70 NATURAL HISTORY OF WASPS. 
in the compound eye by a layer of black paint, as 
fixed as the stops in our microscopes. All is arranged 
at once on the best. plan for the particular object in 
view; the insect can do what it is intended to do 
perfectly, and nothing else. All is as strong as cast- 
iron, so to say, but if anything break, like cast-iron, it 
cannot be mended. 
For a thing of sucha size, this plan, and for another 
size, another plan of construction is preferable. This 
we learn by the common rules of Mechanics. But, 
beyond this mechanism which we can copy, there is 
something which we cannot copy, nor quite under- 
stand, in the adaptation of the functions as well as 
the structural arrangements of the parts to the re- 
quirements of the animal. In the higher animals, still 
keeping to the instance before us, various mechanical 
contrivances are provided, to moderate the admission 
of light into the eye, and to adjust the focus of sight. 
In the insect we find none, or next to none, of these; 
yet, for all that we can see, they are just as neces- 
sary for an insect as for a bird. A wasp, for the 
speed of its flight, needs a range of vision.as long and 
as wide as that of many birds. Again, its habits of 
working alternately in bright sunshine and in almost 
absolute darkness would seem to require as perfect 
an adaptation of its eyes to these alternatives as the 
nocturnal beasts of prey possess. But the adjustment 
of the focus and the mode of the adaptation equally 
baffle us, and we must be content to leave the ques- 
tion as to how these are effected unanswered. 
The form and outline of the compound eyes in 
the wasp tribe, both solitary and social, should be 
noticed. They are separated into two limbs, so 
