STOP LAUGHING THIS IS NOT A JOKE. This is a serious question about this larval dragonfly's grabby-grab face-appendage (technically, it's called a "labial mask") and its b-hole. And the answer is: "Way, way more than you think."
Here's Gwen Pearson, entomologist and Wired's resident bug expert:
The ability of a dragonfly nymph to successfully snatch and grab food is directly related to its anus. The mouth-grabber (labium) is hydraulically activated. The dragonfly draws water in through the anus, clenches, then compresses its abdominal and thoracic muscles against the water-filled rectal chamber. This raises the internal body cavity pressure, and pushes the labium out –in a strike that takes 10 to 30 milliseconds.
Be apprised: The wonders of a dragonfly nymph's rectum are not limited to the inward siphoning of water. According to Pearson, it also uses its anus for jet-propulsion.
(Many thanks to Pearson for the crash course on hexapod mouthpart anatomy.)