Posted by: Rob Viens | June 14, 2012

Giant Spider Encounters

June 14th – Another day of collecting and socializing.   It is amazing how Darwin manages to attend so many social events in a place where he has only been living for 2 months. Darwin’s diary entry today is about the evening obligations (for more on Mr. Aston see Who’s Who Guide to 19th Century European Diplomats):

“Dined with Mr Aston; a very merry pleasant party; in the evening went with Mr Scott (the Attache) to hear a celebrated pianoforte player.— He said Mozarts overtures were too easy. I suppose in the same proportion as the music which he played was too hard for me to enjoy.” (June 14)

I’ve read it over a few times, and I think Darwin is dishing out a good ole zinger in that last sentence. That is exactly the kind of personal sarcasm that does not make it to the official record in Voyage.

Speaking of stinging remarks….back to some more arachnids today…

In doing a little looking online I discovered that the largest fossil spider ever discovered was revealed just last year in a paper in the journal Biology Letters. Here is an image from the paper:

Orb Weaving Spider

It’s impressive, no doubt, but before you start thinking of this…

Tarantula Movie

…let me point out that it’s body was only about 2 inches from end to end and it’s leg span was in the 6-7 inches range. Yeah, it’s a cool fossil, but a little bit of a let down when it comes to prehistoric monsters. The details of the fossil are pretty cool.  Below is a closeup of one of the legs, where you can clearly see the hair.

Orb Weaving Spider

Not bad preservation for a Jurassic (about 165 million year old) fossil!

The fossil has been classified as Nephila jurassica – a golden orb-weaver spider. Modern day golden orb-weavers can actually grow to over 4 inches and have been known to catch bats and birds in their large, often complex, webs (up to 6 feet in diameter). It is the golden color of their silk that gives them their name.

Modern Nephila clavipes (a golden orb-weaving spider species found in Brazil)

Orb Weaving Spider

Darwin encountered several golden orb-weaver spiders in Rio (later identified as Nephila clavipes) and here is what he had to say about them (note that Epeira is an old name for the orb-weavers):

“Amongst the next division Orbiteles.— Epeira is most singularly numerous & interesting: it is a large & numerous family not a genus.— The paths in the forest are barricaded with the strong yellow web. Also others of same division & of ??? are exceedingly abundant.— Number construct their webs over the water: especially one with a red coniceous covering to abdomen.— Many belonging to this latter section are singular by strange form & colour.— The species of Epeira with the tibiæ of 2nd pair of legs enlarged & spinose.— There is no end to the singularity & numbers of this genus.” (Zoological Notebook)

It is actually the female of the golden orb-weavers that grows so large.  These spiders exhibit extreme sexual dimorphism (meaning the male and female have distinctly different sizes). The male is much smaller (as seen in this image from the BBC):

Orb Weaving Spider

Interestingly, in Voyage Darwin writes about “small parasitic spiders” that share the web with the golden orb-weaver.  I suspect these are the males that he was observing!

“Every path in the forest is barricaded with the strong yellow web of a species, belonging to the same division with the Epeira clavipes of Fabricius, which was formerly said by Sloane to make, in the West Indies, webs so strong as to catch birds. A small and pretty kind of spider, with very long fore-legs, and which appears to belong to an undescribed genus, lives as a parasite on almost every one of these webs. I suppose it is too insignificant to be noticed by the great Epeira, and is therefore allowed to prey on the minute insects, which, adhering to the lines, would, otherwise be wasted. When frightened, this little spider either feigns death by extending its front legs, or suddenly drops from the web.” (Voyage of the Beagle)

There are actually now three families of orb-weaving spiders (named after their circular webs).  Golden orb-weavers belong to the Nephilidae family.  The other two are the “typical” orb-weavers (family Araneidae) and the long-jawed orb-weavers (family Tetragnathidae). Altogether they compromise about 25% (about 10,000) of all identified spider species.

As an example of Darwin’s wonderful observation technique and skills, here is the remainder of his description in Voyage regarding orb-weavers.   The footnotes suggest that he is referring to an Argiope sp. of the family Araneidae:

“A large Epeira of the same division with Epeira tubereulata and conica is extremely common, especially in dry situations. Its web, which is generally placed among the great leaves of the common agave, is sometimes strengthened near the centre by a pair or even four zigzag ribbons, which connect two adjoining rays. When any large insect, as a grasshopper or wasp, is caught, the spider, by a dexterous movement, makes it revolve very rapidly, and at the same time emitting a band of threads from its spinners, soon envelops its prey in a case like the cocoon of a silkworm. The spider now examines the powerless victim, and gives the fatal bite on the hinder part of its thorax; then retreating, patiently waits till the poison has taken effect. The virulence of this poison may be judged of from the fact that in half a minute I opened the mesh, and found a large wasp quite lifeless. This Epeira always stands with its head downwards near the centre of the web. When disturbed, it acts differently according to circumstances: if there is a thicket below, it suddenly falls down; and I have distinctly seen the thread from the spinners lengthened by the animal while yet stationary, as preparatory to its fall. If the ground is clear beneath, the Epeira seldom falls, but moves quickly through a central passage from one to the other side. When still further disturbed, it practises a most curious manœuvre: standing in the middle, it violently jerks the web, which is attached to elastic twigs, till at last the whole acquires such a rapid vibratory movement, that even the outline of the spider’s body becomes indistinct.” (Voyage of the Beagle)

Argiope argentata in Brazil (from Wikipedia Commons):

Orb Weaving Spider

(RJV)

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