Exercise XXII 



INTRODUCTION TO GENETICS OF MAN AND FRUIT FLY 117 



most helpful signs of maleness is the possession 

 of "sex combs," consisting of a series of about 

 10 stout, black bristles on the basal (upper) 

 tarsal joint of the first legs; these can be seen 

 with the naked eye. 



Your initial matings were made with virgin 

 females. These are obtained by emptying all 

 the adults out of an active culture of Drosophila. 

 One or two hours later, one finds a few new 

 adults that have emerged in the interim, and 

 have surely not yet mated. If one now segre- 

 gates such virgin females, they can be kept until 

 wanted for mating. The males of course don't 

 require such precautions; they can be taken 

 from the culture at any time. 



Drosophila culture 



Drosophila can be raised in the laboratory in 

 3-inch glass vials closed with cotton plugs and 

 held at a constant temperature of 25°C. The 

 food consists of a cooked-up mixture of corn 

 meal, agar, molasses, water, and a mold-preven- 

 tive. The hot food is poured into vials, and 

 allowed to cool, the mass being stiffened by the 

 setting of the agar into a gel as it cools. A thin 

 suspension of yeast sown on the surface of the 

 food grows rapidly on this medium, providing 

 food for the flies. If the food is too soft or the 

 vial too wet, the adult flies readily stick to the 

 walls or drown. Precautions must be taken to 

 prevent these things from happening. It is im- 

 portant also that flies not be allowed to escape 

 into the laboratory, since adventitious matings 

 could invalidate your results. Containers are 

 provided for the disposal of used vials, and flies 

 with which you have finished, first killed 

 by overetherizing, should be placed in the 

 "morgues" (jars containing kerosene oil). 



Handling Drosophila 



Flies are anesthetized with ether to keep 

 them quiet during examination or transfer. 

 Care must be taken not to overetherize them; 

 there is only a narrow gap between anesthetizing 

 and killing them with ether. Flies killed in this 

 way can be recognized, since their wings are 



drawn up away from the abdomen, the pro- 

 boscis is everted, the legs are stiffly extended 

 and bunched together, and the body is curled 

 and has stopped twitching. 



There are two types of etherizer available. To 

 use the plastic Burco model, put not more than 

 two drops of ether into the chamber through the 

 spout. The ether should last an hour or more. 

 Shake the flies to the bottom of your vial; 

 remove the vial top and place the funnel top 

 over the open vial. Now invert the vial and 

 etherizer together and tap gently to shake flies 

 into the chamber. Immediately after the last 

 fly becomes still, remove the cap at the bottom 

 of the chamber and pour the flies out. 



The other type of etherizer is made of a glass 

 bottle with a tightly fitting cork holding a piece 

 of cotton. To use it, drop a few drops of ether 

 on the cotton, quickly shake the flies from the 

 culture bottle into the etherizer bottle, and 

 quickly close it with the cork, with its ether- 

 wetted cotton inside. (Be sure that the cotton 

 on your etherizer plug is just moistened, not 

 soaked with ether. Any liquid ether that touches 

 the flies is instantly fatal.) Some practice may 

 be needed to do this smoothly. It is helpful 

 first to tap the culture vial sharply against the 

 palm of the hand, so as to shake the flies away 

 from the cotton plug, yet not so violently that 

 they become stuck in the food. Immediately 

 pull out the plug, and set the mouth of the 

 culture vial into the mouth of the etherizer 

 bottle, holding the latter down. Tapping lightly 

 on the upturned bottom of the culture vial, and 

 holding the etherizer bottle toward the light, 

 help to get the flies into the etherizer bottle. 

 Don't tap so hard as to knock pieces of food 

 in on top of them. 



Caution: Since ether is dangerously explosive, 

 there must be no flames or lighted cigarettes in 

 the room. 



At most 10 seconds after the flies in the 

 etherizer have stopped moving, empty them out 

 for examination. If the anesthetization wears 

 off" before you have finished examining them, 

 they can be re-etherized. A re-etherizer is made 

 from one section of a petri dish, with a piece 



