FURTHER REMARKS ON THE COMPOUND EYE. 577 



c. Accessory Optic Phenomena in the Compound Eye. 



Exner says : ' It is manifest that, in so complex a structure 

 as the compound eye, phenomena must occur from refraction, 

 reflection, absorption, etc., which are not directly connected 

 with vision. Owing to the great variations in the structure of 

 this organ, these phenomena vary in different species, but cer- 

 tain phenomena are typical, especially the internal light and 

 pseudo-pupils/ which he proceeds to discuss. 



The Internal Light is a brilliant metallic reflection from the 

 interior of the compound eye, which occupies a greater or less 

 surface of the cornea opposite to the eye of the observer. 

 It is characteristic in the Sphingidae and Noctuae, and is 

 apparently confined to nocturnal and semi-nocturnal insects. 



If a large moth (a Sphinx or Noctuid) is kept in the dark 

 for a few hours, the eyes are seen to shine like those of a cat. 

 In a bright light the luminous reflection gradually contracts in 

 diameter, becomes more and more feeble, and disappears in a 

 shorter or longer time from a minute to five minutes in day- 

 light and more rapidly the brighter the light. The pheno- 

 menon only recurs after the insect has been kept in the dark 

 for from half an hour to three or four hours. 



Leydig described this reflection as similar to a glowing coal, 

 and ascribed it to a reflection from a tapetum. He says 

 that the tapetum is tracheal, and considers that the reflection 

 either depends on a fuller condition of the tracheal vessels, or 

 on a dilated condition of the pupils or perforations in the sub- 

 corneal pigment. 



Max Schultze regards the reflection as being produced by the 

 laminated structure of the visual rods, which he describes, and 

 not by the tracheal vessels. 



Exner [252] says that, by the help of an ophthalmoscope, 

 most faceted eyes, although not all, exhibit a reflex that is, 

 the light which enters the eye returns in exact analogy with 

 what happens in Vertebrates. This reflex is limited to a 

 circular spot, which is nearly or absolutely identical with the 

 optical appearance known as a pseudo-pupil. 



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