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Print edition | Science and technology
Nov 1st 2018

PACHYRHYNCHUS WEEVILS are found on most islands in the eastern Pacific Ocean. If these weevils could fly, that would not be surprising. But they cannot. Why they are so widespread is therefore a mystery. But it is one that Wen-San Huang of the National Museum of Natural Science in Taiwan thinks he has solved.

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One theory, which dates back to 1923, is that the beetles drift from place to place, buoyed up by a tiny air cavity each has beneath its outer shell, which allows the insects to float. Dr Huang’s research into the weevils began when he noted that this theory had never been tested. He discovered, when he put it to the test, that the beetles do, indeed, float. But, as he reports in Experimental Biology, floating in seawater does not do them much good. All 57 adults he tried it with died within two days. Clearly, adult weevils are not good sailors.

That does not, however, mean that weevil young are not. Pachyrhynchus weevils have a predilection for laying their eggs inside the fruit of a mangrove-dwelling plant called the fish-poison tree. This reproduces in a way reminiscent of coconut palms. It drops its fruits into the ocean, which carries them away to germinate on distant beaches. Coconuts are protected from being eaten on their travels by having a hard, thick shell. Fish-poison-tree fruit are also protected from hungry sea creatures. But in their case, as their name suggests, the protection is chemical.
Given the relationship between weevils and plant, Dr Huang wondered how beetle larvae would fare if they were deep inside a piece of fruit floating in seawater. So he tested this as well. His experiment revealed that such larvae are tolerant of saline conditions. Specifically, of 18 grubs thrown into seawater inside a piece of fruit, two survived for six days. Moreover, these larvae went on to develop into healthy, sexually mature adults.
Two larvae out of 18 surviving for six days at sea might not sound particularly impressive. But Dr Huang argues that such numbers would easily support island colonisation. He observes, for example, that the Kuroshio Current, which carries water from the Philippines, past Japan and onwards into the Pacific, moves so swiftly that a piece of fruit caught in it could easily travel 90km in a day. Since it is only 400km from the northernmost islands of the Philippines to the southernmost islands of Japan, a constant migration of larvae in this direction would be easy to maintain—thus solving the question of how a flightless beetle can island-hop so effectively.