58 LABORATORY STUDIES IN ZOOLOGY 



bottle for about a week they will have mated and eggs will have 

 been deposited resulting in the presence of many larvse, some of 

 which will have proceeded as far as the pupal stage. As soon as 

 pupae have formed, the parents must be removed so as not to 

 become mixed with their offspring. These Fi offspring may be 

 collected from such a culture not to exceed ten days after the 

 first flies appear. 



Securing the Fo Generation. — About 15 Fi adults of each sex 

 are then transferred to breeding bottles in order to insure the 

 production of a large F2 generation. The expected Mendelian 

 ratio of 3:1 in the F2 generation resulting from crossing flies in 

 which there is a single Mendelian character involved can be 

 approximated only by having a comparatively large number on 

 which to base a count. As soon as the pupae have formed, the 

 Fi adults must be removed so as not to mix with the F2 flies soon 

 to emerge. Only those flies should be counted which emerge 

 within ten days after the appearance of the first F2 flies in the 

 culture. 



In all holometabolous insects, or those which undergo a complete 

 metamorphosis, four stages in the life cycle are to be expected, 

 namely, the eggs, larval, pupal, and adult. The eggs of the 

 fruit fly will be disregarded because their small size render it 

 difficult to find sufficient numbers for class use. Examine the 

 larva and pupse of this fly under a dissecting lens and make 

 sketches of each upon the proper plates. Look at a pupa case 

 from which the adult has emerged and note the nature of the 

 exit. 



Each table should be supplied with (1) a milk bottle of flies 

 (F2 generation) living upon a banana culture; (2) a glass vial 

 which will fit over the mouth of the milk bottle and into which 

 the flies are to be driven before etherizing; (3) a black cloth 

 which when wrapped about the milk bottle will facilitate trans- 

 ferring the flies to the glass vial on account of their positive 

 reaction to light; (4) a small bottle of ether and a cotton plug 

 to be used in etherizing; (5) a petri dish into which the etherized 

 flies may be poured for examination, segregation, and counting; 

 (6) a camel's-hair brush for moving the flies. 



First, saturate the cotton plug with ether and have it close 

 at hand in order to stop up the mouth of the vial as soon as the 

 flies are transferred to it from the milk bottle. The next step 

 requires much care if all the flies are transferred from one bottle 



