or simple eyes, is at best imperfect when compared with that of the vertebrate animals. Although observation and experiment have shown that insects can dis-

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