From time to time it is possible to get an alien fly into your cultures as soon as they form into flies open the container and let them fly outside. You are trying to keep the wingless strain pure. http/www.killi.co.uk/drosophila1.php
I stole about 10 flies from my BF and made their food/laying matieral with mashed up banana and activated yeast. They thrived! I had a toilet roll tube stuck in the middle and within a wekk or so it was covered in flies.
The worst experience I've ever had with them is when I was pooring a few into my mudskip tank and managed to drop the cup on my floor before getting the lid back on... I had flies ALL OVER my room for a few nights. So annoying. I was armed with a metal rule at my comptuer and must've squished hundreds of the buggers