On purpose or unplanned?
I once sent my children to school with a big apple that was inside a glass bottle with a narrow neck. I told my children to show it to the teacher and ask her how the apple got into the bottle. The teacher said to them that your father is a scientist and he probably knew how to carefully remove the bottom of the bottle and stick it on again once he’d put the apple inside. That, of course, hadn’t been how I’d done it (I had fastened a bottle facing downward, lest water got in, over a tiny apple still on the tree, before it got big). What this has to do with amber and insects inside it? Well, how did they get there?
Amber is used in expensive jewelry, but it’s not a stone or mineral, but the hardened resin that had oozed out of conifer trees like pine and spruce millions of years ago. What interests biologists are their frequent inclusions, primarily arthropods like mites, spiders, centipedes and insects. But even bigger animals like frogs, lizards have been recorded. The oldest ambers with inclusions are those 130 million year olds from Lebanon, while those from Myanmar, known as Burmese Ambers and famous for their biological inclusions, are considered to be approximately 100 million years old. The famous Baltic Amber, used in the “Amber Room” celebrated as the “Eighth Wonder of the World” (the whereabouts of which are still lost), dates back to 34 – 48 million years ago and also contains numerous insects present at that time in Nordic forests. What has been puzzling biologists, and especially entomologists, is the presence of so many aquatic insects in the amber and a variety of suggestions have been made how these insects ended up in the sticky tree resin that ended their lives.
One possibility is that the insects happened to get stuck accidentally, but that idea can quickly be dismissed on the basis of mathematical probability and statistical evidence. Another suggestion had been that coniferous resin flowed into water-filled tree holes and the aquatic insects living in these tiny water bodies were trapped by the resin. However, many of the trapped aquatic insects represented species that do not visit water-filled cavities of trees. Somewhat similar was the suggestion that the resin flowed down the tree’s bark and contacted on its way to (or upon reaching) small water bodies, insects that got encased by the resin. But once again there were objections to that theory just as there were to the idea that wind could have blown the insects onto the resin and caused them to adhere and get stuck never to free themselves again. The three theories have in common that the aquatic insects ended up in the resin by accident, but the fourth suggestion has it that some insects were attracted to the resin by its glistening appearance, resembling water droplets. However, what exactly attracted the dispersing, flying, aquatic species to the resin remained unknown.
To solve that question Horvath and co-workers in Hungary carried out field experiments with sticky surfaces that polarized the light, reflected from the surfaces to different degrees and angles. They then analysed the taxonomic positions and numbers of insects that had got stuck on the surfaces and concluded that it was not simply the “shininess” or “brightness” of the sticky sheets, but the degree and angle of polarization. The result does, of course, agree with the known observation that insects, which depend on water to deposit their eggs in it like mayflies, dragonflies, stone- and caddisflies, identify water bodies not by their colour or smell, but by their polarization characteristics. The result was also in agreement with the published report representing the ancient insect fauna. So, does that solve the question how water-seeking insects ended up in the resin as ‘amber inclusions’?
Perhaps not, because in a recent 2021 publication by Mario Schädel et al., the authors reported that they had examined a piece of Burmese Amber with an assemblage of over 100 specimens of immature stages of a parasitic aquatic woodlouse relative, belonging to a group known as Epicaridea. And these tiny (and of course wingless) specimens did not possess prominent eyes at all and could not fly. So, what attracted them to the resin? Was their entrapment accidental or did the resin attract them? It’s “back to the drawing board”. But that’s science (and keeps us busy)!
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