They're huge. But they only experience this form for a very short period of time! Most of their life is spent underwater in their larval stage, where they hunt other animals and cling to objects to manage currents.
In their mature form, shown, they aren't even really interested or capable of sustaining themselves. Indeed, they just wither away and die. It's sad, but for insects, the dramatic life cycles are part of what makes them so special.
