IN THE GENUS ANTHOPHORABIA. 163 



distinguishing forms depend, exists in the most perfect of the eyes of the male, those of 

 the vertex, I am not yet entirely satisfied. I have certainly detected appearances, in the 

 nymph or pupa state of the male (fig. 10 b), which have led me to think that the lens is then 

 present ; but I have not satisfied myself of this in the perfect insect, and hence the ap- 

 pearances seen may have been due simply to the vitreous body, as it exists in some of the 

 lower forms of the eye among the Annelida. 



Whether, however, the lens does or does not exist, is of little importance with reference 

 to the simple question as to whether these structures in the male Anthophorabia are the 

 true homologues of the eyes in the female. That they are so I have not the slightest 

 hesitation, after what I have shown, in affirming. The presence of a cornea, which covers 

 a chamber lined with pigment, is sufficient proof to the physiologist and anatomist of the 

 nature of the function of the structure. 



The form of the cornea, however, shows that the field of vision is very limited. The 

 cornea, as already stated, is but very slightly convex, being almost level with the surface 

 of the head. This fact may have conducted some to the opinion that these are not visual 

 organs. But neither the actual size of a simple eye, its form, nor the degree of its con- 

 vexity, has any necessary connexion with the simple faculty of perceiving light. The 

 convexity of the cornea has relation only to the extent of angle, ox field of sight. The 

 more convex, and the more elevated the eye is above the surface of the head, the greater 

 proportion of a sphere does it necessarily include; and, as long ago shown by Prof. Muller, 

 the greater the segment of a sphere formed by the eye, the greater is its expanse, ox field 

 of vision ; while, on the contrary, the flatter or more depressed it is the more limited is 

 this field, and the shallower the chamber the shorter is its focal distance. 



The presence of the lens in the simple eye is essential to rendering the sight of images, 

 and the appreciation of form, more or less perfect ; and it does this in proportion to the 

 more or less correct relation which it bears to other conditions coexistent with it. 



With regard to the nerves supplied to these eyes, I may state that although I have not 

 been able to trace those of the vertex so satisfactorily in the male Anthophorabia as I 

 could have wished, owing to the numerous muscular fibres which run parallel to them, 

 yet I have succeeded in tracing the optic nerve (d d) from the side of the cephalic ganglion, 

 or rudimentary brain (d), transversely, in the direction of one of the lateral ocelli (b) ; 

 and I believe, also, that I have distinguished the nerve which goes to the middle eye of the 

 vertex (c). The nervous trunk which is given to the middle ocellus in Insects I have 

 already shown, in my paper on Pteronarcys*, is formed of two closely approximated 

 nerves, one from each cephalic ganglion, as found by a careful dissection of that insect, and 

 also of several Hymenoptera ; and this probably is its condition in all other insects with 

 three ocelli on the upper surface of the head. 



I may here also refer to what is stated in my paper on Meloe'f, that there seems reason 

 to think that in the Arachnida, and probably also in insects, the ocelli originate in the 

 same way as the dermal tubercles, from which they appear to differ chiefly in the mode 

 of development of their nuclei and nucleoli. 



* Linn. Trans, vol. xx. p. 440. t W« v °l- xx - P- 342. 



