Posted by: Rob Viens | June 12, 2012

Red-Hot With Spiders in Rio I

Darwin spent the morning today, June 12th, working “at yesterdays produce”.  He notes:

“a forest is a gold mine to a Naturalist & yesterdays a very rich one.” (June 12)

The rest of the day (and what seems like the next few days) was spent “playing games”.  I’m not sure what the reason was, but it was regatta time in Rio.  Here is what Darwin had to say about the races:

“At one oclock I went to the Admirals for a grand boat race.— The first arriving of the launches, yawls, cutters & other large boats, was an imposing sight.— It immediately made one understand how powerful a flotilla of such boats would be in war.— The racing was rather too long; especially as the Beagle did not come off quite so triumphantly as might have been wished for.— The evening passed away pleasantly, & by moonlight on the beach several foot races were got up between the officers & the crews of Captains gigs.” (June 12)

Poor FitzRoy – I’m sure he was annoyed that the Beagle did not do as well as hoped. But for today, I thought I’d go back to that “gold mine” of natural wonders – the forest.  Along with the many beetles Darwin was fascinated with in Rio (see Beetle Mania! Week!) there where several spider species.  In fact, in a letter to John Henslow (written between mid May and mid June) Darwin notes, “I am at present red-hot with Spiders, they are very interesting, & if I am not mistaken, I have already taken some new genera.”  Upon returning home, Darwin even devoted a significant section of the “Rio” chapter of Voyage of the Beagle to these eight-legged arthropods. He notes:

“The number of spiders, in proportion to other insects, is here compared with England very much larger; perhaps more so than with any other division of the articulate animals. The variety of species among the jumping spiders appears almost infinite.” (Voyage of the Beagle)

So a couple of days on spiders seems in order (starting with some intro today).

Spiders are arthropods (like their cousins the Insects). Along with the scorpions, ticks and mites, they belong to the eight-legged group of arachnids (class Arachnida).

Incidentally, Arachnid comes from the Greek word for spider, which in turn, is related to the myth of Arachne. Arachne was a woman who arrogantly challenged the Goddess Athena to a weaving contest and had the nerve to win. Athena being a Greek god (who, in general, are not known for their good tempers) got angry and turned Arachne into a spider.

The Fable of Arachne (1644–48) by Velázquez:

The Fable of Arachne

Within the Arachnids, spiders belong to the order Araneae. Below are a few common characteristics:

  1. Spiders have two major body sections – a chephalothorax and an abdomen. (Insects have three body segments.) Like all arthropods, their body is covered with an exoskeleton made of chitin.
  2. In addition to four pairs of legs, they have a pair of chelicerae on the head that contain “fangs”.  The chelicerae contain venom glands – giving spiders their age-old bad reputation in human society. (Insects have antenna on their heads, no chelicerae and three pairs of legs.)
  3. Spiders have silk glands and spinnerettes (modified appendages) in their abdomen that allow them to “spin” silk. Silk, by the way, is a protein that is both elastic and extremely strong.
  4. Spiders are predators and use a variety of methods to capture their prey.  Webs and cocoons are the most well-known methods, but there are also spiders that create “lassos”, ambush or run down their prey, or even mimic their food source. (There is at least one spider that is a known herbivore – but that is by far an exception.)
  5. Spiders eat their food in liquid form – so they often have to secrete digestive enzymes to break their food down before “slurping” it up. (Fly smoothie , anyone?)
  6. Most spiders have four pairs of eyes. (Clearly, if spiders were intelligent enough to count, they would use base 8.)

Spider anatomy (from everythingabbout.net):

Spider anatomy

One of the spiders Darwin describes in his notebook was Myrmecia:

“This singular looking spider is not uncommon in the wooded hills, amongst the foliage;— it is the “rufrum”, but the colours vary, especially the black marking.— the abdomen & posterior segments of thorax obscure; the general colour of legs & body is not “fauve” but a mixture of “Orpiment orange & Vermilion red”. Inhabits a leaf curled up; is very active in running & looks singularly like an ant.” (Zoological Notebook)

There are those spectacular colors again – orpiment (an yellowish-orange mineral) and vermilion red.  It’s been months but I still love the way Darwin uses color (for more see A Thousand Shades of Blue).

Darwin is describing an ant mimic spider (later identified as Myrmecium rufum of the Corinnidae family).  These spiders evolved to look just like there prey (ants), allowing them to get close enough to blend in with their potential food source (a classic “wolf in sheep’s clothing”). Some of these mimics even smell like ants, so they can enter the ant nest undetected and make off with some good eats. Yet another great example of evolution in action!

Ant mimic spider – species unknown (from conservationreport.com):

Ant mimic spider

Brazilian ant mimic spider (from treknature.com)

Ant mimic spider

More on spiders (on and off) over the next few days…(RJV)

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Responses

  1. […] The most familiar harvestman are daddy longlegs (I bet you thought they were spiders, didn’t you?) So what makes spiders and harvestmen different?  The easiest way to tell them apart is to look at the body.  In a spider, the two main body parts (the cephalothorax and abdomen) are connected by a narrow bridge called the pedicel.  In the harvestmen, the two segments are connected in such a way that they look like one single body structure (more like the body of a crab, for example). (For a quick review of spider anatomy, see Red-Hot With Spiders in Rio I.) […]


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