Water-spider cocoon (Argyroneta aquatica) - Bildbanksillustration

1. The Underwater HomeUnlike most spiders that live on land, the water-spider builds a "diving bell"—a silk web anchored to underwater plants. They carry air bubbles down from the surface on their bellies and release them into the web until it expands into a bell shape full of breathable air.2. The Role of the CocoonThe illustration you shared likely shows the egg sac (cocoon) which is placed inside or attached to this diving bell.Protection: It keeps the eggs safe from aquatic predators.Oxygenation: Because the cocoon is inside the air-filled bell, the developing spiderlings have a constant supply of oxygen while being submerged.