CHAPTER VII 



Photoreception 



The range of radiations in the electromagnetic spectrum extends from 

 about 10^2 to about 10~^ m,a. Only a very small segment of this spec- 

 trum, from about 253 to 700 m[i, can be detected by organisms. The 

 prime requisite for receiving the radiant energy of this narrow band of 

 the spectrum is a pigment or pigments that will absorb in these wave- 

 lengths. For the organism to derive useful information from this 

 energy absorption, bioelectric potentials must be produced. In insects, 

 cells capable of accomplishing these ends are grouped together in three 

 organs: the compound eyes, the dorsal ocelli, the lateral ocelH. Of 

 these the compound eyes, by virtue of their predominant role in be- 

 haviour and their extraordinarily large complement of cells, are by far 

 the most important. These great sensitive spots on the surface of the 

 head, with no lids to shield them, are in a continuous state of activity 

 from the light impinging upon them during the lifetime of the insect. 



THE COMPOUND EYE 

 Structure 



The principal components of the compound eye are the monopolar 

 neurons that absorb radiant energy and generate action potentials and 

 the transparent areas of the cuticle and internal lenses overlying them. 

 Since cuticle overlies all of the body surface, transparency, or at least 

 translucency, is a sine qua non for the functioning of the photo- 

 receptors. Further modification of the cuticle, however, in respect to 

 shape has given it the added property of gathering light. Together with 

 clear cellular bodies, the crystalline cones, it constitutes a lens system, 

 the dioptric apparatus of the eye. 



The compound eye is a collection of sensilla known as ommatidia 

 (Fig. 6). Each ommatidium is constructed of a corneal lens, a crystal- 

 line cone, a number of primary neurons, and enveloping cells. In 

 primitive insects (e.g., Machilis, one of the Thysanura) the two cells 

 which secreted the cornea lie immediately beneath it, that is, between 

 the cornea and the crystalline cone. In most insects these cells with- 

 draw to a position on either side of the crystaUine cone where, having 

 become pigmented, they are called corneal pigment cells. 



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