124 



NATURAL HISTORY. 



distinct types of eyes found in arthropods, but our space will allow but 

 a brief description of the compound eyes, a form which occurs nowhere 



else in the animal kingdom. As the 

 name indicates, these eyes are com- 

 posed of a number of eyes united 

 together. As shown in the cut, each 

 of these elements consists of a lens 

 for concentrating the light upon 

 the percipient portions, which are in 

 direct communication with nerves 

 from the brain. How an insect or 

 a crab sees with one of these com- 

 plex organs has not yet been settled. 

 At first thought, it would seem that 

 the production of so many images 

 — one to each of the thousands of 

 elements of the compound eye — 

 would result in confusion. It would 

 appear, however, from our present 

 knowledge that each element can see 

 only the object directly in front of 

 it, and hence the impression on the 

 retina may be regarded as a mosaic, 

 ■composed of thousands of distinct images. Further, it is probable that 

 the vision resulting from this complex organ has none of the distinctness 

 of that of the human eye ; form is but very roughly seen, and the principal 

 function of the organs is the recognition of color and differences in the 

 strength of the light. 



While many of the arthropods have a gradual growth, from the egg to 

 the adult, — part after part forming in its proper sequence, thus, at first, 

 sketching out the broader outlines of the adult, and then gradually work- 

 ing in the details, — others at first develop into forms which in nowise 

 resemble the parents, and then suddenly change their whole appearance, 

 i lie succeeding stage in nowise resembling its predecessor. This last is meta- 

 morphosis. Compare, for instance, the silk-worm, with its chrysalis envel- 

 oped in its silken house, and this in turn with the delicate moth which 

 emerges from it. and which lays the eggs from which a new brood of silk- 

 worms are to hatch, and one will see how enormous are the changes 

 involved. One of the stages bears not the slightest resemblance to either 

 of the others, but still generation after generation of these animals go 

 through these seemingly useless modifications. 



Fig. 111. — Section of a portion of a compound 

 eye of a fly (Mused), greatly enlarged. The un- 

 dulations of the upper margin represent the 

 separate lenses, and each of the rods forms a 

 separate optical aud percipient apparatus. 



