ANATOMY AND PHYSIOLOGY Gis 
“ superposition ”’ image being correspondingly bright—an ad- 
vantage, probably, in the case of nocturnal insects. 
Large convex eyes indicate a wide field of vision, while 
small numerous facets mean distinctness of vision, as Lubbock 
has pointed out. The closer the object the better the sight, 
for the greater will be the number of : 
Fic. 143. 
lenses employed to produce the impres- 2 
sion, as Mollock says. If Muller’s 
theory is true, an image may be formed 8 0 | 
of an object at any reasonable distance, 
no power of accommodation being ne- oo 
cessary; while if, on the other hand, 
each cornea with its crystalline cones 
had to form an image after the manner 
of an ordinary hand-lens, only objects 
at a definite distance could be imaged. 
The limit of the perception of form 
by insects is placed at about two meters 
for Lampyris, 1.50 meters for Lepi- 
doptera, 68 cm. for Diptera and 58 cm. 
for Hymenoptera. 
Diagram of outer, trans- 
parent portion of an omma- 
It is generally agreed, however, that  tidium to illustrate the 
% : transmission of an axial ray 
the compound eyes are specially adapted (4) and the repeated reflec- 
1 ° m™| i and absorptia Lime 
to perceive movements of objects. The "0? anc ansiiption ota 
oblique ray (B), which at 
sensitiveness of insects to even slight length emerges at C. p, iris 
movements 1s a matter of common ob- [Canes 
servation; often, however, these insects can be picked up with 
the fingers, if the operation is performed slowly until the insect 
is within the grasp. A moving object affects different facets in 
succession, without necessitating any turning of the eyes or the 
head, as in vertebrates. Furthermore, on the same principle, 
the compound eyes are serviceable for the perception of form 
when the insect itself is moving rapidly. 
The arrangement of the pigment depends adaptively upon 
the quality of the light, as Stefanowska and Exner have 
shown; thus, when the light is too strong, the iris and retinal 
2) 
