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

 

Stingless bees and resin bugs

Stingless bees (Trigona sp.) emerging from trumpet-shaped nest entrance below dipterocarp tree
Stingless bees (Trigona sp.) emerging from trumpet-shaped nest entrance below dipterocarp tree

The peculiar shape of this nest entrance caught my eye. Bees were moving in and out of the trumpet-shaped nest which was located below a large dipterocarp tree, at the foot of Doi Chiang Dao mountain, north of Chiang Mai, Thailand.

Stingless bees (Trigona sp.) emerging from trumpet-shaped nest entrance
Stingless bees (Trigona sp.) emerging from trumpet-shaped nest entrance

These waxy nests are constructed by stingless bees (Meliponini tribe of the family Apidae), a large group of eusocial insects – meaning they live together in colonies with a queen and have different castes – which play an important role in the pollination of crops and wild flowers in tropical countries. Thirty species of stingless bees in the genus Trigona, have been recorded in Thailand; T. collina is the most common species in the north of the country. (1)

Stingless bees (Trigona sp.) emerging from trumpet-shaped nest entrance
Stingless bees (Trigona sp.) emerging from trumpet-shaped nest entrance

As the name implies, stingless bees lack a functional sting, but they have powerful jaws and will aggressively defend their nests against intruders. Non-foraging bees near the nest entrance are there to protect the nest from a range of insects including parasites – which might try to enter. They also deposit fresh resin on the external entrance tubes, in order to deter ants, which are important predators of the bees. (2)

Stingless bees (Trigona sp.) at nest entrance
Stingless bees (Trigona sp.) at nest entrance

The nests of stingless bees are usually associated with a living tree, either in a cavity in the trunk or at the base of the tree, as in this case. The nest architecture is extremely variable between species, but the shape of the external nest entrance, as well as the internal nest features, are often characteristic of a given species. When nests come under attack, hovering bees emerge in force to defend the colony: they ‘face the nest entrance, and engage in aerial fights with non-nestmates, or directly attack larger animals, which retreat with a cloud of defending bees surrounding the head’ (2).

Stingless bees (Trigona sp.) at nest entrance
Stingless bees (Trigona sp.) at nest entrance

Based on looking at different photographs posted on the Internet, the trumpet-shaped nest opening looks like it might be that of Tetrigona binghami (Schwarz, 1937), also called Trigona apicalis variety binghami Schwarz 1937, although this species was only described for the first time in 2005, in Thailand. (1) Such an identification can only be tentative as there is no definitive key available online that I am aware of. The bee’s nest was located near the base of a huge dipterocarp tree, Dipterocarpus alatus, which was festooned with epiphytes.

Dipterocarpus alatus, Chiang Dao
Dipterocarpus alatus, Chiang Dao

Stingless bees live in colonies of somewhere between a few hundred to several thousand individuals. They usually visit many different types of flowers although some species seem to be fairly host specific. The main host plant of T. binghami is said to be teak (Tectona grandis), whereas T. collina has a number of different host plants, including the large dipterocarp resin tree, Dipterocarpus alatus. (1) These trees often have a sort of scar – a tapping hole or resin trap – in the trunk, not far off the ground, that exudes an oily resin.

Resin trap on dipterocarp tree trunk
Resin trap on dipterocarp tree trunk

The resin has a number of traditional uses, including: wood lacquering, drought-proofing of boats, water-proofing of baskets and traditional medicine.  Tapping involves cutting a hole into the trunk of the tree and using fire to stimulate a continuing flow of resin. Tapping can be sustainable, but it depends upon the skill of the tapper. (4) In sites like this one, in Chiang Dao, where these dipterocarps are the only remnants of a cleared forest, the trees will probably be more susceptible to damage and their loss as a shade would be a severe blow to the resorts and houses which exist underneath their wonderful boughs.

Stingless bees (Trigona sp.) hovering above the resin trap
Stingless bees (Trigona sp.) hovering above the resin trap

Stingless bees are called ‘channarong’ in Thai. Some species, such as T. laeviceps – which commonly occurs in suburban areas – are kept by beekeepers for their honey, which is slightly more watery and acidic than western honeybee honey (3). It also ferments. The process of keeping stingless bees is known as meliponiculture.

Stingless bees (Trigona sp.) hovering above the resin trap
Stingless bees (Trigona sp.) hovering above the resin trap
Stingless bees foraging in the resin
Stingless bees (Trigona sp.) foraging in the resin
Ants on dipterocarp tree
Ants on dipterocarp tree

Also lurking in and around the resin trap were a number of so-called resin bugs. These carnivorous assassin bugs (Family: Reduviidae; Subfamily: Harpactorinae; Tribe: Ectinoderini) coat their front legs with sticky tree resin and use this to attract and trap insect prey such as the stingless bees; a strategy called sticky trap predation. Some authors have called them living fly-paper (or bee-paper) or bee-assassins (South American genera). They really are quite strange looking insects and move very slowly.

Resin bug (Reduviidae; Harpactorinae; Ectinoderini), Chiang Dao
Resin bug (Reduviidae; Harpactorinae; Ectinoderini), Chiang Dao

There are said to be 20 species in the Ectinoderini tribe of resin bugs: ten Amulius spp.; and ten Ectinoderus spp.. The species shown here is similar in appearance to Amulius malayus but I have not been able to confidently identify it.

Resin bug (Reduviidae; Harpactorinae; Ectinoderini), Chiang Dao
Resin bug (Reduviidae; Harpactorinae; Ectinoderini), Chiang Dao

There were also one or two smaller assassin bugs, the nymphal stages of the resin bugs, which also looked to be efficient predators (below).

Resin bug nymph
Resin bug nymph

There was a very attractive spider located near the top of the resin trap.  This orb spider, Argiope pulchella, builds a web with a zig-zag stabilimentum (below). It has weaved together its web to create a much denser and thicker X-shaped cross. The spider aligns its legs against the X-shaped stabilimentum, two legs against each arm of the cross. This presumably acts to camouflage, or hide the spider whilst it is sitting on the web, and perhaps the X-shape also attract flying insects into the web. The spider moves off the cross when attending to a catch.

Argiope pulchella, builds a web with an X-shaped stabilimentum
Argiope pulchella, builds a web with an X-shaped stabilimentum

There are probably many other insects attracted to the resin trap, including moths and other sap-sucking species.  It is a fascinating little ecosystem, if that is the right word, and once again a system that is ripe with opportunities for further research.

  1. Klakasikorn, A., Wongsiri, S., Deowanish, S., & Duangphakdee, O. (2005). New record of stingless bees (Meliponini: Trigona) in Thailand. Nat Hist J Chulalongkorn Univ, 5, 1-7.
  2. Roubik, D. W. (2006). Stingless bee nesting biology. Apidologie, 37(2), 124.
  3. Chuttong, B., Chanbang, Y., & Burgett, M. (2014). Meliponiculture: Stingless Bee Beekeeping In Thailand. Bee World, 91(2), 41-45.
  4. Ankarfjard, R. (2000). Ïmpacts from tapping oleoresin from dipterocarpus alatus on trees and timber value in LAO PDR. submitted to the Journal of Economic Botany.
  5. Zhang, J., Weirauch, C., Zhang, G., & Forero, D. (2015). Molecular phylogeny of Harpactorinae and Bactrodinae uncovers complex evolution of sticky trap predation in assassin bugs (Heteroptera: Reduviidae). Cladistics.

Pompilid spider killer

Pompilid wasp, Chiang Dao, Thailand
Pompilid wasp, Chiang Dao, Thailand

I came across this magnificent spider wasp (Pompilidae) feeding on nectar from these flowers beside the steps leading up to Wat Tham Pha Plong, Chiang Dao, Thailand. I have come across this pompilid wasp before in northern Thailand (1), but I am still not sure what species it is. With its orange antennae, it looks similar to the Australian orange spider wasp (Cryptocheilus bicolor) (2), but the head is not orange and the abdomen is black. So perhaps it is another Cryptocheilus species, of which there are twenty-four known. One website provides a check-list of pompilid species from Thailand (3), but none of these seem to fit the bill. Other sites, simply caption photos ‘Pompilidae’, so it is not one that can be identified from of the Internet. If there are any pompilid experts out there, I would love to know what it is!

Pompilid wasp, Cjiand Dao.
Pompilid wasp, Chiang Dao.

Adult pompilid wasps feed on nectar, but they hunt and kill spiders to provide a food source for their off-spring. They sting and paralyse spiders and carry them off to a nest burrow, where they deposit an egg on the hapless arachnid. Each offspring has its own spider to gorge on. The wasp larva hatches out and starts feeding on the living, paralyzed spider. The bigger the spider, the more likely it is that the larvae will develop into a female wasp (which are larger than males).

Pompilid wasp, Chiang Dao.
Pompilid wasp, Chiang Dao.

I would think that there is much to learn about these wasps, particularly species which have been little studied. The are nearly all solitary wasps although a few communal, mud-nesting species exist (4). The hunting behaviour of one group of pompilids, the tarantula hawk wasps – which occur in the deserts of the USA – has been studied: “the wasp rushes at the spider, grabs a leg, flips the spider onto its back, and stings it….” The tarantulas can mount a counter attack, but it seems they are at a disadvantage and rarely succeed in killing the attacking wasp. (5) One can only wonder at how long this evolutionary battle between wasps and spiders has played out over geological time.

Pompilid wasp, Chiang Dao, Thailand.
Pompilid wasp, Chiang Dao, Thailand.

Some pompilid wasps are cleptoparasitoids; they steal the spider prey caught by other pompilid species. They wait until the wasp which has caught the spider puts it down and turns its attention to nest making; they then rush in and lay their own egg on the spider. This egg hatches out before the one laid by the wasp which first caught the spider, and the imposter larva eats the host egg before it hatches. (5) Very sneaky!

Some pompilids prey on species such as this orb spider, Argiope pulchella (6). The spider is sitting in the middle of an X-shaped stabilimentum; an elaborate web decoration or feature which it has constructed out of silk (below).

Argiope pulchella orb spider.
Argiope pulchella orb spider.

I don’t know how poisonous the sting of this particular wasp I photographed would be to humans; and I would not like to find out.

Pompilids are not aggressive and are usually relatively docile (unless provoked), but the sting of the closely related Tarantula hawk wasps is reportedly very intense. The pain has been described as: “like an electric wand that hits you, inducing an immediate, excruciating pain that simply shuts down one’s ability to do anything, except, perhaps, scream.” (7)

One has to admire the skill and tenacity of these wasps, which often prey on spiders which are much larger than themselves, and highly venomous. They have evolved a way of exploiting this prey source and presumably play an important role in regulating spider populations.

Butterflies, like this Clipper (Parthenos sylvia) also enjoy feeding on the flowers of this plant.

wp-1487720950037.jpg
Clipper (Parthenos sylvia)
  1. https://rcannon992.com/2013/11/23/a-two-tone-wasp/
  2. http://www.brisbaneinsects.com/brisbane_vespoidwasps/OrangeSpiderWasp2.htm
  3. http://insectoid.info/checklist/pompilidae/thailand/
  4. http://www.usu.edu/pompilidweb/default.htm
  5. O’Neill, K. M. (2001). Solitary wasps: behavior and natural history. Cornell University Press.
  6. https://www.flickr.com/photos/phil_arach/galleries/72157651182275337/
  7. http://www.desertusa.com/insects/tarantula-hawks.html

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?

You are so nice, I could eat you!

 Golden silk orb-weaver spider (Nephila sp.) from Sulawesi
Golden silk orb-weaver spider (Nephila sp.) from Sulawesi

I came across this huge spider on Bunaken Island, in northern Sulawesi.  The body must have been about 4 cm long and the legs spanned at least 15 cm, maybe more. It is a golden silk orb-weaving spider (genus Nephila) – also called giant wood spiders – which construct large webs to trap their prey. typically flying insects (e.g. bees, wasps and butterflies) but in some cases even small birds have been trapped and consumed!  These spiders, in the suborder Areneomorphae, have superb, pincer-like fangs; which can be seen below the head, in the upside-down position in the photo.  The tiny spiderlings can also be seen crawling over the web; they are too small to trigger a response from Mum!

 Golden silk orb-weaver spider (Nephila sp.) from Sulawesi
Golden silk orb-weaver spider (Nephila sp.) from Sulawesi

The amazing thing about these spiders is their sexual dimorphism; she is huge; he is tiny! I am not 100% sure, but I think that the male is the small red spider which can just be seen to the right of the large black female in the following photo.

 Golden silk orb-weaver spider (Nephila sp.) showing the large black female, and the tiny red male to the right.
Golden silk orb-weaver spider (Nephila sp.) showing the large black female, and the tiny red male to the right.

It is not altogether clear why the males are so much smaller, but one hypothesis is that so they can avoid been eaten by the female, who ignores such small fry!  However, the males do fight it out among themselves for access to the female and size does matter in these battles.  So it seems that there may be competing evolutionary pressures for the males to be large enough to win their fights but small enough not to get eaten (1). Curiously though, if a male does mate successfully with a female, it may be in his own interest to be consumed – now there’s commitment for you – since it can help the success of the fertilization (2) – presumably because the female is well fed!  I guess the trick is to get eaten after mating rather than before!

Oh yes, I forgot to mention that they are venomous!

1)  Mark A. Elgar and Babette F. Fahey (1996).  Sexual cannibalism, competition, and size dimorphism in the orb-weaving spider Nephila plumipes Latreille (Araneae: Araneoidea). Behavioral Ecology  7(2): 195-198.

2) Jutta M. Schneider and Mark A. Elgar  (2001). Sexual cannibalism and sperm competition in the golden orb-web spider Nephila plumipes (Araneoidea): female and male perspectives. Behavioral Ecology 12(5): 547-552.