water spider

water spider, (Argyroneta aquatica), species of aquatic spider that is the only spider known to spend nearly its entire life underwater. The sole member of its genus, the water spider lives in a unique, underwater silk web, which resembles a kind of flexible diving bell. Water spiders are distributed geographically across the northern and central regions of Europe and Siberia, and are found in ponds, slow-moving streams, and other shallow bodies of fresh water, particularly where aquatic vegetation is abundant.

Life in the diving bell

Adult water spiders measure between 8 and 15 mm (0.3 and 0.6 inch) in length and typically are gray to dark brown in color. Their legs and abdomens are covered with fine hairs, which trap air bubbles in the water and give the spiders a shimmering, silvery appearance.

Once a water spider has captured tiny bubbles of air in the hairs on its body at the water’s surface, it transports them to its silk web, which is anchored to underwater plants or other objects, and ejects them into the interior, thereby inflating the underwater house with air. Research has shown that the inflated web serves as a sort of gill, extracting dissolved oxygen from the water when oxygen concentrations inside the web become sufficiently low to draw oxygen in from the water. Slowly, however, the inflated web collapses, and the spider must travel to the water’s surface for bubble renewal, which it does about once each day.

Britannica Quiz

Match the Baby Animal to Its Mama Quiz

Diet and reproduction

Water spiders, which tend to be more active at night than during the day, are carnivorous and prey on aquatic invertebrates, such as water mites, water boatmen, and phantom midge larvae. The spiders are hunters and do not use their webs as snares. Males generally are more aggressive than females, and the bite is reported to be painful to humans.

Most of the life cycle of the water spider, including courtship and breeding, prey capture and feeding, and the development of eggs and embryos, occurs below the water’s surface. Many of these activities take place within the spider’s diving bell. Male water spiders often construct their webs close to those of females and tunnel into the females’ webs to mate. A female may lay between 30 and 70 eggs, which are secured within a cocoon suspended from the upper region of the female’s web. Eggs hatch within several weeks, and the spiderlings disperse into the water. Young water spiders often make use of empty snail shells and similar habitats, which they fill with air, before constructing their own webs.