AND THE MORPHOLOGY OP THE EYE IN INSECTS. 417 



(II.) Eyes in which the dioptron is completely separated from the neuron by the 

 membrana basilaris, but in which the retina is divided info distinct retinulce. This is 

 the condition of the compound eye of the Dragon-fly larva, in the imago of the Gnats, 

 and in the Orthoptera. 



(III.) Eyes in which the dioptron is completely separated from the neuron, and the 

 retina is continuous, but in which the nerve-fibres of the optic nerve are arranged in 

 distinct fasciculi : this condition holds in many Hymenoptera, Hemiptera, and Lepidoptera. 



(IV.) Eyes in which the dioptron is completely separated from the nem*on, in which 

 the retina is continuous, and the fibres of the optic nerve form a single bundle and 

 decussate completely . This condition is found in the Muscidee, the Diurnal Lepidoptera, 

 and in the perfect form of some Dragon-flies {Agrion). 



It would appear therefore that the compound eye is to be regarded as a collection of 

 compound ocelli, in which the tendency to close union is greater nearer to the surface 

 than in its deeper portion, the optic nerves being the last parts to become fused into a 

 single compound structure. 



DESCRIPTION OF THE PLATES. 



The small letters indicate the following parts in all the figures. 



c. The cornea. 



cc. The crystaline cone. 



en. Subcorneal nuclei. 



eh. The chamber. 



c6. Ciliary bodies. 



cl l .cP. Outer and inner cellular 

 layers. 



g x .g 2 . Sections of the optic gan- 

 glion. 



h. Hypodermis. 



Fig. 1. A semidiagrammatic section of the eye and optic ganglion of a Blow-fly. 



1 a. The membrana basilaris of the same insect seen from its neural surface, ped, pedicle of the scleral 

 ring. Both figures show the large muscle m.c. attached to the inner edge of the scleral ring, sr. 



2. A diagram showing the manner in which the image is formed on the Arthropod retina. 



3. An optical section of two of the segments of the dioptron of the Plume Moth (Pterophorus pen- 



tadactylus) . 



4. A diagram showing the optical relations of the same. 



5. A semidiagrammatic representation of a vertical section through the great eyes of a Dragon-fly 



(JEshnia), showing the lymph-channels ; a.a. afferent vessels; b.b. efferent openings. 



6. A section through the scleral ring and membrana basilaris in the region of the efferent lymph- 



path, showing the radiating fibres )/, from the eye of yEshnia. 



7. A similar section from the eye of Agrion. 



8. A section through the scleral ring, s.r., of Agrion, showing a marginal lymph-opening. 



9. The afferent vessels of the dioptron of sEshnia. 



