In the creeks and ponds of the world -- including America -- lives an insect that can reach four inches long and bears a pair of giant pincers and a beak for injective digestive enzymes into its victim. It goes by the name giant water bug. This is what it does for a living.
What you just saw was the ambush hunting strategy of giant water bugs. Once they seize their prey, they inject their poisonous digestive juices, wait a few minutes, and then suck up the resulting slurry. They don't just grab small fish and crustaceans; they've also been caught nabbing snakes, baby turtles, and the occasional human toe (it reportedly hurts like hell). They are crafty, and have even been known to play dead when startled by a much larger predator like a human. They ooze fluid from their anus to enhance the effect, but this is only a feint; they may then suddenly come to life and sting. Their overall hunting and feeding strategy is one put to good use elsewhere by most of the spiders of the world. But of course, the giant water bug does all this underwater. It doesn't have gills -- so how does it breathe while patiently waiting for prey to blunder along?
The giant water bugs are the largest true bugs -- formally called Hemipterans -- in the world. The true bugs can often be recognized by their characteristic proboscis or "rostrum", which they usually use for piercing and sucking. It's a trait put to good use on plants, to the chagrin of the farmers of the world, and, probably, said plants. Others, like the water bug, target animals. The family within the true bugs to which giant water bugs belong, the Belostomatidae, also goes by "toe-biters" or Electric light bugs (ELB). They get this name not for their fusion of Beatles-esque pop, classical sounds, and sci-fi album covers, but for the fact you can catch them at night with lights.
This species, Lethocerus patruelis, is Europe's largest true bug and largest water insect. A recent paper in Zookeys (whose authors filmed the video above) documented the northern advance of L. patruelis in Bulgaria, which the authors interpret as the likely result of climate change. They also found that the giant water bug possesses an unusual (for a true bug) form of sexual cell division.
Though fierce predators, this group of insects has a surprisingly sensitive side. Electric light bugs practice an unusual form of parenting for the animal world: daddy day care. In most Belostomatids, the female deposits eggs on the male's back. He duly trundles around with them, keeping them moist and clean and safe from predators. In the true giant water bugs (Lethocerus), like the guy we saw above, the female merely deposits them on a plant just above the water's surface. There, the male guards and keeps them moist until they hatch, and may even remain aloft on his stem or grass blade to prevent nymphicide by a desperate female looking for a mate. Once they hatch, the little nymphs are on their own.
Which brings us back to their underwater hunting and diving abilities. How does an air-breathing insect stay submerged so long? Insects don't have lungs in the sense that vertebrates do. Instead, they breathe through tiny pores called spiracles spread over their body; the spiracles open onto a series of finely divided tunnels called tracheae that deliver oxygen directly to all the tissues of the body. This differs a great deal from our system, where air is delivered only to the lungs, which supply oxygen to the blood. Our circulatory system then distributes oxygen to all the tissues of the body.
Giant water bugs breathe underwater by taking their air supply with them: like many other aquatic insects, many Belostomatids breathe underwater from air bubbles they trap on their body. Lethocerus has a small space under its wings where it can trap air and breathe air through its spiracles. It's the insect equivalent of a SCUBA system.
I'll leave you with this quiet roundup of hunting giant water bugs. At :14, you can see the proboscis injecting its destructive payload. If you look carefully, you can see a thin filament extending into the dragonfly larva's translucent body. At :18 is the rather macabre sight of the dragonfly nymph starting to bleed from its head. Then, if you look very carefully at the back of the water bug's body at around 1:42-3, I think you can catch a glimpse of the air tank under the water bug's wings. Finally, at 2:06, you get a Zapruder film-like frame-by-frame of the action. Enjoy.
Reference Grozeva S., Kuznetsova V., Simov N., Langourov M. & Dalakchieva S. (2013). Sex chromosome pre-reduction in male meiosis of Lethocerus patruelis (Stål, 1854) (Heteroptera, Belostomatidae) with some notes on the distribution of the species, ZooKeys, 319 119-135. DOI: 10.3897/zookeys.319.4384.app