Stabilmenta: spider’s web decorations

Stabilmentum woven by unknown spider, possibly in the genus Cyclosa

Stabilmenta are conspicuous patterns or decorations made by spiders – particularly orb-web spiders – in their webs. Google ‘stabilmenta’ (singular: stabilmentum) and you will see many wonderful examples of these structures, including crosses, spirals, zigzags and so on.

Stabilmentum made by unknown spider species, possibly in the genus Cyclosa

There are a number of different theories as to why spiders make these structures, including: to attract prey; as camouflage; as a moulting platform to stand on; as a way of warming up the web; and as a warning signal for any potential predators which might want to, or just inadvertently, destroy the web (1, 2, 3). It is possible that they have more than one function, although camouflage seems to be the most popular, or agreed upon, theory (2, 4). Nevertheless, some researchers have shown that more flying insects (apart from grasshoppers) are caught, or intercepted, on webs decorated with stabilimenta (5). Which suggest that they might enhance the efficiency of the web; although other researchers came up with a completely different result (see below).

Circular stabilmentum, possible by a Cyclosa species

Spiders in the Araneid spider-genus Argiope often adorn their webs with these structures. I photographed this stabilmentum (below) made by Argiope pulchella in Thailand. The spider positioned itself over the X-shaped stabilmentum, but moved off it to wrap-up any prey caught in the web.

Argiope pulchella on web with stabilmentum

Some experiments have shown that stabilimentum building is a defensive behavior (3), in effect advertising the presence of the spider’s web and preventing birds from flying through the webs. There is no question that they are highly visible and in some situations, actually reduce the number of prey that are caught (3). This ‘cost’ to the spider can presumably be set against the benefit of not having to rebuild the nest every time a bird flies through it by mistake! Unfortunately for the spider making the stabilmentum, other predatory spiders – such as web-invading jumping spiders – have learnt to recognise the patterns and use them to find their prey (6). Perhaps this is why some spider species make silk replicas of themselves! (7). To fool would-be predators! (8)

Argiope pulchella on web with stabilmentum

As many people may have noticed, spiders webs can be highly visible when covered in dew in the morning, or after a rain shower. I photographed this spider’s web in Spain, after a passing shower.

Water droplets on spider’s web after rain

I agree with another blogger (9), that stabilimenta are probably multi-functional structures, and the fact that they are so common in certain species, must mean that they are being selected by evolution. So the overall benefits must out-weigh the costs.

  1. http://www.mol-ecol.uni-halle.de/research/former_topics/stabilimenta/
  2. Cloudsley-Thompson, J. L. (1995). A review of the anti-predator devices of spiders. Bulletin of the british arachnological society, 10(3), 81-96.
  3. Blackledge, T. A., & Wenzel, J. W. (1999). Do stabilimenta in orb webs attract prey or defend spiders?. Behavioral Ecology, 10(4), 372-376.
  4. https://en.wikipedia.org/wiki/Web_decoration
  5. TSO, I. M. (1996). Stabilimentum of the garden spider Argiope trifasciata: a possible prey attractant. Animal Behaviour, 52(1), 183-191.
  6. Seah, W. K., & Li, D. (2001). Stabilimenta attract unwelcome predators to orb–webs. Proceedings of the Royal Society of London B: Biological Sciences, 268(1476), 1553-1558.
  7. https://www.wired.com/2012/12/spider-building-spider/
  8. http://www.popsci.com/article/science/what-i-learned-hunting-amazonian-spiders-weave-fake-spiders
  9. http://www.bugsinthenews.com/stabilimentum_and_some_notions_on%20function.htm

 

Bright iridescent patches are honest signals!

Purple Sapphire (Heliophorus epicles) male showing iridescent blue patches on upper wing surfaces

Males butterflies in the family Lycaenidae, the so-called Blues, typically have brightly coloured, iridescent colours on the upper (dorsal) surfaces of their wings. Vivid blue iridescence such as this on the Purple Sapphire (Heliophorus epicles) shown here, is usually to do with courtship and mate recognition.

The brightly coloured, iridescent males rely on so-called, structural colouration (described below), which is used both in male-to-male interactions (competition), and in attracting females, via flickering or flashing their bright wings. The females are often dark brown and mostly lacking in these bright structural colours. They may – like female Purple Sapphires – have bright pigmentary colours (orange flashes in this case), but these are probably not secondary sexual characters, i.e. used in courtship and mating. I don’t have a picture of the female, but there are many examples on this website (1).

Purple Sapphire (Heliophorus epicles) side view showing brightly coloured, but mostly, non-iridescent under wings

A variety of different types of microscopic ‘nanostructures’ – extremely small regular structures – have been found to generate blue colours in lycaenid butterflies. Many have so-called multilayers – alternating layers of chitin and air – within the individual scales (2, 3).

Purple Sapphire (Heliophorus epicles) male showing iridescent blue patches on upper wing surfaces and antennae

Butterfly wings are covered on both sides by rows of tiny overlapping scales, a bit like very thin, flat roof tiles or shingles. Scales can vary markedly in size and shape across the wing of a butterfly, but depending on the species, there are about 200–600 scales per square millimetre of wing. The scales are very delicate, typically one or two microns (i.e. one thousand times smaller than a millimetre) in thickness, and are denuded by wear and tear as butterflies age.

It has been suggested that the fact that scales detach so easily is an adaptation to allow butterflies (and moths) to escape from spider’s webs. (4). Scales that are attached to the sticky threads of the spider’s web can be sacrificed to allow the butterfly to regain its freedom.  

Each scale consists of two layers held together by a series of tiny pillars. The lower layer of the scale is flat and smooth between 100 to 200 nanometres (one nanometre is a billionth of a metre) in thickness – whilst the upper layer consists of a series of longitudinal ridges or striae – about one or two microns apart – and transverse crossribs which create a three dimensional lattice, or honeycomb structure with windows into the interior of the scale (5). It is the elaborate 3-D nanostructures so-called perforated multilayers – between the lamellae that cause the structural colours and phenomena like iridescence (3).

The reflected iridescence produced by light scattering from the dorsal wing scales of many lycaenids is highly directional, i.e. it is only observable from a narrow angular window. That is why the blue colour is not visible in some photographs (see below), although the scales can also be denuded.

Purple Sapphire (Heliophorus epicles) male showing no iridescent blue patches, probably due to the angle of the wing

The iridescence produced by male wings of butterflies such Heliophorus epicles, and countless other species, appears to be what is called a secondary sexual character. In other words, female butterflies evaluate these colours when choosing which males to mate with. They have also been called ‘colour badges’ and are thought to be honest signals, or reliable information if you will, of the condition of the males (6).  So the theory is that males with a good pedigree (i.e. genes) and a good upbringing (i.e. favourable environmental conditions) will be bright and showy (!), and females will choose them on the basis that they are more likely to be vigorous and fertile.

Presumably because they are ‘costly’ to produce or difficult to generate, and the scales producing the effect are lost, or worn down as the male butterflies age, then structural colours appear to provide a good indication of male quality and vigour in some species. However, even old and worn males – like the individual shown in the following photograph – still have some iridescent scales with which to attract the ladies!

Purple Sapphire (Heliophorus epicles) male showing worn iridescent blue patches

Although there is, as far as I know, no definitive evidence that female butterflies choose between males on the basis of the quality of the intensity, hue or saturation of their reflective colours, the available evidence supports the idea that brilliant male structural colours evolved as a result of sexual selection (7). It seems that sexual selection in butterflies has homed in on the brightness of these structural colours in the same way that it has in terms of the brightness and ornamentation of the peacock’s tail feathers.

I have focused on the blue patches on the upper sides of the males wings in this blog. The bright yellow and red colours on the undersides also clearly have some function, but it is probably not to do with mating (I’m only guessing!) as the males and females look relatively similar on their undersides. Who knows what really goes on in the minds of these butterflies!

Purple Sapphire (Heliophorus epicles) side view showing brightly coloured, but mostly, non-iridescent under wings

All of these photographs were taken in Thailand.

  1. Mazumder, S. 2017. Heliophorus epicles Godart, 1823 – Purple Sapphire. Kunte, K., P. Roy, S. Kalesh and U. Kodandaramaiah (eds.). Butterflies of India, v. 2.24. Indian Foundation for Butterflies.
    http://www.ifoundbutterflies.org/sp/728/Heliophorus-epicles
  2. Vértesy, Z., Bálint, Z., Kertész, K., Vigneron, J. P., Lousse, V., & Biró, L. P. (2006). Wing scale microstructures and nanostructures in butterflies − natural photonic crystals.Journal of microscopy,224(1), 108-110. 
  3. Wilts, B. D., Leertouwer, H. L., & Stavenga, D. G. (2008). Imaging scatterometry and microspectrophotometry of lycaenid butterfly wing scales with perforated multilayers.Journal of The Royal Society Interface, rsif-2008. 
  4. Eisner, T., Alsop, R., & Ettershank, G. (1964). Adhesiveness of spider silk.Science,146(3647), 1058-1061.
  5. Stavenga, D. G. (2014). Thin film and multilayer optics cause structural colors of many insects and birds.Materials Today: Proceedings,1, 109-121.
  6. Kemp, D. J. (2006). Heightened phenotypic variation and age-based fading of ultraviolet butterfly wing coloration. Evolutionary Ecology Research, 8(3), 515-527.
  7. Kemp, D. J., Vukusic, P., & Rutowski, R. L. (2006). Stress‐mediated covariance between nano‐structural architecture and ultraviolet butterfly coloration. Functional Ecology, 20(2), 282-289.

 

 

 

 

Bluff and deception in Blues

Longbanded Silverline (Spindasis lohita)
Longbanded Silverline (Spindasis lohita)

The Longbanded Silverline (Spindasis lohita), Family Lycaenidae, is a beautiful insect with a remarkable structure – a tail, or ‘false head’ – at the end of its hind wing. There is a bright orange tornal patch – the tornus is the posterior corner of the butterfly wing – on both sides of the wing. There is also a black eye-spot and two pairs of white-tipped, filament-like black tails, or ‘false antennae’, at the end of the wing. Interestingly, many lycaenids have similar black and orange eye-spots and single or double tails.  For example, the Common Tit (Hypolycaena erylus himavantus) also has white-tipped, double tails similar to this species (1). So presumably it was a feature that evolved at sometime during the history of this family. Black and orange make a very eye-catching colour combination.

It is widely assumed that these structures are a ‘false head’ (or ‘fake head’), which acts to divert predatory attacks, e.g. bird pecks, away from the real head (and body) and towards the back of the butterfly. There is plenty of evidence that butterflies really do get pecked at, or on, these hindmost eye-spots (2). It is surprising therefore, that very little rigorous experimentation has been carried out to thoroughly investigate this phenomenon. In other words, the ‘false head’ hypothesis has not been tested scientifically. That is not to say it is not true, it is just a subject that ‘remains ripe for testing’ according to Professor Martin Stevens (3).

Long-banded Silverline (Spindasis lohita) - close-up of false head
Long-banded Silverline (Spindasis lohita) – close-up of false head

The fact that these butterflies invest so much time and energy into producing these deceptive structures and moving the little tails about like false antennae, is to my mind, quite convincing circumstantial evidence for their utility in avoiding predation, or surviving an attack. It is clear how prominent the ‘false head’ is – and might appear to a bird – when looking down on the butterfly from above (see photo below) like a bird might see it. The real head is partly hidden underneath the wings, but the ‘false head’ is very prominent.

Long-banded Silverline (Spindasis lohita), looking down from above
Long-banded Silverline (Spindasis lohita), looking down from above

The lovely wing colours of this butterfly are a tapestry, to borrow a term used by lepidopterists, of tiny overlapping scales of different colours: red, silver, orange, white and black. The scales can be seen in two excellent close-up photographs of the wings of this butterfly on this webpage (4). Males of this species also have bright, iridescent blue patches on both dorsal fore- and hind-wings. It is possible that these, together with the contrasting orange patches, could also act to startle and deter a predator and give the butterfly time to make its escape.

  1. https://rcannon992.com/2015/01/30/pushmi-pullyu-butterfly/
  2. https://rcannon992.com/2016/01/17/peck-me-here-butterfly-predation/
  3. Stevens, M. (2016). Cheats and Deceits: How Animals and Plants Exploit and Mislead. Oxford University Press.
  4. http://butterflycircle.blogspot.co.uk/2010/09/life-history-of-long-banded-silverline.html

Dragons on the beach

Komodo dragon (Varanus komodoensis) on Komodo Island
Komodo dragon (Varanus komodoensis) on Komodo Island

There’s not an awful lot to say about Komodo dragons, other than the fact that they are a huge lizard – the biggest in the world – and give us an indication of the reptilian megafauna that once stalked the earth.

Komodo dragon (Varanus komodoensis) on Komodo Island approaching a water hole
Komodo dragon (Varanus komodoensis) on Komodo Island approaching a water hole

They are not creatures I think anyone could find attractive. Respect, admiration and awe, but not affection. They are just too reptilian, too unknowable. What is going on in that tiny brain when it looks at us. Just the notion, I think, that we are edible!

Komodo dragon (Varanus komodoensis) on Komodo with forked tongue
Komodo dragon (Varanus komodoensis) on Komodo
with forked tongue

I didn’t really like them, with their deer-chomping habits and slimy mouths, but they are magnificent animals and I am very glad to have had the opportunity to see them up close.

Komodo dragon (Varanus komodoensis) drinking from water hole
Komodo dragon (Varanus komodoensis) drinking from water hole

I must confess that they did have an air of dignity about them. When this large one approached the water hole to drink, we scattered, but he remained still, upright and alert (first photo) for some time before moving in for a drink. After all it was his water hole. Was he being cautious or dignified? Maybe a bit of both. He (or she) had probably been drinking there for decades whilst we were only spending a fleeting hour on the island.

137 Komodo dragon (Varanus komodoensis) on beach with deformed foot
137 Komodo dragon (Varanus komodoensis) on beach with deformed foot

The ones on the beach were said to have been fed occasionally, which is why they are very interested and attentive when zodiacs containing tourists arrive to gawp at them and take pictures.

'Shooting' dragons on the beach
‘Shooting’ dragons on the beach
134 Komodo dragon (Varanus komodoensis) on beach
Komodo dragon (Varanus komodoensis) on beach
Komodo dragons (Varanus komodoensis) on beach
Komodo dragons (Varanus komodoensis) on beach
Komodo dragon with open mouth.
Komodo dragon with open mouth.

The young ones, which spend most of their lives up trees to avoid being eaten by the larger ones, are almost cute. Until I saw one catch, kill and swallow a rat (below sequence).

Young Komodo dragon hunting up a tree
Young Komodo dragon hunting up a tree
Young Komodo dragon with freshly killed rat in its mouth (Komodo Island)
Young Komodo dragon with freshly killed rat in its mouth (Komodo Island)
Young Komodo dragon swallowing a rat
Young Komodo dragon swallowing a rat

It stalked it up a tree and then fell to the earth with a thud. It then raced off to a quiet spot (not so quiet on account of the photographers following it with their cameras!) and proceeded to swallow it whole. They shan’t be on my Christmas list. But like them or loath them, they deserve respect; for surviving so long if nothing else!

Baby Komodo dragon peering out of a hole up a palm tree.
Baby Komodo dragon peering out of a hole up a palm tree.
Young Komodo dragon resting on a log
Young Komodo dragon resting on a log

The poor animals which God, sorry evolution, has elected to be food for these giant lizards, are an attractive cervid called the Timor deer. It must be a rum existence never knowing when one of these lizards is going to inflict a venomous bite leaving you hobbling around waiting to be eaten. But perhaps only the weak and sickly get taken. This deer did not seem at all concerned about the presence of the dragons nearby.

Timor deer or Sunba sambar (Rusa timorensis)
Timor deer or Sunba sambar (Rusa timorensis)

All photographs taken in Komodo National Park in October 2016.

Watch out little butterfly!

A little butterfly skipping from flower to flower in the late afternoon sunshine.

Small tortoiseshell (Aglais urticae L.)
Small tortoiseshell (Aglais urticae L.)

Enjoying little sips of nectar. Seemingly oblivious to the cares of this world.

tortoiseshell-ups
Small tortoiseshell (Aglais urticae L.)

Yet there lurks a trap for this innocent little sprite. A spider has cast its net.

Orb spider with butterfly capture
Orb spider with butterfly capture

Nature is not nice, or sweet. The innocent get consumed. Do the fittest always survive? Or does blind chance decree who gets caught and who remains free to fly another day?

Wood Whites go A-Courting!

Wood white (Leptidea sinapis) butterflies courting - male on the left waving his proboscis back and forth
Wood white (Leptidea sinapis) butterflies courting – male on the left waving his proboscis back and forth

In 1988, it was discovered that the Wood White butterfly (Leptidea sinapis (Linnaeus, 1758)) was actually two species, largely overlapping in their habitats, but virtually identical and only distinguishable by microscopic observation of their genital! (3) These so-called cryptic species are widespread in their distribution and occur together throughout the European continent, from the Iberian Peninsula to the Urals (1). Some researchers went back over their collection of museum specimens to separate the two species which were once considered as one! (1)

Wood white on common vetch
Wood white on common vetch (with ant)

Unfortunately, Réal’s wood white – as the new species (Leptidea reali (Reissinger, 1989) is called – has not been found in Britain, although it was found in Ireland in 2001, where it turns out to be commoner and far more widespread than its sister species (L. sinapis). (2) It also occurs in northern and NE Spain (it was discovered in the Pyrenees).

The photographs in this blog were all taken in NW Spain (Galicia); so it is possible that they feature Réal’s wood white, but since I did not capture and dissect them (!), I will never know; which is OK by me. Réal’s Wood White has apparently been described as being a stronger flier and with a preference for more open habitats (2), which was certainly the case with the ones I photographed in Spain (below).

Wood white (Leptidea sinapis) on nectaring on Common vetch in Galicia, Spain
Wood white (Leptidea sinapis) on nectaring on Common vetch in Galicia, Spain

If all this was not exiting enough, in 2011 Réal’s Wood White was itself split! A third species – called the Cryptic Wood White (L. juvernica stat. nov.) – was discovered, based on molecular (mitochondrial and nuclear DNA) markers. (3). So there are now, three so-called ‘sibling’ Leptidea species in Europe. Amazingly, the molecular work allows the scientists to make the claims that they all evolved in the last 270,000 years with a divergence into L. sinapis and L. reali about 120,000 years ago (3). All very recent in geological terms. Unfortunately, the third cryptic species is not found in Britain either (has it been overlooked?) but does occur in Ireland. (3)

Anyway, enough of this preamble, what concerns me in this blog is the behaviour of these attractive little butterflies. My fascination started when I captured some interesting behaviour in a lucky snap I took with a compact camera. I published the picture on my blog in 2014, although I was not sure what was going on (4). I should have guessed it was courtship behaviour. It turns out that quite a lot is known about these Wood Whites which have a characteristic courtship display where the male lands opposite the female and sways his head and waves his antennae backwards and forwards with his proboscis extended (5). I was fortunate to capture the proboscis being waved by the male (below).

Wood white (Leptidea sinapis) butterflies courting detail showing male waving proboscis
Wood white (Leptidea sinapis) butterflies courting detail showing male waving proboscis

Charmingly, the male does not attempt to mate with the female until she has shown some sign of accepting his advances, which she does by lowering her abdomen so that it becomes visible between her wings. (6) She also bends her antennae backwards until they touch her wings (5). Perhaps emitting some sort of pheromone as well. I guess this all means ‘come and get me’! His behaviour has been called  ‘non-insistent’ and her signal to him was called a ‘mating willingness’ signal (6). Isn’t science full of wonderful terminology!

One way of telling the sexes apart in this species (rather these species) is via their antennae. The males have a large patch of white scales on the underside of the antennae, as shown in the illustration taken from Friberg et al. (2008).

Fig. 2 from Friberg et al, 2007. Schematic picture of a male and female Leptidea antenna. (Illustration: Moa Lönn). Springer Press.
Fig. 2 from Friberg et al, 2007. Schematic picture of a male and female Leptidea antenna. (Illustration: Moa Lönn). Springer Press.

The difference in the antennae can also be made out in my snapshot (below) where I have highlighted the different lobes of the male and female (below). The males of both L. sinapsis and  L. reali have a large patch of white scales on the underside of the antennae whilst females only have a few white scales in the midpart of the antenna. (7) Perhaps these white waving wands have the desired effect!

Wood white (Leptidea sinapis) butterflies courting; with antennal tips highlighted. Male on the left.
Wood white (Leptidea sinapis) butterflies courting; with antennal tips highlighted. Male on the left.

Another fascinating detail is that some females waited for over 11 minutes of courtship behaviour by the male – that’s a lot of antennae and proboscis waving by the male! – before giving him the ‘come hither’ signal. Whilst other ladies were happy to give him the nod after only a few seconds! (6) Already mated females were courted for up to 35 minutes by eager males, without them giving any sort of signal; it’s hard not to anthropomorphise! The act of mating itself is quite long-lasting: between 25 and 55 minutes, before the male releases himself from the act of copulation and flies away (5).

The situation is further complicated by the fact that the males do not seem to be able to determine which species they are courting. In other words, they don’t know if she is a L. sinapsis or a L. reali , which means that they can waste a lot of time and effort chatting up the wrong girl! (6). Rather unfairly, it seems that she can tell whether the male is of the same species as herself. All of this confusion is thought to be due to the fact that the species have only recently split apart. They have only had a quarter of a million years to get to know each other!

Links and references

  1. Sachanowicz, K., Wower, A., & Buszko, J. (2011). Past and present distribution of the cryptic species Leptidea sinapis and L. reali (Lepidoptera: Pieridae) in Poland and its implications for the conservation of these butterflies. European Journal of Entomology108(2), 235.
  2. http://www.wikiwand.com/en/Leptidea_reali
  3. Dincă, V., Lukhtanov, V. A., Talavera, G., & Vila, R. (2011). Unexpected layers of cryptic diversity in wood white Leptidea butterflies. Nature communications, 2, 324.
  4. https://rcannon992.com/2014/01/03/fancy-meeting-you-here/
  5. Wiklund, C. (1977). Courtship behaviour in relation to female monogamy in Leptidea sinapis (Lepidoptera). Oikos, 275-283.
  6. Friberg, M., Vongvanich, N., Borg-Karlson, A. K., Kemp, D. J., Merilaita, S., & Wiklund, C. (2007). Female mate choice determines reproductive isolation between sympatric butterflies. Behavioral Ecology and Sociobiology62(6), 873-886.
  7. Friberg, M., Bergman, M., Kullberg, J., Wahlberg, N., & Wiklund, C. (2008). Niche separation in space and time between two sympatric sister species—a case of ecological pleiotropy. Evolutionary Ecology22(1), 1-18.

“Did you hear that?” Said the butterfly.

Blue morpho (Morpho peleides) and Owl butterfly (Caligo atreus)
Blue morpho (Morpho peleides) and Owl butterfly (Caligo atreus)

It used to be thought that butterflies could not hear; that they were deaf. Well I suppose it is understandable, as they do not have ears sticking out from their tiny heads! But it turns out that they can hear – at least some of them can – and they do have ears, but not where you might think. As we shall see, they are on the base of the fore-wings.

It’s long been known that moths (and some butterflies)  have ears which are sensitive to ultrasound – high frequencies above our audible range – and that this trait probably evolved separately numerous times in the family Lepidoptera. Night-flying moths use their high-frequency hearing to detect bats and there is an evolutionary sound war – driven by natural selection – going on between these two nocturnal contestants: predator and prey. The so-called tympanal ears of noctuoid moths, such as the one shown below which I snapped in Thailand, are located on the side of moth (the metathorax) and are said to be tuned to respond to the ultrasonic calls of insectivorous bats.

Asota plana plana (Erebidae). Moths like this have tympanal ears at the junction between the thorax and abdomen.
Asota plana plana (Erebidae). Moths like this have tympanal ears at the junction between the thorax and abdomen.

Butterflies in contrast, evolved into day-flying species, with no need to be able to echo-locate bats like their ancestors did. They have grown bat-deaf! What would be useful for them though, is a way of detecting their daytime predators: birds. It seems that the old bat-detecting ears ears have been adapted to this new purpose in some species like the Blue Morpho butterfly (Morpho peleides).

blue-morpho-morpho-peleides-feeding-on-oranges in a butterfly house
Blue Morpho (Morpho peleides) feeding on oranges in a butterfly house

Ear-like structures have long been noticed at the base of the wings in some nymphalid butterflies. This tiny structure is called Vogel’s Organ. In the Blue Morpho butterfly (shown below) it is an oval-shaped structure composed of inner and outer membranes, which it has been suggested, might allow it to hear two different types of sound frequencies (high and low). It is possible that these butterflies might be ‘listening to the flight sounds of avian predators’ (Lane et al., 2008) and M. peleides may use its two membrane ‘ear’ to ‘detect both singing and flying birds’ (Lucas et al., 2009). It’s not proven yet, but the fact that these butterflies can hear in the range which covers the lower frequency sounds associated with the flapping of bird wings, provides good circumstantial evidence for a putative bird detection system, which can be tested in future experiments (Link 1).

Blue morpho (Morpho peleides) with Vogel's organ at the base of the forewing
Blue morpho (Morpho peleides) with Vogel’s organ at the base of the forewing

The owl butterfly, Caligo eurilochus, also has an ear on the base of its forewings, but according to researchers it is a simpler structure than in the Blue Morpho butterfly. The C. eurilochus ear was most sensitive to sound at frequencies between 1 and 4 kHz, similarly the M. peleides Vogel’s organ is most sensitive to sounds between 2-4 kHz. These could be used to detect the low-frequency components of approaching birds. In other words, they are bird detectors.

Owl butterfly (Caligo atreus) feeding on oranges in a butterfly house
Owl butterfly (Caligo atreus) feeding on oranges in a butterfly house

The owl butterfly is crepuscular, which means that it is most active around dawn and dusk, i.e. during low-light conditions. The ear – or Vogel’s Organ – in C. eurilochus is said to be rather anatomically simple, in comparison to the Blue Morpho.

Owl butterfly (Caligo atreus) - approximate location of Vogel's organ
Owl butterfly (Caligo atreus) – approximate location of Vogel’s organ

We usually know if an animal like a dog or cat can hear us, because it responds in some way to what we say. But it is not easy working out whether something like a butterfly can hear, even if you can find what appears to be its ears.  And when you do work out that they can hear some sounds, it’s not easy to know exactly what they are listening too, and why.

Some butterflies known the ‘crackers’ – Hamadryas spp. – emit surprisingly loud clicks, or ‘clacks’! The clicking or clacking sounds – take your pick – is mostly, but not exclusively, made by males.

Epinome cracker (Hamadryas epinone) from Argentina
Epinome cracker (Hamadryas epinone) from Argentina

A study of the beautiful blue cracker, Hamadryas feronia, in Venezuela, by Jayne Yack (Link 2) and others (2000 paper) at Carleton University (Ottawa, Canada), showed that the males made the ‘sharp clicking sounds’ during chases involving both other males, and females. Typically, a male resting or perching, on the trunk of a tree will take off and fly after another butterfly of the same species as it flies past. If it is another male, they pursue each other, making clicks when they are close to one another. If the male ends up chasing a female, then he ends up conducting what the researchers described as an ‘on-the-wing pendulous display involving continuous clicking’ for the benefit of the female! If she is receptive, then he lands and copulates with her.

Epinome cracker (Hamadryas epinone) puddling
Epinome cracker (Hamadryas epinone) puddling

So it seems that there is a lot more to learn about the sound worlds of butterflies. It is very exciting to think that there may be more complex acoustic interactions going on between butterflies and their avian predators than we ever imagined. So much research has been carried out on the visual markings of butterflies, but it may be that they also rely on sound as well as startling images on their wings to help them avoid the depredations of birds.

All photographs taken by myself either in Argentina or Amsterdam Zoo butterfly house.

Links

  1. http://io9.gizmodo.com/now-we-know-why-butterflies-evolved-to-have-ears-1152166029
  2. https://carleton.ca/biology/people/jayne-yack/

Relevant references

Lucas, K. M., Windmill, J. F., Robert, D., & Yack, J. E. (2009). Auditory mechanics and sensitivity in the tropical butterfly Morpho peleides (Papilionoidea, Nymphalidae). Journal of Experimental Biology, 212(21), 3533-3541.

Lucas, K. M., Mongrain, J. K., Windmill, J. F., Robert, D., & Yack, J. E. (2014). Hearing in the crepuscular owl butterfly (Caligo eurilochus, Nymphalidae). Journal of Comparative Physiology A, 200(10), 891-898.

Conner, W. E., and A. J. Corcoran (2012). Sound Strategies: the 65-million-year-old battle between bats and insects Annual Review of Entomology 57: 21-39.

Ribarič, D., & Gogala, M. (1996). Acoustic behaviour of some butterfly species of the genus Erebia (Lepidoptera: Satyridae). Acta entomologica slovenica, 4(1), 5-12.

Vogel R. 1912. Uber die Chordotonalorgane in der Wurzel der Schmetterlingsflugel. Z Wiss Zool 100:210–244.

Yack, J. E., Otero, L. D., Dawson, J. W., Surlykke, A. & Fullard, J. H. (2000). Sound production and hearing in the blue cracker butterfly Hamadryas feronia (Lepidoptera, Nymphalidae) from Venezuela.Journal of Experimental Biology203(24), 3689-3702.

Yack, J. E. (2004). The structure and function of auditory chordotonal organs in insects. Microscopy research and technique, 63(6), 315-337.

 

 

 

Peck me here! Butterfly predation.

Fluffy Tit (Zeltus amasa amasa) worn and damaged
Fluffy Tit (Zeltus amasa amasa) worn and damaged

It is said that  50% of wild butterflies are killed and eaten before they get a chance to mate and reproduce (1). Poor things! One way to avoid being eaten is to divert the lethal pecks of predatory birds towards body parts that can be sacrificed in the interests of survival. Obtaining direct evidence for the protective utility of eyespots is difficult, but the deflective function of marginal eyespots has been demonstrated in some studies. It has also been shown to work well under low light conditions – such as at dawn and dusk – when birds are most active (2).

Photographs of butterflies often show evidence of extensive damage. Whilst such evidence of ‘beak marks’ – damage caused by a would be predator – is only circumstantial, it is a good indication of the fact that it is a regular occurrence in nature. Indeed, photographs could perhaps be used as a research tool into the intensity of such attacks, although one would only observe the survivors! The others would be inside the birds stomachs. Once pecked, as in the above photograph, the eyespot may be lost, so the butterfly is presumably more vulnerable to subsequent attacks. It has also been shown that eyespots can function in different ways in different seasons: eyespot plasticity! In the dry season, eyespots are a liability, so natural selection has resulted in the evolution of a spotless form – able to blend in well against brown, dead leaf litter – and another form, with eyespots, in the wet season, where they have an evolutionary useful deflective function (3).

Another feature which has become obvious to me as I have taken more and more photographs of butterflies, is that they are seemingly able to fly about and carry out their lives, despite sustaining considerable damage to their wings. Some of this may be simple ‘wear and tear’ as well as predation damage. I have written about this before (4)! It’s a subject which fascinates me for some reason! Perhaps its something to do with our universal fragility? Compare these two images, below. The first, a highly worn and presumably ‘old’ butterfly – a male Clipper, Parthenos sylvia apicalis. The second, a much fresher specimen of the same species – but nevertheless, still sporting a beak mark on the left hind wing – which only became apparent to me after I photographed it and looked at the image on my computer. Yet both individual butterflies were gaily flying around and resting to feed on flowers. Indistinguishable to the casual observer. For some reason, perhaps to do with age, I take comfort in this fact!

Clipper Parthenos sylvia apicalis male, highly worn and damaged
Clipper Parthenos sylvia apicalis male, highly worn and damaged
Clipper (Parthenos sylvia apicalis) male, with small beak mark
Clipper (Parthenos sylvia apicalis) male, with small beak mark

All three images were taken in Doi Chiang Dao, in northern Thailand.

  1. http://www.learnaboutbutterflies.com/Enemies%20of%20Butterflies.htm
  2. Olofsson, Martin, et al. “Marginal eyespots on butterfly wings deflect bird attacks under low light intensities with UV wavelengths.” PLoS One 5.5 (2010): e10798-e10798.
  3. Lyytinen, Anne, et al. “Does predation maintain eyespot plasticity in Bicyclus anynana?.” Proceedings of the Royal Society of London B: Biological Sciences 271.1536 (2004): 279-283.
  4. 4. https://rcannon992.com/2013/12/15/predation-on-butterflies/

A complex caterpillar!

Hyles euphorbiae caterpillar (Macedonia, Greece)
Hyles euphorbiae caterpillar (Macedonia, Greece)

The caterpillars of this species – the Spurge Hawk-moth (Hyles euphorbiae) – are highly variable and there are many different subspecies; some of which are now regarded as separate species (1, 2). In fact, it seems to be a taxonomic nightmare! First of all, the Hyles genus itself comprises “a complex of species, subspecies and forms, all closely related to Hyles euphorbiae” (3). Secondly, the species complex called Hyles euphorbiae “is rather difficult to classify for it would seem to be in the process of diverging into a number of species” (3). Thirdly, some of the subspecies have crossed with each other to form localised hybrid populations in some parts of Europe!

The yellow, black and white larva (with orange prolegs!) shown here is probably Hyles euphorbiae euphorbiae, as it looks very similar to photographs of other larvae collected from the same area (Serres) in northern Greece (3). They feed on species of Euphorbia, which they consume with great gusto and if disturbed, eject “a thick stream of dark green fluid” said to be “rich in phorbol esters, which are potent toxins and irritants” (3). So you have been warned!

This complex of species and subspecies would seem to be a great opportunity to observe evolution in progress, and some researchers are doing just that (5, 6). The problem is that taxonomists have sometimes disagreed as to whether certain forms are completely different species, or just distinct subspecies.  Interestingly, and this is by no means unique in rapidly diverging species, the adult genitalia – features beloved by classical lepidopterists – are of “little use as they show practically no difference between taxa” (3). Fortunately, the colour and patterns of the larval stages do “provide a very good guide to the relationships between the many ‘species’ and ‘subspecies'”, although there is some overlap (5). This is analogous to a study which found ten cryptic species lurking within indistinguishable adults of a neotropical species (in Costa Rica) that did however, have distinctly different caterpillars (and feeding preferences) (4).

Molecular studies – using mitochondrial DNA sequences – are also coming to the rescue of the Hyles euphorbiae complex (HEC) (5). It seems that the different lineages within the HEC complex arose as a consequence of geographical isolation in Pleistocene refuges in Europe, during the Ice Ages (6). But there are unresolved issues concerning the separation of different forms, or haplotypes, using mitochondrial DNA (see reference 6 for a discussion).

Hyles_euphorbiae caterpillar (northern Greece)
Hyles_euphorbiae caterpillar (northern Greece)

References  (all available online)

  1. http://www.pyrgus.de/Hyles_euphorbiae_en.html
  2. http://www.leps.it/indexjs.htm?SpeciesPages/HylesEupho.htm
  3. http://tpittaway.tripod.com/sphinx/h_eup.htm
  4. Hebert, Paul DN, et al. “Ten species in one: DNA barcoding reveals cryptic species in the neotropical skipper butterfly Astraptes fulgerator.” Proceedings of the National Academy of Sciences of the United States of America 101.41 (2004): 14812-14817.
  5. Hundsdoerfer, Anna K., Ian J. Kitching, and Michael Wink. “The phylogeny of the Hyles euphorbiae complex (Lepidoptera: Sphingidae): molecular evidence from sequence data and ISSR-PCR fingerprints.” Organisms Diversity & Evolution 5.3 (2005): 173-198.
  6. Mende, Michael B., and Anna K. Hundsdoerfer. “Mitochondrial lineage sorting in action–historical biogeography of the Hyles euphorbiae complex (Sphingidae, Lepidoptera) in Italy.” BMC evolutionary biology 13.1 (2013): 83.

Flowers and insects: an ancient alliance

Mountain scabious (Pterocephalus dumetorum) from Gran Canaria with resident beetle
Mountain scabious (Pterocephalus dumetorum) from Gran Canaria with visiting beetle

Take a photograph of a flower, examine it closely – or enlarge it on a computer screen – and you will invariably find an insect lurking somewhere in the picture. This is not altogether surprising when we learn that two-thirds of flowers are pollinated by insects. To achieve this, flowers have learnt – OK evolved! – how to bribe, cajole, or trick insects into carrying out this function. Plants don’t walk, so they need animals to carry out a vital function for them; they need them to carry their sperm (in the form of pollen) to another individual where it can fuse with the eggs (ovules) of the other plant. Sex, or to give it another name: reproductive out-crossing!

Psilothrix viridicaeruleus on Hawksbill flower
Psilothrix viridicaeruleus on Hawksbill flower

Some plants self-fertilise – i.e. sperm fuses with eggs from the same flower. Dandelions are among such plants – which can produce seeds without having to be fertilised – although they can also be sexual as well, relying on being fertilised by pollen carried from one dandelion to another by insects. The photo (below) shows a dandelion head composed of dozens of tiny florets, each with pistols bearing pollen, which can be picked up by visiting insects, e.g. bees.

Dandelion flowers (Taraxacum officinale) with a profusion of pollen-bearing pistils
Dandelion flowers (Taraxacum officinale) with a profusion of pollen-bearing pistils

Some plants – the anemophilous ones, lovely word – rely on the wind to carry their pollen to another individual; but if a plant is to rely on an insect to vector its pollen, then it is going to have to have a strategy to achieve this. In practice, thousands of different strategies have evolved over time. If a flower is going to rely an insect to help it reproduce, it needs to do a number of things. First of all it needs to get the attention of the insects; most insect pollinated flowers are large and brightly coloured. Next it needs to offer some sort of inducement, usually in the form of nectar, although the pollen itself is a reward for many visitors.

Small white flower - wind or insect pollinated? (I don't know)
Small, unidentified white flower – wind or insect pollinated? (I don’t know)

A variety of different insects – e.g. bees, wasps, ants, butterflies, beetles and so on – may visit a given flower. Some may be feeding on nectar (butterflies and moths); some might be defending their sap-sucking aphids (ants); some might just be sheltering or hiding in the petals; and some may be eating the plant; but the pollinator species which is best for the plant is the one that helps it to reproduce successfully (i.e. it helps to increase the plant’s fecundity). These are the insects that the flower will evolve to attract. But not all flowers are specialists in this regard; some may be visited by a variety of pollinating bees and butterflies during the day, and by moths during the night. I wonder they ever get any sleep!

Clover weevils within sea clover - this species relies mainly bumblebees for pollination - the beetles are just feeding on it
Clover weevils within sea clover – this species relies mainly bumblebess for pollination – the beetles are just feeding on it

The hummingbird hawk-moth (Macroglossum stellatarum) – which has a very long proboscis – has been seen visiting the wild, Fringed Pink, Dianthus monspessulanus (1, 2). This plant has very long-tubed flowers (below) and emits a strong evening fragrance to attract the moths. It may however, not be the only flower species competing for the moth’s attention!  The most attractive, and sweet-smelling flowers – to a hawk-moth’s nose that is! – will presumably get visited the most. Those flowers will be the ones which are the most fecund in the next generation, and the attractiveness to hawk-moths will continue to evolve. It is interesting that we humans also find the smell of such plants appealing; after all they are not trying to attract us! I guess it demonstrates the universality of the chemistry involved; it involves a compound called linalool, which is a common attractant for nocturnal hawk-moths (3).

Fringed Pink, Dianthus monspessulanus
Fringed Pink, Dianthus monspessulanus

Sea daffodil (Pancratium maritimum) is another flower which relies on hawk-moths for pollination (4).

Sea daffodill (Pancratium maritimum) flowers are pollinated by hawk moths
Sea daffodill (Pancratium maritimum) flowers are pollinated by hawk-moths

Spring Squill (Scilla verna) is a plant that may be ant pollinated. Ants can sometimes be seen feeding on the ovaries (below); they can contribute to pollination by transferring pollen from one flower to another, but they can also be nectar thieves, just feeding on the nectar without carrying out any pollination services in return! (5).

Ants on Spring Squill
Ants on Spring Squill

An alternative strategy is of course to trick the insect into carrying out the needs of the plant. Some orchids do this by looking like bees or flies, but fascinating though this is, we will not follow this further here. Flowers providing rewards of nectar need to ensure that it is not wasted and that the pollen is successfully attached to the insects, for onward transport. All manner of devices and structures are used to make sure that the pollen is first attached – by sticking, brushing or hooking – and then successfully detached and delivered to the receptive female organ: the sticky tip of the pistil, the stigma. Some plants have even evolved ways of selecting the right sort of pollinator for their needs! For example, the guard hairs (below) on the Foxglove flower, Digitalis purpurea, have it is thought, evolved to exclude small bees, which presumably are not strong enough to push past them! The flowers are effectively selecting large bees to pollinate them, especially ones with long-tongues which can reach deep inside the flower to get the nectar (6). It is beneficial to both the bees and the flower to keep the relationship between themselves; it is a mutualistic arrangement which has evolved to suit both parties: the bees get to feed on the nectar, and the flower gets its pollen spread around in an efficient and effective manner. It would not benefit either of them if little upstarts got in and stole the pollen!

Digitalis purpurea flower guard hairs
Digitalis purpurea flower guard hairs

Both the ecology of pollination, and the evolution of the relationships between plants and insects, are vast and much studied subjects. All I want to do here is to illustrate by means of a few photographs, how easy it is to observe some aspects of this biological phenomenon. Most cameras allow close up photograph now, and the results are often surprisingly good – even with a relatively inexpensive camera – if one is prepared to be patient, and capture a detailed image. It is not always obvious that an insect is in the picture! Many of the photos shown here were just ones I had taken of a flower. Only afterwards, when examining the image on a computer screen, did I notice the insect! Most of  the images shown here have been heavily cropped (i.e. by selecting the centre of the photo) to obtain the close up I wanted. I have also included some nice close up images of flowers without insects because I just like the shapes and appearances of these flowers. There is no end to what can be done just by taking a camera out into the garden or countryside (at the right time of year!).

Green mirid on Arctotheca calendula flower
Green mirid on Arctotheca calendula flower

Finally, just a note of caution. Hopefully this blog will have shown how dependent flowers and pollinators are on each other. Anything which affects the pollinators – and bees and butterflies are suffering in our modern world – will affect the plants too. This is particularly true in the case of specialised species, which are dependent on – i.e. adapted to – a particular type of pollinator species. The bottom line is, if the pollinator goes, the flower goes too. It’s an interdependent world and we need to take better care of it.

Sea Holly (Eryngium maritimum) attracts many different species of insects
Sea Holly (Eryngium maritimum) attracts many different species of insects
  1. Willemstein, Sjoert Cornelis. An evolutionary basis for pollination ecology. Vol. 10. Brill Archive, 1987.
  2. https://rcannon992.com/2014/07/19/flower-of-god/
  3. Miyake, Takashi, Ryohei Yamaoka, and Tetsukazu Yahara. “Floral scents of hawkmoth-pollinated flowers in Japan.” Journal of Plant Research 111.2 (1998): 199-205.
  4. https://rcannon992.com/2014/07/29/sea-daffodils-waiting-for-their-hawk-moths/
  5. https://rcannon992.com/2015/10/25/bumbler-bees-and-foxgloves/
  6. https://rcannon992.com/2014/04/29/six-blue-tepals-and-some-nectar-thieves/