THE SIMPLE AND COMPOUND EYES OP INSECTS. 
599 
least, a very feeble one, whilst others have a very convex lens. The same is true in 
the Diurnal Lepidoptera. In Hydrometra the corneal facets are composed of two 
parts of different refractive powers. The outer portion of the cornea is more strongly 
refractive than the inner portion ; it is also more convex on its inner surface than the 
inner surface of the corneal facet, so that it presents the condition of a very powerful 
bi-con vex lens in apposition with a second lens, which is concavo-convex, the two fitted 
together like an achromatic object glass. In Dyticus there is also a remarkable 
arrangement : the corneal facets have a scleral cone adherent to their inner surfaces. 
O 
I have at present examined only dried specimens, but hope to continue the investiga- 
tion. 
The region of binocular vision . — In most insects the field of vision in the two eyes 
has a common portion in the peripheral region in the vicinity of the mouth ; in this 
region the radius of curvature of the cornea is very short. It is, therefore, only 
adapted for the acute vision of very near objects. It is chiefly developed in pre- 
daceous insects. It jirobably serves the insect in judging of the distance of objects 
from the mouth. 
J. Muller, in his classical work ‘ On the Comparative Physiology of Vision,’ states 
that no portion of the compound eye in any of the insects he examined corresponded 
in the direction of the axis of the facets with the eye of the opposite side ; but he 
does not appear to have examined the eyes with sufficient minuteness to have been 
able to detect the slight overlapping of the two fields which I have described. 
Description of the Plates. 
PLATE 52. 
Pig. 1. The ocellus of Eristalis. 
Fig. la. One of the rod cells- of the same. 
Fig. 2. A. vertical section of a portion of the compound eye of the common Crane-fly. 
Fig. 3. A vertical section of a portion of the chamber of the compound eye of an 
immature Crane-fly, showing the rod cells and the facellus. 
Fig. 3a. One of the facets of the same eye seen from without, showing the extremities 
of the rod cells beneath. 
Fig. 4. A transverse section through the middle of the facellus. 
Fig. 4a. A section through the lower extremity of the facellus. 
Fig. 5. The stemon and nervous retina of the same. 
Fig. 6. A vertical section of a portion of the compound eye of the female of Formica 
rufa (from a specimen preserved in spirit). 
Fig. 7. The rod cells, facellus, and stemon of the eye of the Wasp. 
MDCCCLXXVIII. 4 H 
