A web furnished for concealment, Cyclosa conica

Yesterday’s forest walk, along an alarmingly narrow dirt road next to a hundred-foot drop, introduced us to Cyclosa conica

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a spider with an unusual tactic for concealment. On its web it makes a vertical strip of reinforced filaments, called a stabilimentum, to which it adds the husks of its prey. Females often place their egg sacs in the stabilimentum too. Then the spider hides itself at the center of this little visual interference area it has made, while it waits for insects to fly into the web.

The vertical strip of insect remains is clear in the photo above, and here’s a closer look at the arachnid itself.

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The stabilimentum is used in various forms by other orb-weaver spiders (family Araneidae, the builders of spiral wheel-shaped webs).

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Above, the “Writing or Signature Spider”, Argiope sp., photo taken in Singapore. Source.

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Above, stabilimenta of Argiope sp. take different shapes including circular and cross-shaped. Photo from Wikipedia.

What are the functions of the stabilimentum?

Various theories have been propounded as to the effect of the stabilimentum: strengthening the web, preserving the web by causing birds to avoid it, even attracting insects (although it would be natural to think that the more solid-looking stabilimentum might make the webs easier for insects to avoid). The spider we saw makes it into a “decorated” hiding place, but that is most likely an embellishment by this one species upon a structure originally serving other purposes.

In 1998 I-Min Tso, now a professor at Tunghai University in Taiwan, did a field study with Cyclosa conica (the spider we photographed) to find out whether “Stabilimentum-Decorated Webs Spun by Cyclosa Conica (Araneae, Araneidae) Trapped more Insects than Undecorated Webs”. He was able to make the comparison because where he worked (near Pellston, MI), the spiders sometimes omitted the stabilimentum (and 18 out of 24 webs with stabilimenta had no prey included in the “decoration”). This seems odd, as the stabilimentum with prey is cited as a characteristic of the genus Cyclosa, but maybe other observers have failed to notice instances of C. conica webs that lack the stabilimenta, or lack the wrapped prey within them. At any rate, Dr. Tso found that webs with stabilimenta caught more prey than plain webs even when the plain ones were larger. Similar results have been found for other species that add stabilimenta to their webs.

How might this work? At least one species, Argiope argentata (one of the Argiope spp. known as the “Writing Spider”), is said to spin special UV-reflecting silk for the stabilimentum. Theoretically this makes it more visible to insects, like the UV patterns on flowers, which tend to be “bull’s-eyes” surrounding the center where pollination takes place. In a laboratory where the light could be manipulated to contain UV radiation or not, fewer fruit flies flew to webs when the UV light was not present (webs were those of juvenile Argiope versicolor).

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Seen in UV, these flowers have a wide black “target”. Photo by Tom Blegalski/TTBphoto, from geneticarchaeology.com.

Under that theory, insects would fly toward the attractive UV center of the web (the stabilimentum) and not see the less-visible “this is a spider web!” part until too late. The theory fits the Writing spider, which prefers open sunny areas, better than our C. conica, which lives in sun-and-shade forests.

But the theory may be too good to be true, given that we don’t actually know enough about insect vision and behavior, and there is even disagreement regarding how UV-reflective spider silk is. In the real world, light conditions vary from place to place and moment to moment, even as a breeze changes the orientation of a web slightly, making it difficult to assign easy labels like more visible/less visible. And the visual systems of insects vary, with many being (I venture to say) unknown. The Australian spider Argiope aetherea was found to adjust “the quantity of silk decoration… adding more silk decoration when the web was located in dim light rather than bright light.” The authors of this study cite their findings as evidence that is “[c}onsistent with the prey-attracting function”, but of course it would also be consistent with any other function that involved visual perception even without UV involvement, e.g. signaling birds to avoid the web.

As a non-scientist, I’ve probably taken this topic far enough; the visibility and function of web decorations have been argued over for a hundred years, and modern technology seems merely to guarantee that each investigator with a spectrophotometer reaches a different conclusion from the others. One article (1), in 2005, summarizes areas of difference and ambiguity, ending with a possible redirection of emphasis: “The contrast of web decorations is consistent between families and different decoration patterns, raising the exciting possibility that their shape rather than spectral properties might explain variation in receiver response.” But there’s a review of the evidence in a long article not available online (2, abstract only), and now that my curiosity is up, I’m seeking a reprint of it.

1. Bruce, Matthew J, and Astrid M Heiling and Marie E Herberstein. 2005. Spider signals: are web decorations visible to birds and bees? Biology Letters 22 September 2005. 1 (3): 299-302.

2. Herberstein, M. E. , C. L. Craig, J. A. Coddington and M. A. Elgar. 2000. The functional significance of silk decorations of orb-web spiders: a critical review of the empirical evidence. Biological Reviews of the Cambridge Philosophical Society. 75 : 649-669. [abstract]

More photos and information about Cyclosa conica

eurospiders – good photos including extreme closeups of body parts

Range map

Cyclosa is derived from the Greek “kyclos” meaning “round” possibly with reference to its orb-web. Conica refers to the conical shape of the abdomen.

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Form and function: a columbine flower

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Like all columbines, the Western red columbine (Aquilegia formosa) above has a five-petalled flower with unusual “spurs” or tubules on the top. Each spur is formed by one of the five petals, curling into a cylinder as it rises.

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The lower ends of the petals join into a circle, within which are the yellow, pollen-bearing, stamens which extend beyond the petals. [Diagram below from USFS.]

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The sepals (that wrap the immature flower) are not green as in most flowers but red, and extend out at right angles when the flower opens.

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Western red columbine bud.

Flower design is all about getting pollen from the stamens to the pistils (female organs); form definitely follows function.What, then, has led to the development of these seemingly superfluous spurs? One clue is that they are of widely varying lengths. North American columbines range in spur length from from 7.5 to 123 mm (0.35 to 4.8 in.). And, because the first columbine—bearing a flower with short spurs— reached North America via the Bering Strait land bridge, between 10,000 and 40,000 years ago, all this change has taken place in a relatively short time, indicating some big payoff for the plant, in terms of survival or reproduction.

The columbine has both male and female parts in each flower, allowing for self-pollination, but that would not introduce any genetic variation. So the flower of the columbine is an elaborate package which has evolved to get effective pollination from its principal pollinators: bees, hummingbirds, and hawkmoths. And the spurs are an integral part of the process…

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because the knob at the end of each spur contains nectar, and that is a big attraction to pollinators.

The flowers and pollinators have conflicting interests: the visitor just wants the nectar, as much as it can drink, while the flower wants to dole out the nectar bit by bit in order to keep attracting more insects (or other pollinators—bats, birds). One method is by placing the nectar at the end of a passage just barely long enough for the tongue of the pollinators. They can sip but not slurp, and while forcing their way in they make good contact with the pistils and stamens to pick up and deposit pollen.

Darwin was intrigued by an extreme example, a Malagasy orchid which puts its nectar at the end of a 30 cm (11.8 in.) tube, and he hypothesized that flowers and their pollinators evolved together gradually in this regard. The flowers raised the bar, so to speak, a little at a time by lengthening the reach for the nectar, and the pollinating insects gradually evolved longer and longer tongues. In columbines, there are some species with short spurs accessible to bees, others with longer spurs that are mainly pollinated by hummingbirds, and some with even longer spurs for the long-tongued hawkmoth.

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Bee approaching flower, with tongue out. The long tan objects are pollen-bearing anthers, on the ends of the stamens. Photo.

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Hummingbird tongue. Photo.

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A giant hawk moth (Eumorpha typhon) adult with its tongue (proboscis) extended. Image by Alfred University artist Joseph Scheer.

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Photo by Gary Monroe.

Above is the flower of Aquilegia longissima (the longspur columbine of the SW US), which has the longest spurs of any species of columbine. Compare to those on the previous photos of our Western red columbine. The Western red is pollinated primarily by hummingbirds, though it attracts other insects too including bees and butterflies.

In a complex genetic study of North American columbine species published in 2007 (1), Whittall and Hodges found evidence that the ancestral short-spurred columbines had been bee-pollinated, but as they moved south and encountered first hummingbirds, then hawkmoths, had undergone two relatively quick transitions of lengthening spurs to adapt to these new pollinators.When long-tongued pollinators get nectar from a short-spurred flower, they will not need to shoulder their way in, and so won’t contact the stamens and pistils as much. They won’t pick up, or deposit, as much pollen.

And this led to development of different species of columbine. Once flowers in a certain area have gotten longer spurs, so that they mostly depend on a new longer-tongued pollinator, flowers that attract that particular organism better will be more successfully pollinated and produce more seeds. This may mean a change in color, flower orientation (facing up or down), or changes in form. Hodges subsequently studied color preference in the pollinators of columbines:

”What is important in this research is that hawkmoths mostly visit— and pollinate — white or pale flowers,” said senior author Scott A. Hodges, professor of ecology, evolution and marine biology at UCSB. “We have shown experimentally that hawkmoths prefer these paler colors.” When a plant population shifts from being predominantly hummingbird-pollinated where flowers are red, to hawkmoth-pollinated, natural selection works to change the flower color to white or yellow, he explained. [full original article here(2)]

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Photo by SA Hodges, MA Hodges, D Inouye.

This can even be seen in varieties of the same species, as in the case of Aquilegia coerulea, the Colorado blue columbine. Each of the three below is, according to the USDA, the same variety: A. coerulea James.

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Photo from USDA.

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Photo from USDA.

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Photo from USDA.

Here’s the process by which the flowers get lighter in color, as the latitude changes.

Aquilegia coerulea (Colorado blue columbine) ranges in color from dark blue to pale blue to white. Aquilegia coerulea is the southern most (northern New Mexico) occurring dark blue flowered columbine and it occurs at high elevations where colder temperatures generally preclude hawk moths. As Aquilegia coerulea expanded out of the Rocky Mountains into lower elevations and warmer temperatures, the species developed into white or very pale blue varieties. This change to a lighter coloration co-evolved, as hawk moths were available as an alternative pollinator to bees and bumblebees. It is also interesting to note that the spurs on the dark blue Aquilegia coerulea are short, similar to the other dark blue, high elevation columbines whereas the spurs on the pale blue to white Aquilegia coerulea are longer. (USFS)

In this theoretically orderly process whereby bees are excluded from the nectar supplies of long-spurred flowers, it happens that the bees sometimes choose to solve the problem in a “cutting the Gordian knot” fashion, by making holes in the spurs to drink the nectar directly. Unlike a bee who blunders around in the flower and departs with no nectar, the spur-cutting bee contributes nothing to pollination. But bees require both nectar and pollen, and the columbine’s pistils and stamens are easy to reach; so the bee who stops for a quick cheating drink may look in for pollen another time, thus fulfilling the needs of the flower as well as her own. (In honeybees the workers are all females.)

So it seems that Darwin’s idea of a gradual process, with increases in spur length being answered by longer tongues on the pollinator species, is not correct for columbines at least. Based on genetic data, Whittall and Hodges hypothesize a start-and-stop process: the columbines moved into new areas, with new longer-tongued pollinators (e.g. hummingbirds) which could raid the nectar without touching the pollen, and so flowers with longer nectar spurs became more likely to be pollinated and set seed. Instead of the flowers leading the dance by lengthening the spurs, it was the presence of different pollinators that forced change.

But Darwin was proved right in his prediction that an insect would turn up, capable of pollinating the incredibly long-tubed orchid. It’s a hawkmoth called Xanthopan morgani, or Morgan’s Sphinx, and here is its picture.

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Photo from Wikipedia.

More images of these remarkable moths: night-time photo of Arizona moth feeding on Jimsonweed, showing tongue curled up in the air; pictures of the rare British hummingbird hawk-moth, which can hover: A, B; and photos of the African convolvulus hawkmoth.

References

(1) 2007: Whittall Justen B; Hodges Scott A. Pollinator shifts drive increasingly long nectar spurs in columbine flowers. Nature 2007;447(7145):706-9.

(2) 2009: Hodges, Scott A.; Derieg, Nathan J. Adaptive radiations: From field to genomic studies. Proceedings of the National Academy of Sciences June 16, 2009; 106 (suppl. 1): 9947–9954.

Attack of the mourning cloak butterfly larvae

That title sounds contradictory, doesn’t it? Butterflies are beautiful, innocuous, always to be protected. If only the world were as Walt Disney told us it was! [NOTE: I’ve learned from readers of this post that this caterpillar has a toxic substance in its hairs or spines that can cause a very painful reaction if you touch it, so be careful—indeed of any hairy or spiny caterpillar. See below,  https://nosleepingdogs.wordpress.com/2010/06/01/attack-of-the-mourning-cloak-butterfly-larvae/#comment-40639  and https://nosleepingdogs.wordpress.com/2010/06/01/attack-of-the-mourning-cloak-butterfly-larvae/#comment-40872 ]

The first title of this post was “Attack of the tent caterpillars”, because of what I saw. First the caterpillars,

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then their “tent”. The black balls visible are probably frass (caterpillar excrement). A few caterpillars are under the tent; some species retire periodically to their tent for protection from the elements and birds.

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The closer I looked the uglier they were to me.

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They were chowing down on the leaves of our little grove of aspens, planted a few years ago and much cherished.

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Birds, including 6 pairs of nesting tree swallows (Tachycineta bicolor), usually keep insect pests under control around our house. But nobody showed any interest in this concentration of food on the aspens; too spiky, or maybe bad-tasting. Caterpillars eat so much so fast, they can defoliate trees. I went looking for something to spray them with and found we had no insect spray. Finally I used 409 cleaning spray, it certainly smells toxic. The next day most of the caterpillars were still alive and eating. Finally a better idea occurred: cut off the branches they were on and bag them up. Since the infestation had spread to just 3 branches, I was able to do that.

It was only afterwards that I succeeded in identifying the caterpillars. I had looked at all the so-called “tent caterpillars”, and others, without finding anything that matched. Then there they were: they would have grown up to be mourning cloak butterflies (Nymphalis antiopa).

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Photo from milesizz on flickr.

You can imagine how bad I felt. I’ve since thought that maybe I could have cut the branches and then lodged them in among the branches of some other tree. Or kept some and fed them until they pupated. The favored food trees for the larvae are elm, willow, hackberry, and trees of the genus Populus: cottonwood, poplar, birch, and, yes, aspen. Except for occasional cottonwoods and shrubby willow along the river, none of these are native around here. But we do see the occasional mourning cloak, one of which must have laid the eggs earlier this spring—this species overwinters as adults, emerges to mate and lay eggs in spring, then after 10 days or so the caterpillars hatch out, eat, pupate and emerge as butterflies before fall. Given how much caterpillars eat, harvesting enough willow from the riverbanks to keep them fed doesn’t sound practical, at least not for very many individuals. But if there is a next time I think I will try it.

Here are a few closeups of the caterpillars. Identification was hard, maybe because they go through 5 “instars” or stages, shedding their skins each time and so perhaps different instars look a bit different. Some of the photos of this species showed much hairier-looking caterpillars, whereas the ones here were extremely spiny but with few hairs.

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Note the red dots on the back, and the red legs (arrows).

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The eggs would have looked like this.

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Photo from Canadian Biodiversity Information Facility. For an excellent series of photos showing a female laying eggs, changes in the eggs as they get close to hatching, and the tiny new caterpillars, see this backyardnature.com page by Bea Laporte.

And each spiky black voracious caterpillar, after eating its fill of the tender leaves of our aspens, would have toddled off to some sheltered place to pupate, making a chrysalis like this.

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Photo from bugwood.org.

Since mourning cloak adults overwinter, they are one of the earliest butterflies to appear, and regarded as a sign of spring. The “mourning cloak” refers to their dominant wing color, dark rusty red bordered with black—though it’s lightened with blue jewels and cream-colored edges.

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I’ll close this tale of butterflies-never-to-be, with a melancholy ballad in which the mourning cloak appears, perhaps in the role of one of the Greek Furies, haunting one who has done wrong. Usually such messengers of vengeance and doom have unpleasant appearances, as did the Furies, but to the guilty heart a bright butterfly might be even more menacing than a dark spiky caterpillar.

The Mourning Cloak
(Karah Stokes/Spruce and Maple Music 1)

One fair morning late in June
The sun shone on the daisies white
When a messenger of sorrow deep
Came into my garden bright

Wings of deepest velvet black
Bound with gold and sapphires rare
A butterfly, a Mourning Cloak,
Like one a wealthy widow’d wear

He promised me a golden ring
But he gave it to a rich man’s child
He craved the ease wealth would bring
Above a love both true and wild

So I called him to our trysting place
“Since there’s no help, let’s kiss and part”
He took me in a sweet embrace
And he felt a penknife in his heart

He looked at me with fading eyes
I left him there as he left me
The dawn next morning brought the news
That he’d been set upon by thieves

Oh, butterfly, why do you haunt?
Know you the secret in my breast?
I pierced his heart as he pierced mine
I slew the one I loved the best

One fair morning late in June
The sun shone on the daisies white
When a messenger of sorrow deep
Came into my garden bright

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Paper wasps and their nest

I found a group of paper wasps working on a nest, on top of our porch swing.

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Behind the active nest is another larger one, apparently abandoned––or maybe the young have already emerged from it.

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A few days later both nests had been knocked down by some creature that probably ate the wasps and any eggs or pupae; nothing left but one dead wasp.

In North America there are 22 species of paper wasps, genus Polistes, according to Wikipedia. [More about paper wasps, including their life cycle: 1, 2, and Bugguide has photos of about 18 different species from N. America.] They are quite common around our place, and generally ignore us if we do the same. I’ve gotten stung twice this summer though: once when removing a nest made in the recess of the car door hinges; and once when I was replacing a hummingbird feeder without noticing the wasp clinging to the bottom––I touched it and was stung. (Paper wasps feed on nectar, so the hummingbird food attracts them; they also prey on caterpillars and other “garden pests” so they’re generally considered “beneficial insects” in our narrow human way of thinking.) I caused both of these incidents, so I have no gripe against the wasps, just a resolve to be more careful. As you can see, these wasps let me get quite close with the camera.

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Don’t expect such tolerance from some other insects that look very similar. Hornets and yellowjackets are irascible and can sting more than once. Stings from any, including the paper wasps, can cause severe reactions (anaphylactic shock) in allergic individuals.

A few wasp-related byways

More good pictures of paper wasps, taken by a backyard naturalist in Michigan, are here. The common wasp builds quite large nests, also of paper, but they are spherical and the cells are not visible as they are in paper wasp nests.

And here’s something I enjoyed discovering: a bird, Pernis apivorus, which may have wasp repellent. It’s called “Honey Buzzard”, but it is not a buzzard and feeds more on wasps (adults and pupae) than on bees. It’s believed to have some chemical on its feathers that dissuade wasps from stinging!

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[Painting by John Gould, English ornithologist and artist]

This beauty winters in Africa and summers in Europe and Asia, so we won’t be seeing it around our house. It has a very unusual display in flight: “The most striking version of their soaring displays involves a characteristic wing quivering which looks as if the bird is clapping its wings together above its head.”

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[Photo of a wasp-eating Honey Buzzard in Sweden, by Omar Brännström]