Why is a stick insect like a coconut?

Stick insects cannot fly far – so how did the species spread across the Pacific? Yao Hua Law reports.

With their stumpy wings, Megacrania tsudai females cannot fly. Their dispersal across the Pacific has relied on sea-faring eggs.

With its short stumpy wings, the bright green stick insect Megacrania tsudai is not cut out for long distance travel. So how has the species spread across the Pacific? By adopting the same strategy as the coconut, new research suggests.

Reporting in Ecological Research, entomologist Shun Kobayashi at the University of Ryukyus and his colleagues have shown that M. tsudai’s eggs can spend a year in seawater without reducing their hatching rate. The finding supports the idea that the insect’s eggs spread from Taiwan to Japan’s Ryukyu archipelago, a distance of 400 kilometres, by riding the prevailing currents.

“I always believed that the eggs dispersed to Japan through the ocean from Taiwan,” says Chia-Chi Hsiung, curator at the Lyman Museum at McGill University, who has been studying Megacrania for two decades.

Stick insects have long been known to employ interesting egg dispersal methods. In some species eggs resemble edible seeds which ants gather and carry to their cosy, parasite-free nests but then find them to be quite inedible.

Previous research had already shown the eggs of M. tsudai are well adapted to the insect’s perilous coastal habitat. M. tsudai females drop their eggs on to the lower leaves and ground where tides and yearly typhoons often inundate the eggs. The shells are lined with a crystalline material that prevents water seeping in. A spongy inner filling makes them buoyant so they avoid being buried in the sands and raises their chances of a safe landing on a higher leaf or twig.

Entomologists suspected these egg modifications might also render the eggs sufficiently ship-shape to travel across the Pacific and colonise new lands.

Kobayashi and his colleagues collected more than 900 M. tsudai eggs and floated them on seawater for 30, 60 or 90 days. The experience did not affect the hatching rate but they did find an interesting side effect – the longer the eggs were exposed to seawater, the slower they developed. While unexposed eggs hatch after four months on average, those that had floated for at least three months took up to two weeks longer to emerge, probably a result of reduced oxygen diffusing into the partly submerged egg.

Kobayashi suggests that eggs riding the Kuroshio Current might also benefit from the extra ten to 12 days to make landfall before they hatch. He estimates that it takes less than seven days for the eggs to travel on the current from Taiwan to the Ryukyu Archipelago and that it can take up to 90 days for eggs floating from Southeast Asia.

His next plan is to use genetic analysis to “clarify the relationships between M. tsudai distributed on different islands”, he says. This may reveal the insect’s dispersal route across the Pacific. Some suspect the large green stick insects found in the Philippines may also be members of the same species.

The sea-faring capability of M. tsudai eggs is unique – the first documented case among stick insects. But it is important for another reason. The one notable habitat on Earth that insects have failed to dominate is the open seas. The eggs of M. tsudai show that some insects at least have the potential to ride the waves.

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