Stick insects expand territory after being eaten by birds

May 28, 2018, Kobe University

It's commonly assumed that when insects are eaten by birds, they and their unborn young have no chance of survival. However, a team of Japanese researchers hypothesized that the eggs within insect bodies can pass through birds undigested. They tested this hypothesis with stick insects, known for their hard eggs, and found that some eggs are excreted unharmed and successfully hatch. Stick insects cannot travel very far by themselves, so being eaten by birds could even contribute to expanding their habitat.

The research team was led by Associate Professor Kenji Suetsugu (Kobe University Graduate School of Science), Associate Professor Katsuro Ito (Kochi University), and Associate Professor Takeshi Yokoyama (Tokyo University of Agriculture and Technology). The findings were published in the online edition of Ecology on May 28.

Plants cannot move around, so they have developed various ways to distribute their seeds. The most common is seed dispersal by animals, who eat the fruits and excrete the seeds whole. For many birds, insects are also one of their main food sources. If insect eggs can pass through birds unharmed, we could say that insects, just like plants, are using the birds as a means of long-distance transport.

To achieve this, several conditions must be met: the eggs must be strong enough to pass through digestive tracts unharmed, the insect young born from these eggs must be able to fend for themselves, and the eggs must be viable without fertilization. Stick insects fulfil these conditions. The insect eggs are only fertilized just before the eggs are laid, using sperm stored within the seminal vesicle. However, females of many stick insect species are parthenogenic, enabling them to produce viable eggs without fertilization. In addition, like plant seeds, stick insect eggs have a very hard shell. They lay these eggs by scattering them on the surface of the ground, and after hatching the young locate suitable plants for food by themselves.

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Cambridge scientists getting to grips with what makes stick insects stick.

It's long been a matter of some conjecture - how the stick insect is able to stick. Well, scientists in Cambridge may have discovered the reason.

They've found that geckos, tree frogs, spiders and insects all share a special skill – they can walk up vertical surfaces and even upside down using adhesive pads on their feet.

But geckos have ‘dry’ feet, while insects have ‘wet’ feet. Scientists have assumed that the two groups use different mechanisms to keep their feet firmly attached to a surface, but new research from David Labonte and Dr Walter Federle in the University of Cambridge’s Department of Zoology provides evidence that this isn’t actually the case.

“It has generally been assumed that the fluid on their feet must be involved in helping insects like stick insects adhere to a surface by capillary and viscous forces – in the same way that a beer glass will stick to a glass table if it’s wet on the bottom.

"Our research shows that the fluid is likely used for something else entirely – it may even help insects unstick their feet.”

– DAVID LABONE, REPORT AUTHOR

By measuring how much force was required to detach the foot of a stick insect from a glass plate at different speeds and applying the theory of fracture mechanics, Labonte and Federle found that only a ‘dry’ contact model could explain the data.

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Biggest stick insect colony in the UK discovered in a garden in Cornwall

The colony came to light after bug-spotters responded to a call for reports of the spindly creatures.


Charity Buglife last month launched a public survey to record Cornwall's stick insect population.

The creatures are not native to these shores, but are known to thrive in the more sheltered parts of Cornwall and can be seen during the autumn when they try and find warmer spots in gardens.

The survey located new stick insect colonies in Padstow and Shortlanesend but the most exciting find was Mandy Rance-Matthews's discovery of 140 stick insects in the garden of her home in Stenalees, near St Austell.

Buglife's South West conservation officer, Andrew Whitehouse, said: "This is an incredible find.

"For every stick insect spotted there are often many more that are not seen – hidden deeper in a bush or hedge – this colony could support more than 300 stick insects."

Mrs Ranee-Matthews said: "We count them every year and are amazed to find that we have the biggest colony in the country."

Malcolm Lee, a Cornwall-based stick insect expert, said: "Our wild stick insects are originally from New Zealand and are restricted to mild areas of the county as their eggs are unable to survive hard frosts."

He said fears that the population would have been affected by the last two very cold winters had proved to be unfounded.