A Giant Lichen Orb Weaver in Helotes, Texas 18

— This article by Jerry Cates and Dawn S., first published in July 2007, was last revised on 21 May 2012. © Bugsinthenews Vol. 08:07.

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Giant Lichen Orbweaver (Araneus bicentenarius), Dawn S., Helotes, Texas, 07.19.07–Dorsum

Dawn Wrote:

“Jerry–This lady is out on my front gate. I live north of Helotes, Texas, off of Hwy 211 (outside of San Antonio) We took these pictures about 7pm this evening.

I have never seen one like it before and don’t seem to be able to find anything to match it on the web. I’m hoping that you can help.

Thank you, Dawn S.”

Besides the green lynx (Peucetia viridans) and a few small crab spiders, not many arachnids are bright green in color. This species, the giant lichen orbweaver (Araneus bicentenarius) is an exception.

Its common name derives from its appearing, to one who happens upon it in the wild, as a displaced chunk of bright green lichen caught, as it were, in a web.

On closer examination, the “chunk of lichen” scampers up its web to a place of safety, or drops to the ground to hide in the grass, revealing that it is, in fact, a living animal, and more specifically a spider of some repute.

The genus Araneus is comprised of  “angulate and round-shouldered orbweavers”. This refers to the conspicuous protuberances on the anterior dorsal abdomen.

Giant Lichen Orbweaver (Araneus bicentenarius), Dawn S., Helotes, Texas, 07.19.07–Dorsolateral

Those protuberances are particularly obvious in the photo shown at left.

Another important anatomical feature of this genus is the architecture of the thoracic groove. In the genus Neoscona, and in many other spider genera as well, a longitudinal groove separates the thorax, behind the head, into right and left halves. In the genus Araneus, this longitudinal groove is absent, and is replaced by a transverse groove at the base of a somewhat abbreviated cephalic groove.

In the photo at left, the transverse groove can be seen as a darkened mark at the posterior terminus of the cephalic groove that separates the pars cephalica into left and right halves.; in the above photo (at top of page), note that the thorax is smooth where the longitudinal thoracic groove would normally be found (were this a member of the genus Neoscona).

Spiders in this genus construct orb webs with open hubs, in contrast to those is the genus Argiope, whose orb webs have closed hubs that are typically decorated with conspicuous stabilimenta.

The giant lichen orbweaver (Araneus bicentenarius) is found in the United States and in Canada, and was first described by the Reverend Henry C. McCook (1837-1911) in 1888.

McCook, a renaissance man of immense intellect, served in the Union Army as a first lieutenant and chaplain with the Illinois Volunteers during the Civil War. He did so alongside many other members of his family, who were collectively known as “The Fighting McCooks”.

Giant Lichen Orbweaver (Araneus bicentenarius), Dawn S., Helotes, Texas, 07.19.07–Lateral

Later, in 1870, McCook became the minister at Tabernacle Presbyterian Church, Philadelphia, Pennsylvania. Though an active theologian the remainder of his life, in 1873 McCook began a thorough study of spiders and ants, and eventually became president of the American Entomological Society (1898-1900).

McCook’s three-volume work, “American Spiders and Their Spinning Work—A Natural History of the Orb-Weaving Spiders of the United States, with Special Regard to Their Industry and Habits”, remains a treasured treatise on spider behavior.

It is said McCook originally intended to write a natural history of all North American spiders, in one volume. As his studies continued he soon recognized the futility of that endeavor, and finally settled on a less ambitious study of orb weavers alone.

Now, on to other things:

As I am often wont to do–being never completely satisfied with the delectable offerings provided by those who so kindly send their beauteous photographs to me–I asked Dawn if she might attempt to photograph the ventral anatomy of this creature. It is always a thing of wonder to me when, as happens more often than n0t, my pointed requests for additional photos are honored. Those who come to this website to learn about the topics featured here tend to be an incredibly responsive, resourceful group, a fact for which I am very grateful.

Giant Lichen Orbweaver (Araneus bicentenarius), Dawn S., Helotes, Texas, 07.19.07–Lateroposterior

Now, it happens that, of all the photographs yet published of this species, none shows the ventral abdomen well. Inasmuch as that portion of orbweaver anatomy often (see, for example, the markings adorning the various Neosconae) shows markings, not to mention structures, of important distinction, I was anxious to have a look.

Dawn sweetly answered my request in the affirmative, and now her illuminating photographs are published below, as a part of this page.

Dawn is a superb photographer. She, her husband, and daughter, are equipped with, it turns out, several excellent cameras. Of course, her subject, the Giant Lichen Orbweaver, was also quite photogenic. We should expect to see this somewhat rare species in greater numbers than usual this year, owing to all the rainfall we’ve had.

Some specimens of this species have been reported with abdomens measuring an inch or more in length, rivaling the size of the yellow garden spider (Argiope aurantia). High rainfall levels have set the stage for large numbers of flying insects, like crickets and grasshoppers, to emerge during the summer and early fall months. They will then feed these spiders in such bounty that they will likely become quite large. If so, they will be more obvious than before and perhaps others will send me photos of them as well (it turned out, however, that this was the only one of this species whose photographs were sent in to me in 2007).

Giant Lichen Orbweaver (Araneus bicentenarius), Dawn S., Helotes, Texas, 07.19.07–Ventrum

Dawn Wrote again:

“Jerry–I’m sorry that it took me so long to get back to you. I apparently sent the original e-mail via my husband’s computer and he doesn’t check his email as frequently as I do mine.

We received over 4 inches of rain yesterday and I quite honestly didn’t think that the spider would still be there today. But luckily we found her nestled in the leaves. I think we got a couple of shots of her underside but she was pretty curled up.

Actually my husband, daughter and I were all taking shots of her the first night with 3 different cameras. We used a Nikon D70, an Olympus C-750 and the one that actually got the best shots was the Kodak Z612.

My husband used the macro setting and forced the flash and at times we used a flashlight for lighting as well. Thanks–Dawn”

Editor’s Notes: Did you notice how much work Dawn and her family went to in order to take these photos? I hope you are as grateful to them as I am. We owe them a debt of gratitude.

Here we can see the ventral body, and the underside of this spider is quite interesting. It sports a marking in the center of its ventral abdomen that looks like a yellowish “T” flanked by whitish splotches.

Giant Lichen Orbweaver (Araneus bicentenarius), Dawn S., Helotes, Texas, 07.19.07–Ventrum 2

Note that this marking bears no resemblance whatever to the markings on the ventral abdomens of the spiders we’ve seen from the genus Neoscona. In the latter spiders, the central mark is dark, and is surrounded by pale yellow or white markings, while here the central mark is reversed, having itself a pale yellow and white coloration, and being  surrounded by a dark border. The genus Eriophora, by comparison, has a central mark that is a dark red or maroon, and is surrounded by a pale border.

The purpose for this marking (there doesn’t have to be a purpose, but methinks there should be some practical value to it, inasmuch as a modicum of genetic investment is required to retain its shape and coloration) is open to conjecture, but one likely candidate seems particularly obvious.

Enlarging the photo at left, to reveal additional details of the ventral abdomen, shows that the upper margin of the yellowish “T” lies just posterior to the epigynum (the darker semicircular structure just above the “T”). This is precisely where the male inserts his semen-laden pedipalp during mating.

The male orbweaver must maneuver adroitly, as a single misstep places him at risk of being swathed in silk, killed, and consumed. Such does not forgive the slightest error, either in timing or, so to speak, targeting. Perchance the ventral marking assists the male in visually locating the female’s epigynum, so that a pedipalp may be flawlessly inserted and its semen deposited, whereupon the male may make a safe exit without inciting the ire of his mate.

Giant Lichen Orbweaver (Araneus bicentenarius), Dawn S., Helotes, Texas, 07.19.07–Epigynum

On the other hand, one wonders how he might visualize this marking while in the act of mating. Could it be that the marking is observed prior to mating, while the spiders are some distance apart? And that the male uses the visualization, thus received, first to stimulate his mating instincts, and then to orient himself, in his mind, to the location of the epigynum relative to other markings on the female’s lateral body? Ah, were this so, we would have to apportion more intellect to the male than most arachnologists are wont to do. It would make for an interesting study, though, don’t you think?

It is worthy of note that the structures on each side of the epigynum, which appear as lateral slits in the abdominal skin, are pulmonary spiracles that serve as respiratory orifices. Air drawn through these slits passes over a series of minute lamellae that resemble the pages in a book. Thus they form what is termed “book lungs”. Book lungs are internal structures, and are not visible without dissecting the abdomen. However, their location can be inferred by the wrinkled, slightly darker (and in this specimen, reddish colored) region just anterior to the pulmonary spiracles.

The exoskeleton in this region actually participates in the structure of the book lungs, inasmuch as the latter are invaginations of the former, and elaborated structurally so that they form the scaffolding for the lamellae through which the gaseous exchange of respiration occurs.

Also worthy of note is the structure of the complex of spinnerets posterior to the marking we have been discussing.

Giant Lichen Orbweaver (Araneus bicentenarius), Dawn S., Helotes, Texas, 07.19.07–Spinnerets

This complex consists of three pairs of spinnerets, an arrangement common to all spiders (except for certain Mesothelae, which have four pairs, and certain Orthognatha which have only two or, in a few species, merely one pair). The two anterior spinnerets are visible in the foreground of this photo, and the two posterior spinnerets are visible just beyond them.

A median pair of relatively diminutive spinnerets, positioned between the posterior pair, is hidden from view (though in this photo the fan of silken fibers may be emanating from their spigots). Each spinneret is attached to an even more elaborate system of abdominal silk glands by ducts that end in multiple spigots on each spinneret. As many as six different kinds of silk glands are present in the spider’s abdomen.

The terminal regions of each spinneret are studded with spigots, each attached to a particular kind of silk gland, and each is capable of producing a separate silk line on command (the liquid silk solidifies on compression within the spigot, dependent strictly on internal tensioning). The musculature affiliated with each spinneret allows the spider considerable positional latitude as it spins multiple silk lines from each spinneret. As might be imagined, this latitude attains amazing complexity, rife with incredible flexibility for the spider.

For a detailed discussion of the anatomy of the spinning structures of the spider, see Rainer F. Foelix’s book 1996 book “Biology of Spiders”, pg. 110-122 (now in its 3rd Edition, published in 2011).

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Dawn, her husband, and her daughter, are ALL superb photographers, and are equipped with excellent, but quite different, digital cameras. Dawn mentioned that the best shots were taken with a Kodak Z612, a 6.1 megapixel camera with a 12:1 optical zoom, priced today (22 July 2007) below $300.00. The Nikon D70, another 6.1 megapixel digital camera, but with SLR capabilities and a multitude of optional lens, runs under $1,000, but significantly more than $300 (a refurbished, used model, can be had on the Internet for just under $700). The Olympus C-750 is a 4 megapixel digital camera with a 10x zoom, and a price tag around $430.00.

This isn’t the place to review digital cameras (though maybe it should be), but I am not surprised that the least expensive camera of the lot took the best photos. One reason the Kodak Z612 bested the Nikon D70 might have been the added complexity of the latter over the former. I often use a Canon EOS 10D SLR, with 6.3 megapixels, to photograph insects, spiders, and wildflowers in macro, and am sometimes disappointed in the results. That camera and its retinue of specialized lenses cost, not very long ago, in the neighborhood of $6,000, but it is regularly bested by cameras valued at less than $100. My old Sony Mavica FD91, for example, with only 0.8 megapixels, always produces excellent macros, though I have to carry a bunch of floppies to the field in order to make good use of it…