Like humans, insects communicate with each other by “telephone” and can even leave messages, according to Dutch researchers.

No special electronics are needed because the bugs literally use green technology – plants – to communicate.

According to the new study from the Netherlands Institute of Ecology and Wageningen University, insects which live in and above the ground use a plant as a telephone by eating its roots. That changes the chemical composition of its leaves, which in turn causes the plant to release alarm signals into the air.

These signals tell other insects not to eat that plant and move along, in order to avoid any competition between insects. The signals also warn others of possibly-dangerous chemical compounds in the plant.

If a bug isn’t around to immediately receive the message, the study shows another insect can leave a ‘voicemail’ message in the soil itself, through various soil fungi, by leaving specific remains in the soil after eating from the plant.

In the Netherlands Institute of Ecology greenhouse, plants and plant-feeding insects are put together to assess their ability to store 'voicemail messages' in the soil. (Photo: Olga Kostenko/NIOO-KNAW)

Unlike our own ‘voicemail’ messages which disappear at a push of a button, these warning messages live on to serve future generations of insects.  Any new plants that happen to grow on the same spot, according to the research, can grab these same signals from the soil and again communicate the message to other insects.

The messages left in the soil can be rather specific . The new plant could warn bugs that its predecessor suffered from conditions that could be harmful to insects.

In their experiments, the researchers grew ragwort plants in a greenhouse and then left the plants open to threatening insects, such as leaf-eating caterpillars or root-feeding beetle larvae.  They later replaced those plants by growing new ones in the same soil and again the plants were left open to the hungry insects.

“What we discovered is that the composition of fungi in the soil changed greatly and depended on whether the insect had been feeding on roots or leaves,” explains researcher and study author, Olga Kostenko. “These changes in fungal community, in turn, affected the growth and chemistry of the next batch of plants and therefore the insects on those plants.”

The researchers are working to find an answer to how long the warning messages remain in the soil and just how widespread this occurrence is throughout nature.

The ancient giant griffinfly Meganeura monyi had a wingspan of up to 75 centimeters (Artwork: Dodoni)

Some 300 million years  super-sized insects – some as large as  hawks – swarmed the Earth.  The largest of these mega-insects was a predatory dragonfly-like creature with a wingspan of up to 75 centimeters.

That was during the late Carboniferous- early Permian periods, when the atmosphere was rich with oxygen. But then, 150 million years ago, birds showed up and the downsizing of insects began.

Scientists at the University of California, Santa Cruz believe high concentrations of oxygen in the atmosphere – over 30 percent of the air was 0² versus the 21 percent we have in today’s atmosphere – were responsible for the insects’ large size.

Insects have small breathing tubes instead of lungs, so the higher oxygen levels allowed them to take in and use more of the life-sustaining gas which encouraged their super sizes.

This fossil insect wing (Stephanotypus schneideri) from about 300 million years ago measures 19.5 centimeters. For comparison, the inset (left) shows the wing of the largest dragonfly of the past 65 million years. (Photo: Wolfgang Zessin)

To reach their findings, the scientists examined data from more than 10,500 fossil wing lengths taken from various published records.  They checked the size of the insects versus oxygen levels as they evolved over a period of hundreds of millions of years.

“Maximum insect size does track oxygen surprisingly well as it goes up and down for about 200 million years,” said Matthew Clapham, an assistant professor at UC Santa Cruz, who c0-authored a study published online in Proceedings of the Academy of Science. “Then right around the end of the Jurassic and beginning of the Cretaceous period, about 150 million years ago, all of a sudden oxygen goes up but insect size goes down. And this coincides really strikingly with the evolution of birds.”

With all over those hungry birds around, insects needed to become more maneuverable.  Survival was a driving force in the evolution of flying insects.  As result, the insects became smaller, which allowed them to survive and thrive, while their giant relatives died off.

Another transition in insect size took place more recently, at the end of the Cretaceous period some 90 and 65 million years ago, according to  Clapman and Jered Karr, a UCSC graduate student who co-authored the study.  They think several factors, such as the continued specialization of birds, along with the evolution of bats, and a mass extinction at the end of the Cretaceous period, may be behind this evolutionary transition.  A shortage of fossils from that period has made it difficult for scientists to track insect sizes.

Drawing of the Ichthyornis dispar, a bird from the Late Cretaceous of North America (Artwork: ArthurWeasley via Wikimedia Commons)

“I suspect it’s from the continuing specialization of birds,” Clapham said. “The early birds were not very good at flying. But by the end of the Cretaceous, birds did look quite a lot like modern birds.”

Clapham emphasizes their study wasn’t about determining average insect size during this time period, because the fossil records tended to favor the larger sized insects over the smaller ones, but instead concentrated on changes in the maximum size of insects over time.

“There have always been small insects,” he said. “Even in the Permian when you had these giant insects, there were lots with wings a couple of millimeters long. It’s always a combination of ecological and environmental factors that determines body size, and there are plenty of ecological reasons why insects are small,” said Clapman.