Biodiversity & Evolution

Arachnid

The truth about daddy longlegs

The arachnid order Opiliones has several common names in English including daddy longlegs, grand daddy longlegs and harvestmen.  I have many wonderful things to share with you regarding the natural history of Opiliones so I’m going to post it in a series of about five posts. Here is the first fun fact to whet your appetite.  Keep an eye out for the others very soon!

Longlegs fact #1: Daddy longlegs are NOT spiders.

They are also not insects.  So what are they?  Well, I’m glad you asked.  Daddy longlegs are arachnids belonging to the order Opiliones.  The subclass Arachnida includes spiders, scorpions, mites/ticks, daddy longlegs, and several smaller groups, all of which belong to different orders.  So daddy longlegs are distantly related to spiders, but are actually more closely related to scorpions!

These incredible arachnids are easily overlooked, as they are most active at night and many species are very secretive in their behavior.  Here’s a few examples of species from Costa Rica:

Prionostemma sp. feasting upon what remains of an insect

Cynorta marginalis eating an earthworm as a second individual sneaks up.

Poecilaemula signata (male) with enlarged chelicerae

The orange harvestman, an undescribed species

For more arachnid photos see my Arachnid gallery.


Ogre-faced spiders

This is a fascinating group of spiders, and there’s so much I could tell you about, but I’ll keep it short!

The net-casting spiders belong to a small family (~60 species in 4 genera) of cribellate spiders named Deinopidae – from Greek deinos + opsis, meaning “terrible appearance”.  They are distributed worldwide with the majority of species being found in tropical and subtropical regions.  Spiders of this family are also often called ogre-faced spiders – I suppose because an ogre’s face also has a fearful appearance.  But also because these spiders have an excessively large pair of median eyes while the other six pairs are significantly smaller.  Although its creative, honestly I think its a bit of a stretch to say they have a similar appearance to an ogre (but then again, I’m no expert on ogre morphology).

So what is so fascinating about them?  Well…

The really remarkable thing about these spiders is their unique behavior for capturing prey!  In order to catch prey these spiders spin a web, the “net”, that is held between the first, second and third pairs of legs.  The spiders dangle from a strand of silk attached to something above (say, a small branch).  Ogre-faced spiders have great night vision with their large pair of eyes.  When a suitable prey item, usually some insect, wanders below, the spider drops down, stretches the net and casts it over the unsuspecting prey to ensnare it.  Finally, the spider delivers a venomous bite to subdue the prey.  Keep in mind that although venomous, these spiders are not considered dangerous to humans!

Check out these great spiders below!  Both species were photographed in Costa Rica.

It’s also amazing how these spiders just seem to disappear during the day!  They hide under leaves and in dark crevices.  But its no surprise how cryptic they can be after you observe one nearly disappearing right in front of you!  Accidentally spook them while they’re waiting to capture prey and they will pull all their legs appearing as if they were just a small stick (below).  Incredible!

 


Orb-Weaving Spiders

The next time you are outside in your garden or yard and you see a spectacular orb-web, just stop to think about the incredible spider capable of engineering such an intricate design.  Look at it very closely.  Perhaps you will see a thickened zig-zag pattern in the middle of the web.  Or maybe you will notice one very large spider along with a smaller, but nearly identical, spider – a case of sexual size dimorphism, where the male is usually much smaller than the female.  Maybe you will even get to witness the spider in the process of building the web!

The orb-web you see was most likely built by a species of spider belonging to the family Araneidae, typical orb-weaving spiders.  With over 3,000 described worldwide, the family Araneidae make up the third largest family of spiders after Salticidae (jumping spiders) and Linyphiidae (the sheet web spiders).  Araneid spiders are grouped together with other closely related families of orb-weaving spiders in the superfamily Araneoidea.  Examples of other orb-weaving families in this group include the Theridiidae (the cobweb spiders; this family includes the widow spiders), Nephilidae (includes the golden silk orb-weavers) and Linyphiidae (sheet web spiders).

Below: A web built between two trees by an orb-weaving spider in Costa Rica.

Some spiders build webs with a very noticeable thickened pattern (zig-zag, circular, spiral, X-shaped) near the center.  This structure in the web is called a stabilimentum (seen in the following few photos) and was originally thought to add structural support (i.e., stabilize) the web, but this idea has received little support.  The precise function of the stabilimentum is still up for debate, and there are several very good hypotheses each supported by some degree of empirical evidence.  The stabilimentum may: help to make the web more visible to larger animals, such as birds birds and mammals, to prevent them from accidentally destroying the web; provide camouflage for the spider; make the spider appear larger; or attract prey species by reflecting ultraviolet light.  There are also other theories as to the function.  It is highly likely, however, that more than one theory is correct and that perhaps the stabilimentum has evolved to function in more than one way.

Below: Argiope savignyi building a circular stabilimentum, La Selva Biological Station, Costa Rica.


After building the stabilimentum the spider moved to the center of the web and remained still.

It is quite obvious how the stabilimentum in this case makes the spider appear larger as well as camouflages the white body of the spider.  If you saw this web from a distance you would also notice how the web is effectively made more visible to our eyes and this might prevent, say, a bird from flying into it.

Below: This stabilimentum is X-shaped and when the spider is at rest its legs also assume this same shape, once again showing that camouflage might be a factor.

Here are some other spiders of the family Araneidae to show the great diversity of this family!

Below: Micrathena sp. from Costa Rica.

These spiders are great to have outside your house and around the garden as they act as a biocontrol, providing a natural way of managing insect populations.

Below: An araneid spider eating an insect, Lafayette, Louisiana, USA.

Below: An araneid spider on a radial line near the edge of the web, Price Lake, North Carolina, USA.

The next time you are outside and see an orb-web take a closer look.  And give these fascinating animals some credit!