Most species are insects, and insects have a decent claim to be the most successful land animals. But for all that, the ocean remains almost entirely closed to them.
Water striders in the genus Halobates are the only truly marine insects, though a few hundred other species live in salty coastal environments. So why aren’t there more insects in the sea?
Why are there so few insects in the sea?
One theory is that it’s for the same reason that there are so few fish on land – the habitat is already bursting with their relations. Recent work on the evolutionary history of insects shows that they evolved long ago from a small group of primitive shrimp-like crustaceans, in the same way that land vertebrates – amphibians, reptiles, mammals and birds – evolved from a small group of fishes.
Crustaceans, from tiny krill and water fleas to enormous crabs and lobsters, throng anywhere there is water. Though a few crustaceans routinely live on land (such as woodlice and land crabs), the Earth’s surface and the air have become dominated by insects. Crustaceans’ external gills have been replaced by insects’ internal breathing-tubes, and some of the many appendages crustaceans carry around on their multi-branched legs might have evolved into the wings of modern flying insects.
Because insects almost certainly evolved on land, many of their adaptations – from reproduction to the physiology of eating and breathing – are tailored to a terrestrial existence. Even those insects that live much of their lives in fresh water cannot stray too far from dry land.