If you want to go with gills instead of lungs, you could have large inscet analogs on an alien planet who live in the water.
They would probably have to have a more advanced circulatory system like vertibrates have and having a lot more oxygen dissolved in the water than in modern seas would be very helpful.
If the planet's atmosphere has much more oxygen than Earth, there could be arthopods on land or in the air much bigger than modern insects, as there once were on Earth.
So if those giant insect like creatures are also vemonous like some insects, a human explorer (possiblyone with an nsect phobia) might jump into a body of water to secape from really killer bee like creatures. Only to find that the arthopods in the water can grow much larger, and that it is only the juveniles who crawl on land or fly in the air, to escape predators when they are young and small.