9 8 



ENTOMOLOGY 



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and enclose the retinula proximally. All these 

 parts are hypodermal in origin, as is also the 

 fenestrate basement membrane, through which 

 pass tracheae and nerve fibers. The nerve 

 fibrillae, which are ultimate branches of the 

 optic nerve, pass into the retinal cells the 

 end-organs of vision. Under the basement 

 membrane is a fibrous optic tract of com- 

 plex structure. 



Compound eyes are of three types: (i) 

 eucone, in which the cone-cells form solid crys- 

 talline cones; (2) pseudocone, in which the cone- 

 cells contain a transparent fluid; and (3) acone, 

 in which there are no cones, though the cone- 

 cells are present. 



Physiology. After much experimentation 

 and discussion upon the physiology of the com- 

 pound eye the subject of the monumental 

 works of Grenacher and Exner Miiller's 

 " mosaic" theory is still generally accepted, 

 though it was proposed early in the last cen- 

 tury. It is thought that an image is formed 

 by thousands of separate points of light, each 

 of which corresponds to a distinct field of 

 vision in the external world. Each ommati- 

 dium is adapted to transmit light along its axis 

 only (Fig. 145), as oblique rays are lost by 

 absorption in the black pigment which sur- 



. . , . . . 



rounds the crystalline cone and the axial 

 rna bdom. Along the rhabdom, then, light can 



tion through middle region; reach and affect the terminations of the optic 

 s a ; ^"reft nerve. Each ommatidium does not itself form 

 membrane; c, cornea; , a picture i it simply preserves the intensity and 



nucleus; nv, nerve nbrillae; . 



pc, pseudocone; pg l , pg*, cells color of the light from one particular portion 

 cTc a onSfniS S r S^e^; <* the field of vision; and when this is done by 

 r, one of the six retinal cells hundreds or thousands of contiguous ommati- 



which compose the retinula ;rh, ,. . A n .-, . , 



rhabdom.composedofsixrhab- dia, an image results. All that the painter 



domeres; I. trachea ; to, tracheal does, who Copies an object, is to put together 



vesicle. After HICKSON. r J 



patches of light in the same relations of qual- 

 ity and position that he finds in the object itself and this is essen- 



PIG. 144. Structure of an 

 ommatidium of Calliphora 

 vomitoria. A, radial section 

 (chiefly); B, transverse sec- 



