THE SIMPLE AND COMPOUND EYES OP INSECTS. 
5‘J I 
In the eye of the Herald moth (tig. 38) I have found some very remarkable drop- 
like appendages at the inner extremities of some of the cones (a"), but I have not been 
able to make out their nature. I almost suspect they are the result of the rupture of 
the axial threads of the rhabdion, and are produced by the contraction of these threads, 
which, if such is the case, are viscous in the recent condition.* The thread-like 
prolongations of the cone are seen to end at their inner extremities in very long 
fusiform cells (c), which like the rhabdia are sometimes contained in tubular sheaths of 
chit in (fig. 41). The inner extremities of the fusiform cells are connected with stellate 
ganglion cells ; but the whole of the deeper structures in the few species I have 
examined are so deeply pigmented that I am not able to give any satisfactory details 
concerning*’ the ganglionic retina. 
o o o 
XI. General Remarks on the Morphology of the Eyes of Insects. 
Three forms of eye have been recognised in the Arthropoda since the time of 
J. Muller’s investigation of the subject : the simple, the aggregate, and the compound 
eye. 
In the simple eye there is no difficulty in recognising the signification of the rod-like 
elements which are situated beneath the cornea, or their epithelial origin. 
There can be little doubt but that we have the highest development of the 
aggregate eye in the so-called compound eye of the Nematocerous Diptera and of the 
Hymenoptera. 
As far as the cornea is concerned, these eyes do not differ from the true compound 
eyes of other insects and of many crustaceans ; but as I have shown, the deeper parts 
are similar to those of the simple eye in a high condition of differentiation. This 
form of eye is therefore to be regarded as a highly developed form of a connecting 
link between the simple and compound eye. 
I am at present unable to point out in a satisfactory manner the nature and 
morphological relations of the facellus. Although this structure is present in the eyes 
of many Lepidoptera, it is apparently absent in the true Diptera, unless the facelloid 
layer of the retina can be regarded as its representative. The morphological repre- 
sentative of the facellus is more probably found in the pigmented bundles at the 
inner extremity of the compound rhabdia of the periphery of the eye in these 
insects. 
A comparison of the parts in the aggregate eyes of the Nematocerous Diptera and 
Hymenoptera with the structures in the true compound eye is not difficult, but it 
can only be made in a tentative manner until the development of the aggregate eye 
has been more thoroughly worked out and a comparison has been made with the 
A similar contraction Las been observed by Max Schultze in tbe inner extremities of tlie rods of 
Vertebrates, as a post-mortem condition. (Arcbiv., band iii., p. 220.) 
MDCCCLXXV1II. 4 G 
