It seems a long time ago, but we did briefly achieve our annual Welsh fifteen moment in the garden this year, when more than fifteen butterflies, of the usual 3 suspect species – Red Admiral, Small Tortoiseshell and Peacock had a brief chance to gather and nectar on the flowers of our Buddleia davidii.It seems a struggle to keep theses shrubs alive here for more than a few years. If you look at the very useful Buddleja national collection website, click here, you’ll see that like many plants they can suffer from a range of problems, of which I suspect eelworm is a significant issue for us, given the often cool and damp weather here. I must try the suggested method for looking for this pest next year, by cutting up sickly leaves, leaving in a glass of water and looking for the tiny wriggling parasites. Though as with many diseases, the only approach seems to be to remove and destroy affected plants.
I’m tending now to leave a few plants to set seed in the hope that some of the seedlings may prove to be more suited to our conditions, or resistant to the eelworms. One form in particular, above, seems to be very valuable with flowers late in the season, but this year apart from a few dry days it’s been a really poor year for our butterflies.
Even greater excitement was seeing and being able to photograph a Hummingbird Hawkmoth, Macroglossum stellarum, in the garden, late afternoon on July 29th. We only ever see these every few years here, but I had spotted one earlier in May as we sat on the terrace with a cup of tea, post shearing. However this time, it obliged me by hanging around for long enough for me to take several usable photos as it visited the Buddleja flowers for 10 minutes or so.
Viewing the many images got me thinking about how brilliantly these large bodied moths control their proboscis to probe into the tubular depths of the Buddleja flowers. Butterflies and bumblebees do this as well of course, but they have the easy task of gripping onto the petals with their 3 pronged talons. The hawkmoth prefers to hover in front of the flower, even whilst being buffeted by wind, and direct the flexible proboscis tip into a quite narrow opening which may itself be moving around in the wind.
Try to imagine a drone, or fixed wing aeroplane, or even a helicopter managing to achieve a similar task. If it could be done, it would require extremely sophisticated controlling software, and probably weeks of specialist operator training to manage it.
So what about the moth? How does it do it, and do its control systems vary significantly from night flying moths?
A few issues intrigued me – visual perception of flowers in a daytime flying moth; proboscis manipulation, and moth flight control.Click here for an interesting study by Goyret and Kelber (Chromatic Signals Control Proboscis Movements during Hovering Flight in the Hummingbird Hawkmoth Macroglossum stellatarum). This paper discusses many of these issues and in particular the ways that animals perceive visual cues – either using brightness, colour or pattern recognition. Or a combination of all three.
These researchers used recently emerged adult moths and established that whereas night flying moths use mainly brightness, olfactory and mechanical cues to probe flower tubes, hummingbird hawkmoths principally rely on their 3 types of visual photoreceptors. Like many other insects, these work with peak sensitivities in the ultra violet, blue, and green spectral ranges, to select and locate nectar sources. Using colour, rather than just the brightness of an image, seems to enable the emerged moths to learn more rapidly which flowers are the best food sources, since colour is a constant, whereas the “brightness” of a flower varies under different light conditions.
But if you look at the paper flower patterns used in their experiments, illustrated below, you’ll see how similar they are to the basic Buddleja flower pattern, with a central yellow or orange eye.
The image above, from their paper shows the actual flower model colours used on the top line. The artificial nectary tube was located at the centre of the coloured cross. The next 2 lines show the statistically expected ratio of first contact made by moths, (as a pie chart) if it was completely random. This reflects the roughly 40 percent of coloured surface in the central cross, relative to the overall flower area. The second row, labelled as primary contact, shows the actual observed first contact by 223 different moths. The text below is an edited version of the authors’ assessment of recently emerged, “naive”, moths’ learning processes…
After finding 3–4 nectaries, moths learned to find any subsequent nectaries in 2 seconds or less. Nevertheless, while all the flower models could be learned, the efficiency with which moths foraged on the different models was affected by the innate bias to probe on the yellow areas of bi-colour models
In models with a yellow cross pattern on blue (yB, Yb and YB), moths found the first nectar tube relatively quickly (medians between 4 and 5 seconds) and continued to decrease the inspection time thereafter, resulting in high foraging efficiencies.
In models with a blue cross on yellow offering both chromatic and achromatic (brightness) contrasts (By and bY), moths were initially slower, with median probing times until first success of 14 and 9 seconds respectively The bias to probe on the colour yellow also resulted in more moths finding fewer nectaries. Thus moths were less efficient overall, attaining significantly fewer nectaries when foraging in bY than in Yb.
Similarly, completely emptied “nectary tube” numbers were lower in By than in yB. These results suggest that the observed tendency to probe on the small cross mark regardless of its colour is relevant for the first(s) contacts only, and that subsequent inspection movements are primarily affected by colour differences.
In bB flowers (providing only achromatic or brightness, and not colour, contrast) moths found their first nectar tube relatively fast, and most animals could empty a high number of flowers. On the other hand, when inspecting Bb flowers the probing times until first successful event were twice as long as in bB, fewer moths found nectaries and, consequently moths could empty fewer flowers than in bB. This suggests that after the first contact, moths directed their proboscis towards the brighter background areas, thus being “mislead from the nectar tube” by their innate achromatic control of inspection movements. This could also be directly observed during the experiments (personal observation).
I’ve written before on this blog about the fascinating anatomy and function of the proboscis, triggered by earlier sightings of humming bird hawk moths. Click here for more. There’s also a very good review of the evolution and function of the Lepidopteran proboscis, which you can read by clicking here. (Feeding Mechanisms of Adult Lepidoptera: Structure, Function, and Evolution of the Mouthparts- Harald W. Krenn).
But I haven’t considered before how a hummingbird hawkmoth, manages to fly and hover so perfectly in front of the flowers.
This took me into quite complex territory, so there are a lot of links for those interested to follow up on. Frustratingly I researched these a month ago, but we’re now moving into the annual autumnal internet slowdown, when never mind streaming (which is impossible for us anyway) even trying to get a pdf research article to download takes ages. So what follows will be a bit of a vague precis.
A good starting point is to read this pdf paper, click here, (Antennal Mechanosensors Mediate Flight Control in Moths by Sane, Dieudonné, Willis, and Daniel – 2007). They describe how any flying insect requires some system of neuro-sensory monitoring and feedback whilst in flight in order to fine tune wing beats and even body shape, to respond to changes in environmental conditions – e.g. gusts of wind. The hard chitinous external skeleton of any insect (exoskeleton) apparently also tends also to deform under flight, with twisting forces known as Coriolis forces, which would then impact on how unstable its flight became. Such deformation requires some degree of monitoring and corrective movements, to maintain controlled flight. (Click here for more on Coriolis forces – though as a non physicist I struggled to really grasp the detail).
Flies, which have just one pair of usable wings, use their modified stubby rear “wings”, or halteres (which look like small clubs), to monitor this exoskeleton deformation during flight, and these halteres then send feedback signals to muscles affecting wing control, to modify their movements.
But moths (and butterflies) have 2 pairs of wings, and no halteres, so how do they carry out this in flight monitoring of potentially disruptive Coriolis forces? The answer is that at least in some moths the antennae, which historically were thought of as sensory appendages principally used to detect smells, or pheromones, are key mechano-sensory receptors during flight.
At the base of the antennae are 2 groups of structures, which monitor their position. One group, called Bohm’s bristles, monitor the actual position of the antennae base relative to the head. The other group of sensors called the Johnston’s organ monitors tiny vibrations of the antennae during flight, which relate to airflow over the antennae. But things are even more complex, since it turns out that the moth actually vibrates the antennae during flight at a frequency mirroring the frequency of the wing beats.
This monitoring isn’t even a simple neuro-sensory phenomenon. In the paper above, you’ll read that if the antennae of moths were experimentally removed, the moths could still fly, but their flight become less stable, and they couldn’t hover or execute complex manoeuvres – they lose this vital feedback and the consequent fine tuning of wing beats. However if the antennae were then stuck back with superglue, the moth regained its normal flight capabilities!
Coping with pitching, rolling and yawing took me back to a long day at RAF Shawbury in the early seventies. After theoretical lessons on flying over many weeks in the RAF corps of school CCF, the moment finally came to experience it in practice. Unfortunately a surname beginning with “W” meant I had the last slot, which at the end of the day, was a simple second seat for a Chipmunked take off, circle of the runway and immediate landing.
I was unimpressed, and have rarely taken to the skies since.
There is another less complex review on aspects of hummingbird hawkmoth flight which you can read here, and also a fascinating interview video with one of the scientists doing research in this field of moth flight. (Luminance-dependent visual processing enables moth flight in low light by Sponberg, Dyhr, Hall, Daniel)
There is another fascinating video which I can share (though not include) of an insight into one of the most interesting evolutionary arms races in the natural world – that of bat predation of moths, involving the bat’s usage of sonar detection. Certain moths have evolved strategies to escape capture. Click here to view the work of William Conner, and see some fabulous nightime footage, and click here to read more about this arms race.
This reminded me of one of the most amazing things I’ve witnessed in the natural world. Years ago when I was doing some moth trapping for my Moth DVD ROM Diary, on a forest track a short distance from home, I stood and watched as moths flew, in typical butterfly type flight in the direction of the strong mercury vapour lamp light. As is usual, it didn’t take long for bats to appear on the scene, and I watched as many moths were easily caught by the bats, with seemingly no effort to escape. A few moths, mainly of the “Carpet” type, used the strategy illustrated in Conner’s video, of dropping like a stone to the ground, just in front the bat, as it zeroed in on them. Our bats don’t seem to have mastered the wing scooping movement to counter this strategy, which you can see in the video.But on 3 separate occasions I saw a large moth flying in leisurely, butterfly style flapping flight, perhaps 15 feet off the ground. Until the shadow of a fast approaching bat approached within about 8 feet of the moth. Suddenly something changed (presumably the moth picked up the bat’s sonar echoes), and its flight changed into an amazing series of tight loop the loops, closely followed by the bat, still glued to it’s flight path. After a brief whirl of dramatic dog fight action, the moth ended up safe on the forestry track, and I could see what it was. In each case it was an Angle Shades Moth.
Look at this moth at rest, and unlike all other local macro moths (apart from the Small Angle Shades !) …… you can appreciate how it folds its wings with longitudinal ridges which makes it look in profile very much like a carefully folded paper aeroplane. My suspicion is that at the point when the moth picks up on the bat’s presence, it changes how its wings are held, perhaps pulling them, even partially, into this sort of position, which enables such fast and tightly constructed loops to be executed. Perhaps a unique evolutionary response over millenia to defeat even the best of bats?
Years ago I mentioned this behaviour to Mike Gunton, the BBC producer who has worked on many of David Attenborough’s natural history programmes, and happened to stay with us overnight whilst fishing the Teifi, and discussed how great it would be to film this behaviour. Perhaps now, the way technology has developed, it is a realistic prospect for someone with the appropriate gear. Or maybe it has already been filmed??
I was happy to get conclusive proof this week that the animal that ripped up one patch of our upper hay meadow last year, and this year has already done 6 times as much damage, is indeed a badger. Let’s just hope it stays in the field and doesn’t start to trash the garden…Finally I just checked that the last time we had more than 3 dry days in a row was in the middle of June. And only 12 dry days since the first of July.
Very good conditions for moss, mushrooms and some foliage…But the consequence of an environment in which foliage is rarely drying out is most obvious with many of the native trees round about… In many recent years, most leaves haven’t fallen by November 5th, because of mild conditions. But if you enlarge the photo above, a view of woods across the valley from us, count the number of trees with few leaves left – almost 2 months before that date.
Ash Die back is rampant. Dying Larch are everywhere, even Hazels and Oaks are suffering this summer. Scale on Oaks is dramatic, and leaves were being shed in late August. I’ve just finished reading ” The Company of Trees” by Thomas Packenham. A renowned tree enthusiast from Ireland, his book follows the year 2013. He was fully aware even then of the risks to his extensive arboretum of novel fungal diseases, and I would recommend the book to anyone interested in trees, as an enjoyable read.I’ve always, probably simplistically, associated all trees with extreme longevity and there is something comforting about growing a tree from seed, and expecting it to be around long after the nurturer has shuffled off. Packenham’s book, being centred around a landscape that has been owned by his family for centuries, has many examples of such arboreal longevity, and frequently records his sadness as elderly, treasured trees are lost through storms, or disease.
But to suddenly see so many young and mature, not ancient, trees succumbing all around us is deeply troubling. I do hope that those now planning and subsidising the reforestation of many of the upland areas nearby are aware of the current state of affairs. Click here for a contemporary official assessment of one of these diseases, Phytophtora ramorum, by the Forestry Commission. Is it over optimistic?