PROBLEM I. The Khids of Ajiiffials of the Earth 



4' 



Fig. 50 These are representatives of each of the five principal groups of the Arthro- 

 pod Phylum. Which cormnon afiinial is an example of each group':' 



PHYLUM - ARTHROPODS 



Jointed-Legged Invertebrates 



A glance at the arthropods. The in- 

 vertebrates with jointed legs, or ar- 

 thropods, are the most complex inverte- 

 brates. You can recognize an arthropod 

 by two characteristics: they have jointed 

 legs and they have an external (outside) 

 skeleton made not of bone or cartilage, 

 but largely of a material called chitin 

 (ky'tin). Most of the arthropods can 

 be classified in five groups or classes. 

 Examples of these five classes are repre- 

 sented in Figure 50: the insects, the 

 spiders, the hundred-leggers, the thou- 

 sand-leggers, and the crustaceans (crus- 

 tay 'shuns) which include crabs and 

 lobsters. 



What is an insect? Let us begin our 

 study with the most common arthro- 

 pods, the "insects. Insects differ from the 

 other arthropods in that they have six 

 legs and three distinct body parts: a 

 head with feelers called anteTi?iae (an- 

 ten'nee), a thorax with three pairs of 

 legs, and an abdomen (ab-doh'men). 

 The abdomen never has legs. In the ab- 

 domen you can see distinct rings called 



seginents. Most insects have two pairs 

 of wings, but you cannot depend on 

 this as a way of recognizing insects, since 

 some insects have only one pair and 

 others have no wings at all. The wings, 

 legs, and feelers are called appendages 

 (ap-pend'a-jes). If you examine Figure 

 59 and the other pictures of insects, you 

 will see the parts mentioned here. 



Most insects have large eyes, called 

 compound eyes because each eye con- 

 sists of many six-sided lenses. Insects 

 can hear, too. Some have eardrums; some 

 seem to use the feelers as organs of 

 hearing. But the feelers seem to serve 

 also as organs of smell and touch. 



Insect flight is very different from bird 

 flight. In most insects the wings move 

 with astonishing speed. The house fly's 

 beat is about 3 30 times a second. You can 

 understand why it makes a buzz. How- 

 ever, the speed is not the same in all in- 

 sects. The grasshopper has been timed at 

 twenty miles an hour. The "darning 

 needle" can fly at the rate of sixty miles 

 an hour but no insect flies far without 

 stopping. 



The life story of an insect. Let us trace 

 the life story of a common insect, a 



