96 EVOLUTION AND GENETICS 



mesothorax thrust in between the normal meso- 

 thorax and the abdomen. It carries a pair of wings 

 that do not completely unfold. On this new meso- 

 thorax the characteristic arrangement of the bristles 

 is shown. Thus at a single step a typical region of the 

 body has doubled. The character is recessive. 



In the second figure ( h ) the dark color of the fly 

 is due to a factor called ebony. 



The size of adult flies varies according to the 

 amount of nourishment obtained by the larva. After 

 the fly emerges its size remains nearly constant, as in 

 many insects. Two races have, however, been sepa- 

 rated that are different in size as a result of a genetic 

 factor. The first of these, called dwarf, is represented 

 by figure 42 (c). The race is small but variable in 

 size, depending on food and other conditions. The 

 same figure shows the presence of another factor, 

 "sooty," that makes the fly dark. 



In the fourth figure (d) another mutation in size 

 is shown. It is called "giant." The flies are twice the 

 size of wild flies. 



In the fifth figure {e) the mutant dichaete is 

 shown. It is characterized bv the absence of two of 

 the bristles on the thorax. Other bristles may also be 

 absent, but not so constantly as the two just men- 

 tioned. Another effect of the same factor is the 

 spread-out condition of the wings. 



In the sixth figure (/) the wings are curled up 

 over the back ; the character is called curled. 



