When I am out and about I will see something that raises a smile, an insect feeding on a flower, the tree-filled view across our beautiful valley, a tiny springtail busying itself among moss. Our sight is a wonderful gift, but we never see the full picture. Our eyes are limited to the light spectrum that ranges from red to violet, the colours of the rainbow, but there is ‘light’ or radiation beyond that, infra-red and ultra-violet.
Our skin can detect infra-red which it feels as heat, and ultra-violet which triggers a tanning reaction, but other living things can respond by seeing these light ranges.
Pit vipers get their name because they have two pits on their face which can ‘see’ infra-red light with sufficient clarity to strike at warm blooded prey even in pitch darkness. British snakes can’t do this, they can rely on picking up scent and ground vibrations to detect prey but normal sight to strike. Vampire bats use infra-red sight to home in on their victims.
Many insects cannot see red light nor infra-red, but they can see light at the blue and violet end of the spectrum and they can see ultra-violet rays too. Many flowers have nectar guide patterns that only show in ultra-violet light that we cannot see, but pollinating insects can and so they can home in on their food.
We cannot see either directly, but we have developed devices that can detect infra-red and ultra-violet rays. They have a host of uses in industry and naturalists use them when surveying nature.
In the same way that pit vipers and vampire bats can see warm blooded creatures in the dark, infra-red cameras can show up mammals at night. Last year the Biodiversity Group organised a bat walk in the Byes. Apart from being able to hear the bat calls using special microphones, we were delighted to watch the Daubenton’s bats skimming insects from the top of the river using an infra-red camera.
The high energy ultra-violet light can produce interesting effects. There are several pigments in nature that respond to ultra-violet light by glowing visibly, they fluoresce. Electrons in the pigment molecules absorb the energy and are ‘excited’ to a higher state. As they drop back to their proper state they release the energy but as light that we can see.
Many lichens can be very confusing because they look very similar to each other in visible light, but some have pigments that fluoresce in ultra-violet light. Lichenologists can use this to clear the uncertainty. It can also produce stunning pictures.
The latest use for ultra-violet in nature surveying is looking for Brown Hairstreak butterflies. The adult butterflies tend to remain high up in trees and naturalists are limited to searching for eggs and caterpillars in the shrubbery below. The eggs are tiny and the caterpillars are nocturnal, hiding camouflaged under leaves during the day. But, when they come out to feed, the caterpillars fluoresce in UV light, they glow a pale green blue which makes them easy to see. The Biodiversity Group is hoping to use this to survey the hedgerows in the Knapp later this year.
Ed Dolphin
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