146 



THE SENSE ORGANS 



[CH. 



A. T.S. through part of 

 compound eye of Diplacodes 

 haematodes Burm. (x 30) to 

 shew transition from pigment- 

 ed to unpigmented ommatidia. 

 B. T.S. through median eye-line 

 of Aeschna brevistyla Ramb. 

 ( x 45). ep epicranium ; fm fe- 

 nestrate membrane; hy hypo- 

 dermis. Original. 



groups is marked in the living insect by a change of colour, the 



upper part of the eye being usually 



of a reddish hue, the lower greenish 



or yellowish. In the Aeschninae the 



two groups of ommatidia are present, 



and can be separated by the difference 



of colour, the upper portion being 



usually a brilliant green or blue, the 



lower more greyish. But there is an 



intermediate zone in which the size of 



the facets is graded, and the amount 



of pigmentation gradually decreases Fig. 63. 



from below upwards. Ommatidia of 



both groups appear to be all of equal 



length in the Aeschnidae. 



The differentiation of two groups 

 of ommatidia is clearly correlated 

 with a difference in the use of the 

 two portions of the eye. During flight, 



the lateral and lower portions of the eye receive the impression 

 of the surrounding objects, while the upper portion of the eye 

 receives only rays of direct sunlight. It is not, therefore, necessary 

 for this latter portion to be able to distinguish clearly the form 

 of objects, provided that it is able to note the change in the 

 intensity of light from above, produced by the veiling of the sun 

 by clouds, or by the shadow of a possible enemy. While the 

 absence of pigment undoubtedly renders this part of the eye 

 unable to perceive definite forms clearly, it must also tend to 

 mitigate the direct glare of the sun, by allowing the rays of light 

 to diffuse equally in all directions. 



The Zygoptera, on the other hand, never hawk about in the 

 way that the larger Dragonflies do. It is essential for them, 

 whether at rest or flying close to the surface of the water, to see 

 surrounding objects clearly in all directions. Hence no differen- 

 tiation of the upper ommatidia has ever been needed in their case, 

 and none has arisen. 



The Median Eye-line. We have already mentioned the fact 

 (p. 11) that the large eyes of most Libellulidae and Aeschninae 



