THE DKAGON FLY. 45 



If these parts are difficult to see, compress a maxilla 

 between two glass slips, and examine it under slight magni- 

 fication. Compare this mouth with the mouth of a butter- 

 fly. To what method of feeding is this mouth adapted? 



II. Thorax and Abdomen. Find in thorax and abdomen 

 all the parts as mentioned for the butterfly, noting these 

 special points : 



1. The relative development of the thoracic segments. 

 Which is smallest, and which largest? 



2. The approximation of the legs. Why are the three 

 pairs thus bunched together; i.e., what advantage is there 

 to the animal in such arrangement? 



3. The richly veined, naked, transparent wings. The 

 costal vein is on the anterior margin of each wing, and the 

 subcostal and median veins are parallel with it as far as 

 the middle of the wing. There a stout cross vein (the 

 nodus) crosses these three, dividing the wing in halves. 

 On the costal margin beyond the nodus is a thickened 

 dark spot (the wing spot, or ptero stigma). 



The close correspondence between the appendages of the 

 dragon fly and those of the butterfly will have been noticed 

 ere this by every student. That the legs and wings and 

 other parts have essentially the same structure in the two 

 animals, is very apparent. Had the origin of these parts 

 been studied in the developing young of each animal, a 

 similar correspondence would have been noted. When 

 the parts of two animals show correspondence in structure 

 and origin, such parts are said to be homologous; when 

 they correspond merely in use, and not in either structure 

 or origin, they are said to be analogous. 



The legs of a butterfly are analogous with the cilia of a 

 slipper animalcule, because both structures have similar 

 use as locomotive organs ; but they are not homologous. 

 The wings of the dragon fly and of the butterfly are homol- 

 ogous, although very different in appearance; for they 



