1878.' 
The Senses of the Lower Animals. 
301 
tain blind species — occupy a position similar to that of the 
eyes in the Vertebrates, but, instead of consisting each of a 
simple lens, they are formed of an aggregation of lenses, 
varying greatly in number, and in some groups amounting 
to many thousands. From each of these lenses or facets a 
crystalline rod radiates downward to the nervous ganglia. 
The ocelli, or small eyes, placed on the top of the head are 
simple in structure, absent entirely in many species, and in 
others covered over with hair. 
We can scarcely doubt that a structure so widely differing 
from that which prevails among the higher animals must be 
accompanied with corresponding differences in function to 
some of which we have already referred. We know that 
the sight of many inserts must require a wide range. If 
they are to find their way back to their nests, hives, or other 
haunts, they must be able to recognise objedts at a very 
considerable distance, whilst in other cases their vision to 
be useful must be almost microscopic in its character. In 
accordance with this twofold need it has been observed that 
in many groups the upper lenses of the facetted eyes are 
considerably larger than the lower. Hence the former may 
possibly serve for the recognition of distant objedts, and the 
latter for the examination of such as are near at hand. 
Graber, however, maintains that the compound eyes are 
telescopic in their fundtion, whilst the ocelli are adapted 
solely for the perception of proximate objedts, whence their 
strong convexity. He remarks that these simple eyes occur 
especially in insedts whose locomotive powers are feeble, and 
whose entire sphere of adtion is restridted. To this view 
exception must be taken : the Neuroptera, such as the 
dragonflies, — perhaps the most locomotive of all insedts, — ■ 
the great majority of the Hymenoptera and Diptera, all of 
them swift, strong, and agile on the wing, possess these 
organs ; whilst in the Coleoptera, comparatively clumsy and 
imperfedt flyers, they are generally absent. 
The ocelli, in the Hymenoptera at least, are, according to 
F. Muller, adapted to a very feeble illumination, their size 
increasing as the habits of the species are more nodturnal. 
But even insedts in which no eyes can be traced — such as 
the blind grubs of Lucilia C cesar, L. evistalis, and other flies 
— have, according to Pouchet, a perception of the intensity 
and the diredtion of incident rays of light, diffused appa- 
rently over the whole surface of the body. 
The eyes of spiders are not facetted like those of insedts, 
and occupy a position more corresponding to the ocelli than 
to the compound eyes of insedts. This position, awkward 
