30 ON THE ANATOMY OF THE FLY. 



the great similarity of the contents of these follicles to those of 

 the spleen and other ductless glands in the vertebrata, I have 

 not the slightest doubt as to the similarity of their functions, 

 which is the elaboration of the circulating fluid. 



Section X. The Organs of Special Sense. 



The organs of special sensation in the fly are the compound 

 and simple eyes, the antenna, the halteres, and probably the 

 frontal sac, the cephalo-sternum, the lobes of the proboscis, 

 and maxillary palpi. 



With the exception of the visual organs, which closely 

 resemble the essential parts of those of the vertebrata, great 

 difficulties occur in determining the functions of the sensory 

 organs, and the removal of any of these, even when this can be 

 done without injury to other parts, is no test of its function. Flies 

 seem completely helpless when the antennee are removed, and 

 quite unable to fly if the halteres are cut off, yet this is no proof 

 that the quickness of the insect is due to its antenna), or that 

 flight is a function of its halteres. All the senses seem so closely 

 bound together, or correlated to each other in insects, that we can 

 hardly expect any to be normally exercised when one^is damaged. 

 Even in man himself, we know that nothing but experience 

 enables him to judge correctly of objects with a single sense, and 

 we should hardly expect a dog to act normally, if one of its 

 senses were suddenly removed. Apart from the pain which may 

 accompany the removal of an organ, the new relations with 

 the external world might be expected to produce an abnormal 

 manifestation in a creature, almost all the acts of which are 

 probably purely reflex. So that structure, analogy, and the habits 

 of the insect, are really the only guides in such investigations. 



The great compound eyes each consist of between four and 

 five thousand facets; each facet being a circular biconvex lens, the 

 thousandth of an inch in diameter, set in a flat hexagonal frame. 



