Species Spotlight: Blue Stick Insects
Hi everyone,
Back to Species Spotlight today after a little Science Spotlight detour earlier in the week. I will occasionally do blog posts about non-species related topics, particularly as we move into the colder months (in the UK), as some information about wildlife spotting and helping wildlife in the winter may be of use.
Today we are going to look at a species that isn't actually on our Wild Watch page, but that I saw and thought was absolutely fascinating - BLUE stick insects! I spotted these at Chester Zoo yesterday - you can see the pictures on my Twitter by clicking on this link https://twitter.com/kieranscience/status/1433436864569135105
I haven't uploaded them to the project page because iNaturalist is only for wild organisms, not captive organisms. However, I wanted to talk about them because there are some really cool answers to the question I'm sure you're all asking yourselves...
As you can see from my Twitter post, I was a little shocked that they were blue! And not some vague blue, but proper, in-your-face azure! Stick insects, as we all know, are camouflage experts; indeed it is the key to their success, acting as their primary defence mechanism from predators. Many species* do possess other defence mechanisms, such as: spraying harmful chemicals; stabbing predators with spines; mimicking other things more threatening than sticks (this is done by adopting poses that resemble the outline of other animals, such as ants or scorpions); or even, surprisingly, by flashing bright colours in order to become extra conspicuous, often to warn or confuse would-be predators. However, in all stick insect species, it is their uncanny resemblance to the humble twig that is their greatest survival asset.
*There are over 3,000 stick insect species - who knew there were so many ways to look like a stick! They belong to the order Phasmatodea, which also contains the leaf insects (if you haven't heard of these, I'm sure you can take a good guess as to what they look like)
So WHY ON EARTH are these things blue? I don't know about you, but I don't see many blue sticks or turquoise trees knocking about. It is a question that has stumped biologists as well, and the honest answer to it is... "we don't know!" However, "we don't know" is one of the most exciting phrases in science, opening up a world of possibilities, and it is no difference in this case. Whilst entomologists haven't been able to say for definite why these stick insects are so cerulean, they do have some interesting hypotheses, each of which will tell us something interesting about the world of anti predator defences!
Just before we dive in - the stick insects in question are part of the Achrioptera genus of sticks insects. I'm not sure exactly which species was present in Chester Zoo, but the males of all the species of this genus are brightly coloured (many are blue), which makes this whole mystery extra fascinating - as this colouration is present in multiple species, there is DEFINITELY a good reason for it to exist! Achrioptera is a genus found primarily in Madagascar and Comoros (both island nations - an extra EXTRA layer of fascination, as island species can evolve in especially weird and wonderful ways, as they are free from the influences of the rest of the world). So without further ado, let's go...
THEORY 1 - BOYS IN BLUE
As it is only the adult males that exhibit the blue colouration (juveniles and adult females look like proper sticks), it could be that the blue is something to do with mate choice/sexual selection. Basically, the blue could be something to do with attracting females. However, there is no other evidence for this theory - there are no observations of females choosing mates based on their blueness, and, if the blue colour does indeed make the males more conspicuous to predators, then they would be goners before they had the chance to attract a female. This suggests that there must be an anti predator aspect to it after all...
THEORY 2 - ON THE MOVE
Stick camouflage is great... until you're on the move. I've never seen a blue stick, but I've never seen a walking stick either (you know what I mean). Achrioptera males tend to be more mobile than females, therefore the males could be conspicuously coloured because, when they're moving, the bright colours act as disruptive colouration. This is actually a type of camouflage - it works by breaking up the animal's outline, which is great if you want to be camouflaged and mobile (the military use this sort of camouflage a lot). Now this might well be the case for some Achrioptera species in which the males are green and yellow, as this may help them camouflage against the leafy foliage, but it is unlikely to be helpful for the blue species we're interested in today.
THEORY 3 - I'M WARNING YOU
Most highly conspicuous organisms possess bright colours in order to warn other animals. The bright colours are their equivalent of wearing a sign saying "WATCH OUT, I'M DANGEROUS" (poison dart frogs are a classic example). Of course, as discussed in my last Species Spotlight, it may also be that an organism is just pretending to be dangerous. Achrioptera males may have evolved to be able to digest some of the highly toxic plant species present in their localities, meaning that, like their frog counterparts, they are poisonous and are giving fair warning to anything that fancies a snack! More research needs to be done to assess whether or not this is the case, and whether the females eat the same plants. If they do, then this theory would look a lot weaker, as you would expect the females to be similarly coloured.
THEORY 4 - I'M WARNING YOU... AGAIN
Both the males and females of Achrioptera species are equipped with their own chemical weapons, possessing the ability to spray repellent chemicals on any attackers. Both sexes are also equipped with brightly coloured wings, although they are often small and generally hidden. When threatened, males and females will unveil these wings and make stridulating noises (similar to cricket chirps) to warn attackers about their harmful substances. However, this doesn't explain why males are ALWAYS conspicuously coloured, whereas the females remain drab and stick-y until threatened.
Whilst we're not sure which, if any, of these theories are correct, each gives us a glimpse into the variety of anti-predator defence mechanisms in the animal kingdom. What we can say with a high level of confidence is that there is a good reason for them to be blue: if there were no survival and/or mating advantages to being blue, natural selection would have seen the disappearance of these individuals a long time ago. As it is, the fact that it is a common feature of males across many species, shows us that it's definitely good for something... we just don't know what. This is undoubtedly an exciting area of future research of any budding biologists among you!
I'll be back next week with a new post about a Wild Watch species. In the meantime, keep your eye out for blue sticks - you might just help us solve this mystery!
Diolch
Kieran
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