Posted by: Rob Viens | June 25, 2012

Who’s Your Daddy (Longlegs)?

After spending almost a quarter of the year in Rio, Darwin started to get a little sentimental about his last few days in Brazil.  It turns out he would not leave for another 10 days, but expecting an earlier departure he wrote:

“In the evening took a farewell stroll to the Lagoa, & saw for the last time its waters stained purple by the last rays of twilight.” (June 25)

An uncommonly “non-specific” use of the word “purple” there, Charles.  Don’t we get something from Syme’s book?  Maybe a “plum purple” or an “imperial purple” or even a “bluish lilac purple”?

Anyway, I thought I would write about a short entry in Darwin’s Zoological Notebooks today and say a few things about an arachnid that most people assume is a spider – the harvestman.  Darwin simply writes:

“And lastly under rotting wood Phalangium is abundant: & still more the sub-genus Gonoleptes.— I found one strange species, at superior base of hinder legs was a claw, & also corresponding ones on the hips, which together formed a pair of posterior pincers with which the insect seized any object.” (Zoological Notebook)

The taxonomic names that Darwin uses are no longer in use, but what he is referring to here is the order Opiliones – the harvestmen.  I’m sure it is much more innocent, but these always sound to me like “reapers”, as in “grim” kind.  But in reality, these arachnids are generally a lot less predatory than their cousins the spiders, mites and scorpions, and completely harmless to humans.

Harvestman species – Phalangium opilio (from Wikipedia Commons)

Phalangium opilio

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.)

There are other differences, too, that are less obvious to the naked eye.  Harvestmen do not have silk or venom glands (so they don’t spin webs).  And they only have a single pair of eyes. Many, like daddy longlegs, have long legs compared to their body size (though this is not always the case) and the second pair of legs actually contains sensory organs used to explore their environment.

An exotic harvestman from Brazil (from Troy Bartlett’s NatureCloseups.com – a site with beautiful arthropod photography)

harvestman species Brazil

When it comes to eating, harvestmen are not restricted to slurping up their food like their spiders cousins.  They can eat solids, too. And they tend to eat a lot of different things. Many are omnivores (eating insects, plants and fungi), some are scavengers, and others are detrivores (eating animal waste and decaying material). Since they don’t inject venom (as spiders do), the chelicerae have adapted to form little “claws” for grasping food. Here is a picture of these little “hands” at 200x magnification (from Wikipedia Commons):

harvestman chelicerae

And because I can’t resist the “geologic context”, it is also worth noting that harvestmen are an ancient order of living things (adding to their “reaper-like” sense of mystery).  Not only are the earliest fossils of harvestmen over 400 million years old (!) but fossils from the Jurassic (about 165  million years ago) have been found that are virtually identical to modern species. (Which means they survived at least three of Earth’s major mass extinctions. Not a surprise as there was probably a lot for the scavengers and detrivores to eat.) Hey if there is one thing evolution tells us – when a body plan works, there is no reason to change the design!

305 million-year-old harvestmen fossil scan- click the image to see more info and a rotating 3D visual of this fossil (from Imperial College London);

harvestman fossil

Speaking about ancient harvestmen fossils – one last bit of trivia.  The 400 million year fossil harvestmen that have been found in chert deposits in Scotland have the distinction of having the first penis in the fossil record.  There is also evidence to suggest they lived on land – making them one of the earliest animal colonizers of the continents.  These two “firsts” make sense, as a penis would have been an adaptation to life on land.  Hey, in evolution, everything has got to start somewhere… (For more details, see the report in National Geographic News.)

(RJV)

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Responses

  1. My brain is exploding… not only are daddy long-legs NOT spiders, they have graspers (all) and penises (males) and don’t spin webs? How could I have missed all that?

    • I know – crazy, isn’t it?
      FYI – Some people use the term “daddy longlegs” for similar looking species. There are some spiders (I have some in my cellar) that have long legs and look similar. But if you look closely you see that they have two distinct body segments (and are usually found in a web) – those are spiders. (I think I have heard them called “cellar spiders”.)


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