STRUCTURE OF THE HOUSE-FLY 45 



which latter are said to possess each four thousand 

 facets. The compound eyes of the male fly are 

 proportionately larger than those of the female ; it is 

 quite observable that they approach each other more 

 closely at the top of the head, a feature of sex differenti- 

 ation which is shared with bees, wasps, and many 

 other insect creatures. It is thought that a single 

 brain image arises from the combined views of the 

 four thousand facets of the compound eyes blending 

 with the view conveyed through the "ocelli." How- 

 ever, it is a most curious fact that it is the inconspicuous 

 ocelli which are of supreme importance visually. The 

 compound eyes have doubtless some special function, 

 but throughout the insect world the size of compound 

 eyes is not a certain indication of keenness of sight. 

 The vision of the fly is good for distinguishing the 

 movement of any broad mass, but it is rather in- 

 effective for observing a thin line, as may be proved 

 by slowly lowering a knife blade, with a steady hand, 

 when its body may be severed before the fly takes 

 alarm. It is a remarkable fact that the family of 

 Tabanida (blood-sucking breeze flies), w r hich are 

 destitute of " ocelli," are the dullest sighted of all flies ; 

 in fact, at least semi-blind. Moses Harris observed 

 that a blue-bottle became practically blindfolded when 

 its ocelli were covered with an opaque pigment. 

 Probably this is the case with other insects. Bees, 

 which require long distance^ sight for home returning, 

 are well provided with ocelli. Butterflies, however, 

 without the use of ocelli have a distinct faculty of day- 

 light vision for a moderate distance. The investigation 

 of sight by blindfolding is very difficult in flies. 



