Water boatmen and backswimmers are both small, boat-shaped aquatic bugs that look remarkably similar at first glance, but they differ in one immediately obvious way: backswimmers swim upside down, while water boatmen swim right-side up. That single difference hints at deeper distinctions in their anatomy, diet, and behavior that matter if you’re trying to identify what’s in your pond or pool.
Both belong to the order Hemiptera, making them “true bugs” rather than beetles or other aquatic insects. But they sit in separate families: water boatmen are Corixidae, and backswimmers are Notonectidae. Adults of both typically measure less than half an inch long (5 to 16 mm), and both use flattened, oar-like hind legs to row through the water. Beyond those basics, they’re surprisingly different animals.
How to Tell Them Apart
The fastest way to identify which bug you’re looking at is to watch how it swims. Backswimmers spend their entire lives rowing around belly-up, with their light-colored back facing downward and their darker belly facing the sky. This is the reverse of most aquatic insects, which are dark on top and light underneath for camouflage. Backswimmers have essentially flipped the usual color scheme to match their inverted lifestyle: a pale back blends with the bright sky when seen from below by fish, while the dark belly blends with the pond bottom when seen from above by birds.
Water boatmen swim in the conventional orientation, dark side up. They’re also often seen in a distinctive head-down position, grabbing onto submerged plant stems. Their body shape is slightly flatter and more oval compared to the more triangular, keel-shaped profile of backswimmers.
Another reliable clue is the beak. Backswimmers have a long, prominent beak designed for piercing prey. Water boatmen have shorter, less conspicuous mouthparts suited to their very different diet.
Predator vs. Scavenger
This is where the two bugs diverge most dramatically. Backswimmers are active predators. Each set of legs serves a distinct purpose: the front legs snatch prey, the middle pair holds it in place, and the flattened, hair-fringed hind legs propel the bug through the water. They hunt mosquito larvae, other small insects, tadpoles, and even small fish. Their piercing beak injects digestive enzymes into prey, then sucks out the liquefied contents.
Water boatmen, by contrast, are primarily herbivores and detritivores. They feed on algae, plant material, and microscopic organisms they scrape from submerged surfaces. Some species are omnivorous and will eat small invertebrates, but they don’t hunt in the aggressive way backswimmers do. This dietary difference is the main reason water boatmen are harmless to handle, while backswimmers can deliver a painful bite if you pick one up. The sting is sometimes compared to a bee sting, which has earned backswimmers the nickname “water wasps” in some regions.
How They Breathe Underwater
Neither bug has gills. Both carry their air supply with them, but the mechanism is elegant. Their bodies are covered in rough, waxy hairs that repel water and trap a thin layer of air called a plastron. This bubble sits along the thorax and head, and the insect breathes through tiny openings called spiracles that connect to the air layer. Oxygen from the surrounding water diffuses into the bubble as the insect uses it up, and carbon dioxide diffuses out. The plastron essentially functions as an external gill, allowing the bug to stay submerged for extended periods without surfacing.
You’ll still see both species break the surface periodically to refresh their air supply. Backswimmers typically hang from the water’s surface film, belly up, with their abdomen poking through to collect air. Water boatmen surface less conspicuously, often just long enough to trap a fresh bubble before diving back down.
Role in Your Pond or Pool
If you find these bugs in a swimming pool, the distinction matters. Water boatmen are attracted to algae, so their presence usually signals that your pool has an algae problem, even if you can’t see it yet. They won’t bite you and aren’t interested in you at all. Backswimmers show up because they’re hunting the water boatmen and other small prey already in the water. Removing the algae that attracts water boatmen will eventually drive both species away.
In natural ponds and wetlands, both play useful ecological roles. Backswimmers are among the most effective natural predators of mosquito larvae. Research quantifying predation rates found that backswimmers were the second most aggressive predators of mosquito larvae among aquatic insects, trailing only giant water bugs. Water boatmen also consume some mosquito larvae but at much lower rates. Both families of bugs are recognized as environmentally friendly tools for biological mosquito control, particularly in regions where mosquito-borne diseases like malaria are a concern.
Water boatmen serve a different ecological function as grazers. By consuming algae, they help regulate algae blooms and cycle nutrients through the food web. They’re also an important food source for fish, frogs, and wading birds. In some parts of Mexico, water boatmen eggs are harvested and eaten as a traditional food called “ahuatle.”
Quick Comparison
- Swimming position: Water boatmen swim right-side up; backswimmers swim belly-up.
- Coloring: Water boatmen are dark on top, light underneath. Backswimmers are the reverse: light back, dark belly.
- Diet: Water boatmen eat algae and plant matter. Backswimmers are predators that hunt larvae, insects, and small aquatic animals.
- Bite risk: Water boatmen do not bite. Backswimmers can deliver a painful sting comparable to a bee sting.
- Beak: Backswimmers have a long, piercing beak. Water boatmen have short, blunt mouthparts.
- In pools: Water boatmen arrive for algae. Backswimmers arrive to eat the water boatmen.