Compound eye 



Lens of ommatidium 



Perforated 

 supporting 

 membrane 



Retinal 

 pigment 



Retinal 

 cells 



Corneal lens 

 Cone 



Iris cells Lens- 



growing 

 cells 



INSECT EYES 



The head of a locust showing the compound eye with its many facets, each repre- 

 senting the exposed surface of an ommatidium, or single eye, and an ommatidium 

 seen in section cut lengthwise. In the arthropods, or animals with jointed legs, there 

 are compound eyes, as well as simple ones 



on the front of the head. The mouth, at the lower end of the head, con- 

 sists of several distinct parts. 



The thorax, which is covered by the wings when the animal is at rest, 

 is made up of three more or less distinct segments, or rings. Each segment 

 carries one pair of jointed legs. Two of the segments carry one pair of 

 wings each, and the anterior (forward) wings cover the posterior (hind) 

 ones when at rest. 



The abdomen, like the thorax, is distinctly segmented. Indeed, the name 

 of this class of animals. Insects, refers to the fact that the body is "cut in", 

 or segmented, like the body of a caterpillar. This is easily observed in the 

 abdomen of dragonfiies, bees, moths and beetles (see illustrations oppo- 

 site). The foremost segment has on each side a small tympanum, or drum, 

 which is actually an eardrum (see illustration opposite). The hindmost seg- 

 ment bears special structures that have to do with the removal of refuse, 

 other structures with reproduction. In the female these terminal parts to- 

 gether constitute the egg-laying organ, or ovipositor. 



The bodies of insects and of mammals, like the bodies of plants, consist 

 of many distinct parts or organs. And if we take the time to watch any ani- 

 mals over a long period, we see that they too, like plants, undergo regular 

 changes in form and in behavior. 



Comparing The moment we begin to compare carefully, we dis- 

 cover that structures can correspond in many ways and yet not be the 

 same, even if we call them by the same name. Thus parts may be "alike" 

 in relative position— as the "tail" of a cat and the "tail" of a dragonfly, 



15 



