4 STRUCTURE OF THE LEPIDOPTERA 



you will generally see a ground colour of grey, blue, brown, or 

 black ; but when viewed at certain angles in a strong light the 

 most gorgeous hues of metallic brilliancy gold, copper, and bronze 

 are to be observed. All such colours are due to the reflection of 

 light from the colouring matter that lies between the numerous 

 conical compartments. 



A glance at _the section of a compound eye will show 

 you that all the little cones radiate from a common centre. 

 And, as each little compartment is surrounded by opaque 

 colouring matter, it is clear that perpendicular rays only are 

 capable of penetrating to its base and exciting the nerve fibre that 

 lies there. Thus each little division of a compound eye forms its 

 own image of the object that happens to be exactly opposite its 

 facet. But how many facets do we find in a single eye ? Some- 

 times only a few hundreds, but some- 

 times as many as seventeen or eighteen 

 thousand ! We must not, however, 

 conclude that the nature of the vision 

 of butterflies and moths is necessarily 

 very different from our own. We have 

 two eyes, but the images formed by them 

 are both blended, so that we do not see 

 double. We can understand, therefore, 

 FIG. 4. SECTION OF THE tnat tlle thousands of images formed in 

 EYE OF AN INSECT. a single eye may be blended together so 



as to form one continuous picture. Still 



there remains this difference : while in our own case the two images 

 formed by the two eyes are practically the same, in the caseof 

 insects every one of the little conical tubes of a compound eye 

 forms an image of an object that cannot possibly be formed by any 

 one of the others. Thus, if the lepidopterous insect sees a continuous 

 picture of its surroundings, such a picture is produced by the over- 

 lapping and blending, at their edges, of hundreds or thousands of 

 distinct parts. 



There is yet another interesting difference between the vision 

 of these insects and that of ourselves. As already stated, our two 

 eyes are both turned toward the same point at the same time. 

 But look at the butterfly's eyes. Here are no movable eyeballs, 

 and the two eyes, placed as they are at the sides of the head, are 

 always turned in opposite directions. The corner, too, are very 

 convex ; and consequently the range of vision is vastly wider than 



