150 THE SENSE ORGANS [CH. 



Optical Phenomena of the Compound Eye. 



When a Dragonfly is caught and held in the hand, the eyes 

 are seen to glow with a most beautiful light, generally of a semi- 

 metallic green or blue colour, sometimes red, brown or grey. 

 This is a reflection of light from the interior of the eye, and is 

 called the Internal Light. 



Again, if we peer into the eye, there will be seen, apparently 

 deep down in the eye, a very distinct black spot, surrounded by 

 six less distinct but similar black spots. These are the pseudo- 

 pupils. The central pseudo-pupil is always exactly opposite the 

 eye of the observer, and moves with great rapidity as his position 

 changes. Its formation is due to the almost complete absorption 

 of the direct light passing straight in through the corneal lens, 

 while part at least of the oblique light is reflected back. Thus 

 the central area appears black, the surrounding parts bright. 

 The six secondary pseudo-pupils are probably diffraction-images 

 corresponding with the six sides of the hexagon surrounding the 

 central pseudo-pupil. If the curvature of that part of the eye 

 gazed into be not approximately symmetrical (as, for instance, 

 near the border of the eye), the pseudo-pupils will not appear 

 rounded, but distorted and elongated. 



Further, we can easily remark, in the eye of a live Dragonfly, 

 two or three irregular greyish areas, which, like the pseudo-pupils, 

 move when the observer moves. These are pseudo-pupils of the 

 third order, derived from the secondary pseudo-pupils in the 

 same way that the latter are derived from the central one. They 

 are actually twelve in number, but only two or three can fall 

 within the limits of the cornea, for any given position of observation. 



The Eyes in the Larva. 



The compound eyes are functional, and of considerable size, 

 in the newly-hatched larva. The ocelli, on the other hand, only 

 appear at metamorphosis in the Anisoptera, and late in larval 

 life in the Zygoptera. In the case of the compound eye, the 

 number of ommatidia, as well as the length of the retinulae, 

 increases at each ecdysis, while the optic ganglion becomes 

 gradually more separated off from the brain proper, and increases 



