THE FLY. 87 



begins with the thorax, which consists of three divisions or seg- 

 ments. Each of these has a name : the first is called the pro-thorax, 

 and bears the first pair of legs; the second, the meso-thorax, which 

 is much the largest of the three, bears the second pair of legs and 

 the two wings ; and the third, the meta-thorax, bears the third 

 pair of legs. This segment has also a pair of curious organs called 

 halteres or balancers. They are very much shorter than the wings, 

 and end in a knob. Their use is not known. As they occupy 

 the place of the hind wings in four-winged insects, some suppose 

 they are rudiments of these wings. It is known that in flying they 

 are in quick vibration, and some therefore have concluded that 

 they help to steady the fly, and have called them balancers. This 

 is one of several problems which the little House-fly offers for 

 solution. 



The abdomen has nine segments. Six of these are easily dis- 

 tinguishable ; not so the other three. Vv^e have thus made out 

 the thirteen segments. But it should be mentioned that some 

 authors call the number seventeen. They say the head is com- 

 posed of five segments instead of one, so the difference lies in the 

 head only. Of the skin, or integument of the fly, it will suffice 

 to say that it is at once a marvel of lightness, thinness, strength, 

 and elasticity. 



Turning again to the head, there will be seen, standing out 

 in front, two small organs called antenna, having several joints; 

 the most conspicuous of which is like a short club with a feather 

 fixed to one of its sides. The use of these organs has hitherto 

 baffled investigation. The general opinion seems to be that they 

 are feelers, and have a sense with which we are unacquainted. 



The eyes are immovable and compound, having lenses or 

 facets hexagonal in form, numbering altogether about four thousand. 

 Though so numerous, they each have a tube leading to the 

 brain, called a pyramid or cone, because of its length and form. 

 Wonderful to relate, these are all isolated from each other by 

 red-coloured pigment. It is by these cones that the impression of 

 objects in the field of view is conveyed to the brain. An opinion 

 prevails that no two lenses see the same object at the same time; 

 but when it is remembered that we ourselves see an object with 

 both eyes without confusion, it is not difficult to conceive that 



