INSECT VISITORS 



141 



This theory of Mosaic Visum in insects was propounded as early as 1826 

 by Johannes Miiller, the Berlin physiologist. Its correctness was afterwards doubted, 

 and the suggestion was made that each component of the compound eye receives 

 a complete but reversed image of the external object, so that the insect perceives the 

 object as many times as there are facets (Gottsche, 1852). More recent investigation 

 (Grenacher, 1879; Exner, 1875, 1881, 1889, 1891) showed this theory o{ Multiple 

 Vision or Theory of Images to be untenable, and Miiller's theory of mosaic vision 

 to be correct. The theory has undergone, however, an important modification, 

 it now being believed that a number of crystalline cones are concerned in the 

 perception of each point of light, so that in consequence of refraction, a dioptric 

 but erect picture results (Claus, op, cit., p. 569). 



The insect, therefore, perceives only a part of the object by means, of each 

 facet and, since each facet sees a different part, the result is a compound, mosaic, 

 erect picture. 



There is a great deal of variation as to the number and size of the facets, 

 but the numbers appear to be tolerably constant within the various orders of 

 insects. The house-fly, for example, possesses about 4,000 facets, the goat-moth 

 (Cossus) 11,000, the death's-head-moth 12,000, some Neuroptera 12,000, a dragon-fly 

 (Aeschna) 20,000, and a beetle (Mordella) 25,000. Since the recognition of an 

 object is only possible by the summation of the separate activities of the individual 

 facets, distinctness of vision is in proportion to the number of facets (Exner). 

 The smaller the facets and the longer their crystalline cones, the fewer (but at 

 the same time the more definite) are the rays of light which affect them, and the 

 more limited are the parts of the outer world which can be perceived. The larger 

 the facets and the shorter their crystalline cones, the more numerous (and the 

 more intense and widely distributed) are the effective rays of light, but at the same 

 time perception is more diffuse. Many small facets diminish the intensity of light, 

 but increase the distinctness of vision, or power of localization. If the whole eye 

 is strongly curved it receives light from many different directions, and the field 

 of vision is enlarged. At the same time fewer facets are met by the rays of light 

 from one and the same point of an object, and so the field of vision of one 

 facet is more sharply marked off from that of others, and is consequently more 

 distinct. 



It follows from these views of Exner that insects which fly in darkness (e.g. 

 moths) possess larger and more strongly curved facets than diurnal insects (e.g. 

 butterflies). 



It appears from the calculations of Notthaft that insects cannot recognize 

 clearly objects which are more than 60 cm. away, so that they are extremely 

 short-sighted. Beyond the limit named vision is dim, though movements, both 

 of light or dark objects, can be distinguished at greater distances. According to 

 Plateau the visual range of insects never greatly exceeds 2 m. On an average, 

 Lepidoptera can see the movements of a large body at 1-5 m., flies at o-8 m., 

 and Hymenoptera at 0-5 m. Plateau is therefore of opinion that insects are guided 

 to flowers exclusively by the sense of smell. 



That insects are short-sighted is confirmed by the observations of Delpino 

 ('Ult. OSS./ Atti Soc. ital. sc. nat., Milano, xii, 1869, p. 10). On a meadow near 



