INSECT VISITORS I4t 
This theory of Mosaic Vision 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 of Multiple 
Viston 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 
