Given the overwhelming diversity that characterizes the insect world – there are millions of species currently in existence, and they could constitute up to 90 percent of all animal life forms on earth – it may come as a surprise that up until very recently, there were believed to be no amphibious insects. This long-held assumption changed when researches in Hawaii discovered caterpillars that were just as comfortable living on land as they were underwater. The caterpillars, of which there are several species, belong to the genus Hyposmocoma, which encompasses about a third of all butterflies and moths found in Hawaii. (Members of the genus are exclusively found in Hawaii as well.) Below we provide an overview of these amphibious caterpillars, the only known insects in the world, and possibly the only animals in the world, that are adapted to live on land and underwater.
The discovery of the unique caterpillars was made when two entomologists, Daniel Rubinoff and Patrick Schmitz, noticed caterpillars eating algae underwater in a mountain stream. Their location in the stream was peculiar because of how far into the water they were – they had to have been there for a matter of days, not hours. Although there are some aquatic caterpillars, they are rare, so discovering a marine caterpillar species would be interesting in its own right. However, what made Rubinoff and Schmitz’s discovery so extraordinary is that the caterpillars in the streams had previously been observed on land. Thus, they couldn’t only be (very rare) marine caterpillars, as they evidently were capable of living on land.
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Tests in the lab confirmed the uniqueness of the two researchers’ find. The caterpillars that were brought back to the lab lived contently for several weeks in an aerated aquarium, but they proved equally content in a dry petri dish, where they also lived for several weeks. So, the caterpillars were evidently amphibious, the first of their kind discovered. They might even be unique among all animals. There are plenty of amphibians, but these creatures can’t simply live indefinitely either on land or in water. Instead, amphibians can essentially endure land or water for a period of time, but each species is ultimately a terrestrial or aquatic creature.
The amphibious caterpillars of Hawaii developed their singularly flexible lifestyles because of the environment in which they evolved. Essentially all creatures live on solid ground or beneath the water, but the Hawaiian caterpillars developed in something of a liminal space specific to the volcano rocks on which they live. These rocks are often dry for extended periods of time, but they can also be submerged a few feet underwater for equally long periods (like after a flash flood sweeps through the landscape and leaves a lingering aquatic aftereffect). When the caterpillars are on dry ground, they breath through small holes on their bodies called spiracles. When they are submerged, they absorb oxygen through their skin from the water that surrounds them. The water most be relatively fast-flowing, as only this water is aerated enough to support the caterpillars. (As an interesting side note, they resist being swept away in the current by attaching themselves to underwater rocks with silk threads.)
It is their double method of breathing that makes the caterpillars so interesting and possibly unique. If they were fully aquatic caterpillars, they would have gills, and if they were fully terrestrial, they wouldn’t be able to breath underwater by absorbing oxygen directly through their skin. Interestingly, earthworms can live underwater for quite a long time by diffusing oxygen through their skin that is taken from the surrounding water – earthworms always get their oxygen through their skin because they have no lungs to breath – but when they are submerged, they are basically slowly suffocating to death because there is not enough oxygen. This is not the case with the Hawaiian caterpillars, and that is why their discovery is exciting.
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Of course, it remains to be seen if there are other creatures out there that are as thoroughly amphibious as the caterpillars in Hawaii. We live in a vast world, and there are many species left to discover. However, the fact that none have been found is a testament to the rarity of Rubinoff and Schmitz’s discovery, as well as a powerful reminder of the beautiful diversity of life that is tucked within virtually every ripple of the globe.