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.

 

 

 

 

Red Admirals – European migrants

Red Admiral (Vanessa atlanata) dorsal side. Galicia, Spain. 11 June.

Migrant Red Admirals Vanessa atalanta (L.), usually arrive in the UK during May and June each year. Like the closely related butterfly, The Painted Lady, Vanessa cardui (L.), these migrations of Red admirals originate from countries around the Mediterranean – possibly as far south as the North African coast. (2) The butterflies fly north on southerly winds to feed on new growth as it becomes available in the Spring (1).

The Painted Lady, Vanessa cardui (L.) on Aesculus californica. Barcelona, Spain. 6th June.

Most European Red Admirals  migrate north in the Spring and – after producing a new generation – migrate south again in the Autumn. (3)  This seasonal movement appears to occur right across Europe and western Asia, although this still needs confirmation from many regions, with waves of migrants moving north, for example up into Finland, northern Norway and northern Russia. (4, 5, 9, 10).

Red Admiral (Vanessa atlanata) feeding on thistle. Galicia, Spain, 11 June.

Red Admirals arriving in the UK, mate and lay their eggs mainly on stinging nettles (Urtica diocia); a new generation emerges sometime over the period, August to October.  A small number of Red Admirals remain to overwinter in the British Isles (mainly in southern England) – although numbers appear to be increasing with climate change – whilst the majority elect to migrate. (3) How does this choice to migrate or not work in practice? “Should I stay or should I go now”?! (6). Perhaps a small proportion of the population are genetically programmed not to migrate?

The Red Admiral (Vanessa atalanta) on bell heather. Galicia, Spain, 28th August.

Of those individuals that remain in the UK, it is not thought that they hibernate in a physiological sense, although many sites state that they do hibernate, I think it is true to say that they merely remain dormant, since they can become active on sunny days throughout the winter. (5)  Some of these remaining butterflies must mate in the autumn, as there are records of V. atalanta larvae developing slowly over winter. In other words, a second generation gradually develops over the period from autumn until the following spring. This is exactly what happens when the migrants arrive back in Spain in October and early November as well; ‘larval development occurs throughout the winter until a first annual generation of adults appears in early spring’ (Stefanescu, 2001). (3)

The Red Admiral (Vanessa atalanta) on bell heather. Galicia, Spain, 28th August.

The small proportion of the UK population which do not migrate south are in effect opportunists, which presumably do well in mild winters but suffer heavy mortality in cold ones. The home-grown adults appear in early spring in the UK, well before the next wave of migrants arrive from southern climes, but the overall contribution of these overwintering individuals is thought to be minimal; populations in northern Europe were considered to be entirely dependent on immigration which determines abundance (8). This situation may however, be changing as the climate warms.

The Red Admiral (Vanessa atalanta) on bell heather. Galicia, Spain, 28th August. Wings held in typical 3/4’s open position (See Link 11).

Red Admirals flying southwards in September, in Finland, were found to migrate on sunny days when cool northern winds were blowing (13). Red Admirals take about 5 weeks to fly the 3,000 km from Northern Europe down to the countries surrounding the Mediterranean (1). Circumstantial evidence from meteorological radar observations suggests that they migrate at high altitudes (up to 2,000m or more), where temperatures may be as low as 2-3 deg C! Once they arrive in the south again, in places such as the Catalonia lowlands in north-east Spain – in October and early November, they start breeding a new generation. (2)

The Red Admiral (Vanessa atalanta) on bell heather. Galicia, Spain, 28 August.

Not all Red Admirals migrate over long distances. Studies in Spain by Stefanescu (2001) have shown that some individuals fly much shorter distances towards nearby locations of a high altitude. The butterflies shown here (e.g. above and below) feeding on bell heather were photographed in late August at one such location, near the peak of a hill in Galicia, Spain.

The Red Admiral (Vanessa atalanta) on bell heather. Galicia, Spain, 28 August.

Citizen science projects, such as the one on Red Admiral migration run by the Insect Migration & Ecology Research Group, at the University of Bern, Switzerland (13), offer enormous potential for gathering information on insect migration. People all over Europe can record sightings on a plethora of citizen science portals – some of which are configured as easy to use Apps – allowing researchers to build up unprecedented data bases of records in time and space. It will be fascinating to see what they can come up with in terms of new findings.

  1. Stefanescu, C., Alarcón, M., & Àvila, A. (2007). Migration of the painted lady butterfly, Vanessa cardui, to north‐eastern Spain is aided by African wind currents. Journal of Animal Ecology, 76(5), 888-898.
  2. Brattström, O., Bensch, S., Wassenaar, L. I., Hobson, K. A., & Åkesson, S. (2010). Understanding the migration ecology of European red admirals Vanessa atalanta using stable hydrogen isotopes. Ecography, 33(4), 720-729.
  3. Stefanescu, C. (2001). The nature of migration in the red admiral butterfly Vanessa atalanta: evidence from the population ecology in its southern range. Ecological Entomology, 26(5), 525-536.
  4. Fox, R. & Dennis, R. L. (2010). Winter survival of Vanessa atalanta (Linnaeus, 1758)(Lepidoptera: Nymphalidae): a new resident butterfly for Britain and Ireland?. Entomologist”s Gazette, 61(2), 94.
  5. Bolotov, I. N., Bochneva, I. A., Podbolotskaya, M. V., Gofarov, M. Y., & Spitsyn, V. M. (2015). Butterflies (Lepidoptera: Papilionoidea and Hesperioidea) from meadows of Vinogradovsky District, Arkhangelsk Region, northern European Russia, with notes on recent intense expansion of the southern species to the north. Check List, 11(5), 1727.
  6. https://www.youtube.com/watch?v=BN1WwnEDWAM (The Clash video).
  7. http://nurturing-nature.co.uk/wildlife-garden-videos/red-admiral-butterflies-do-they-hibernate-in-britain/
  8. Pollard, E., & Greatorex-Davies, J. N. (1998). Increased abundance of the red admiral butterfly Vanessa atalanta in Britain: the roles of immigration, overwintering and breeding within the country. Ecology Letters, 1(2), 77-81.
  9. Brattström, O. (2007). Ecology of red admiral migration. Department of Animal Ecology, Lund University.
  10. Brattström, O., Åkesson, S., & Bensch, S. (2010). AFLP reveals cryptic population structure in migratory European red admirals (Vanessa atalanta). Ecological Entomology, 35(2), 248-252.
  11. Peter B. Hardy. The Butterflies of Greater Manchester. http://www.oocities.org/pgll@btopenworld.com/bgm/bgm.htm
  12. Mikkola, K. (2003). Red admirals Vanessa atalanta (Lepidoptera: Nymphalidae) select northern winds on southward migration. Entomol. Fenn., 14(1), 15-24.
  13. https://insectmigration.wordpress.com/red-admiral-migration/

Bee-flies: the dipteran narwhals

White-tailed bee-fly (Bombyliidae) showing wing venation
White-tailed bee-fly (Bombyliidae), possibly Bombylius posticus, showing wing venation

I always enjoy seeing bombyliids (bee-flies). They sound like little helicopters, hovering and buzzing about, and their furry appearance gives them a certain cuteness. They are flies pretending to be bees!

Not the easiest of insects to identify from photographs though. This one looks rather like Bombylius posticus, which has a wide Palaearctic distribution, but I am not sure if it is found in northern Thailand, where I took the photograph. This species has prominent white tufted scales at both the base and apex of the abdomen. (1). Alternatively, it might be a variant of Bombylius major, which is found in Thailand.

White-tailed bee-fly (Bombyliidae), possibly Bombylius posticus, showing white scales
White-tailed bee-fly (Bombyliidae), possibly Bombylius posticus, showing white scales

Why would they want to mimic bees? One reason might be that they avoid predation by other insects which think that they are bees, i.e. armed with a harmful sting. Although they don’t have a stinging apparatus like a bee, they do have a very prominent, needle-like proboscis sticking out in front of their heads. They use this stiff, unretractable organ to penetrate and probe flowers for nectar. It almost looks like they are carrying a little spear or javelin; the dipteran equivalent of a narwhal! According to Wikipedia, some people in East Anglia call them beewhals. (2)

Bee=fly (Bombyliidae) in flight, showing proboscis sticking out in front
Bee-fly (Bombyliidae) in flight, showing proboscis sticking out in front

Another reason why they might benefit from resembling bees, is that they lay their eggs in the nests of bees and wasps. Indeed, they actually flick their eggs into the nests of some solitary bees, whilst hovering above the nest opening. (See links 3 and 4 for videos of this behaviour). Flicking, or shooting eggs from a safe distance, as one blogger aptly put it! (5) The tufts at the end of the abdomen are reportedly used to collect dust prior to flicking the eggs, something that would be fascinating to watch!

The bee-fly larvae are ectoparasitic, meaning that they attach onto the outside of the bee larvae in order to feed on their body fluids. Perhaps their bee-like appearance helps the adult bee flies get close to bees nests without being attacked? Different species are also parasites, and hyper-parasites, on a wide range of insects, including butterflies, grasshoppers, wasps, other flies, beetles and cockroaches!

White-tailed bee-fly (Bombyliidae) resting.
White-tailed bee-fly (Bombyliidae) resting.

The adults feed on pollen and nectar and are important pollinators, indeed some plants species depend upon them for their survival. There is a nice little blog about bee-flies in a Scottish garden (6).

  1. http://diptera.info/forum/viewthread.php?thread_id=61710
  2. https://en.wikipedia.org/wiki/Bombyliidae
  3. http://www.brc.ac.uk/soldierflies-and-allies/bee-flies
  4. http://abugblog.blogspot.co.uk/2015/04/awesome-bee-flies.html
  5. https://livingwithinsects.wordpress.com/2012/04/22/bee-fly/
  6. http://www.leavesnbloom.com/2012/04/bombylius-major-bee-fly-aerodynamics.html

Termites on a trail

Termites (possibly Hospitalitermes sp), Doi Chiang Dao, Thailand
Termites (possibly Hospitalitermes sp.), Doi Chiang Dao, Thailand

I came across these termites moving along a water pipe on the Gully Trail at Wat Tham Pha Plong, Doi Chiang Dao, northern Thailand. The pipes provide water for the monks and their guests at the temple, but they also provide a convenient thoroughfare for the termites, helping them to move through the forest without having to traverse the ground!

Termites on water pipes, Gully trail, Doi Chiang Dao, Thailand
Termites on water pipes, Gully trail, Doi Chiang Dao, Thailand

These are nasutiform termites, which is to say that the soldiers have a pointed snout – a nasus – and are bit smaller than the workers. Presumably they make up for their lack of stature by being armed with a chemical spray gun, which can shoot a noxious aerosol of glue or repellent at any foe, particularly ants.

Termites (probably Hospitalitermes sp) nasute soldier outlined
Termites (probably Hospitalitermes sp.) nasute soldier outlined

There are at least 92 species of termite (Isoptera) in Thailand, including 7 genera in the subfamily Nasutitermitinae. (1) Most species in this subfamily are found in jungles and forests; they do not usually attack man-made structures, but rather feed on lichen, lichen-bark, dead leaves, branches, twigs, and other plant matter. (2)

I am not certain, but I think these are a species in the genus Hospitalitermes (Holmgren). There are six Hospitalitermes species in Thailand (1) and they forage openly during the day, as here. Other possibilities from northern Thailand are Nasutitermes, Bulbitermes, Aciculitermes and Havilanditermes. Lovely names!

Termites (probably Hospitalitermes sp.), large worker
Termites (probably Hospitalitermes sp.), large workers and slightly smaller, nasutiform soldiers

It is possible that the termites were heading out from their nest to forage in the jungle. I could only find one worker carrying a food-ball in the photos I took (below). Termites are of course decomposers; Nature’s way of cutting up and breaking down wood and vegetation. But they don’t do it alone; they need the help of microbes, symbiotic protozoa which they carry in their guts, to break down cellulose.

Termites (possibly Hospitalitermes sp.). One working (outlined) is carrying a food ball.
Termites (possibly Hospitalitermes sp.). One working (outlined) is carrying a food ball.

What termites lack in size they make up for in numbers. One study of termite abundance in Thailand, found that they were on average 6,450 individual termites per square metre, weighing about 10.7 g. If we scale up this termite biomass, which comprised many species, it gives a total weight of 10.7 million g or 10,700 kg per square kilometre. If I have got my arithmetic right, that is a fantastic ten tonnes per square km! The same as two or three large elephants!

Termites (probably Hospitalitermes sp.) walking along a water pipe
Termites (probably Hospitalitermes sp.) walking along a water pipe
  1. Sornnuwat, Y., Vongkaluang, C., & Takematsu, Y. (2004). A systematic key to termites of Thailand. Kasetsart J (Nat Sci), 38, 349-368.
  2. https://www.termiteweb.com/the-nasutitermitinae-termites/
  3. Inoue, T., Takematsu, Y., Hyodo, F., Sugimoto, A., Yamada, A., Klangkaew, C., … & Abe, T. (2001). The abundance and biomass of subterranean termites (Isoptera) in a dry evergreen forest of northeast Thailand. Sociobiology, 37(1), 41-52.