THE INSECTS— ARTHROPODS 



213 



rounding the thorax, and remain distinct with independent movement in 

 the abdomen. 



Insects have three pairs of legs. This is the most distinctive charac- 

 teristic of the insects because no other group of animals has six legs and 

 all of this group have this number. There is one pair attached to the 

 ventral surface of each segment of the thorax. 



Courtesy General Biological Supply House 



Fig. 15.3. A tomato worm parasitized by the bracoid flies. Some insects are highly 



beneficial through destruction of other insects that are harmful. This tomato worm 



will never complete its metamorphosis into a sphinx moth. A bracoid fly has laid its 



eggs in the caterpillar and the young have now formed cocoons. 



Insects usually have two pairs of wings, one pair attached to each 

 of the two posterior segments of the thorax. There are a few, however, 

 that have no wings at all and others with only one pair, so this is not a 

 universal characteristic of the group. 



Insects breathe by spiracles. These are tiny breathing pores found 

 on the side of the abdomen, one pair on each segment, and usually two 

 additional pairs on the posterior thoracic segments. These connect in- 

 ternally with tiny tubes, trachea, which carry the air directly to the parts 



