I believe Malakyoma means you suck on the straw to vacuum the fruit flies in and you blow on the straw to evacuate them into the mantid containers.
Another method you can employ is the refrigerator. If you dump a bunch of flies into another container, seal it, and put it in the refigerator for a few minutes, the flies become sluggish. If you place them in there for a few minutes longer, they'll completely stop moving and appear to be dead, but will revive when they warm back up. When they're immobile, you can easily parse out specific numbers of fruit flies into separate mantid containers with a cold spoon to prevent them from reviving during the transfer. One tip for preventing the flies from getting out when you open the culture and from getting crushed when you close it is to tap the container a few times, this will dislodge the flies that're on the lid or on the walls of the container. You'll have to move quickly as they'll start swarming up again, but if the culturing container is large enough, it shouldn't be difficult.