BUTTERFLIES AND BEES 



35 



mouths, we shall again 

 find many basic simi- 

 larities in spite of 

 the great differences. 

 Among animals that 

 are built on substan- 

 tially the same plan the 

 corresponding parts are 

 said to be homologous. 

 Thus, the thorax of one 

 insect is homologous 

 with the thorax of 

 another insect. More- 

 over, the thorax of a 

 butterfly is said to be 

 homologous with the 

 three distinct segments 

 immediately behind the 

 head of the caterpillar. 

 We may also say that 

 the three pairs of legs 

 on one insect are homol- 

 ogous with each other, 

 since they originate and 

 develop in the same 

 way, in spite of differ- 

 ences, as between the 

 hind leg of the grass- 

 hopper, for example, 

 and the front legs. In 

 the same way we con- 

 sider the "balancers" 

 of a fly homologous with the hind wings of the butterfly and 

 the wing covers of the beetle homologous with the front wings. 

 The idea of homology helps us to understand a certain same- 

 ness among organs that appear to be very different in structure 



Fig. 1 6. Homology in the appendages of 

 the lobster 



In the Crustacea all the appendages are built on 

 the same plan, but each segment of the body (rep- 

 resented by Roman numerals) has a distinctive 

 organ. I and // are sensory; III-V combine sen- 

 sory functions with food-getting; VI-VIII are 

 chiefly food-getters, but are also related to breath- 

 ing; IX is the nipper; X and XI are both grasp- 

 ing and locomotor organs; XII and XIII are 

 walking legs. The abdominal appendages XIV- 



XVIII are called swimmerets and probably assist 

 in slow swimming. XIV and XV are also related 

 to reproduction in the male, and in the female all 

 the swimmerets carry the hatching eggs and larvse. 



XIX and XX spread out into a flat tail-paddle, 

 used in swimming backward suddenly 



