Spiders: The Eighth Level of Diversity   2 comments


Wildlife, nature, fauna and flora of Costa Rica.

The rainfall of April, May and June from 2016 was more or less exactly the same in terms of monthly totals for the same periods in 2107 but this week has proved to be very wet.  The rain has been incessant, both day and night with very little of the sun having been seen.  A total of 15 inches rain fell and now the creek is running quite fast, so no water problems this year.

Spiders:  Not Everyone’s Favorite

I have to admit that many people I take out would rather just walk straight past spiders and ignore their presence.  Some people, despite their revulsion, are intrigued by their natural history, they are, after all, fascinating creatures.  I find them interesting to observe, understand and photograph.  They are mini carnivores with a wealth of different life histories.  They are also ubiquitous, they can be found just about anywhere on the planet, (apart from maybe Antarctica), and in abundance.  It is often stated that at no point in your life are you any more than six feet away from a spider and I can demonstrate the proof of that fact quite adequately every evening when I take people on the night walk.

There are approximately 40,000 species of spider distributed worldwide which makes them the eighth most numerously named group of animals on the planet.  Although they are all, (apart from possibly one species), carnivores and use venom to subdue and kill their prey, they are not dangerous to humans expect for those well documented species, the numbers of which you could count on the fingers of your hands.

Spiders divide up into two broad groups.  The first group are the Tarantulas, Trapdoor Spiders, Funnel-web Spiders and the Purse-web Spiders.  The second group are all other spiders and they are subdivided depending of the mode of food capture.  There are the Web Weavers; those spiders that produce the familiar silken webs but those webs also come in different forms.  There are Orb-web weavers, Sheet-web Weavers, Tangle-web Weavers and Lace-web Weavers.  Then there are Ambush Spiders such as Crab Spiders as well as Wolf Spides, Huntsman Spiders, Lynx Spiders, Fishing Spiders, Spitting Spiders, Jumping Spiders, Woodlouse Hunters and Wandering Spiders.  All in all, if there is a way of catching prey, then spiders have it covered.

A Living Tetrahedron

I noticed this spider last week crossing the road in front of me.  The bright yellow coloration was in marked contrast to the grey grit across which it was scurrying.  As I bent down to take the photograph, the spider went into a defensive posture and tucked its legs up against the body.  This made it easy for me but gives it a somewhat unnatural pose.  I think this individual must have fallen from an overhanging tree branch, maybe escaping becoming a meal for a larger animal.  Being bright yellow might work to disguise it amongst foliage but not against the dark grey of the road surface.

Rainforest arachnid. Rainforest areneae. Costa Rica spider.

The strange tetrahedron shaped body of this unidentified spider

You can gauge how small the creature was by the size of the grit next to it which is only about 1mm across.  When looked at close-up though you can see the spider is remarkably shaped.  The body is drawn up into an almost polygonal shape, a tetrahedron, rounded along its edges.  There are raised red nodules along the rear edge.  You would think that these would make it an easy spider to identify, but no such luck.  Extensive searching has not provided an I.D.  I would hazard a guess that it is one of the many tropical orb-weavers.  If anyone has a name I would be very grateful to receive it.

A Silver Orb in the Sun

Further down the road, in an open sunny area I saw a large web with the weaver sitting in the center, its eight legs forming a cross of four pairs.  Because of its location I could not get around to the opposite side to get a dorsal view so had to settle for the ventral view.

This species I had seen before, on numerous occasions.  This was a Silver-orb Spider, (Argiope argentata).  The fluting of the body rim along with the black and yellow tiger striping of the legs are examples of disruptive form and coloration which serve to make the creature less spider-like.  It is an effective form of camouflage used throughout the animal kingdom.

Rainforest Silver-orb Spider. Rainforest animals. Argiope argentata Costa Rica. Spider web. Spider silk.

Ventral view of a Silver-orb Spider, (Argiope argentata)

The web itself is constructed from silken threads.  Silk is an amazing material and deserves a book devoted solely to that subject.  Silk is both elastic and strong.  Orb-weavers can produce upto five different kinds of silk, each variety having a different use.  Silk is produced as a liquid in the silk glands, it is then secreted from the spinnerets where it is drawn out by the hind legs into the long threads.

To construct the web, the spider first has to create a framework of threads radiating from the center.  Then starting on the outside, it spins a spiral thread, round and around towards the center.  The spiral silk is coated in a sticky hygroscopic glue which serves to catch any prey item unfortunate enough to fly or fall into it.  The elasticity of the threads takes out the kinetic energy of an impacting large flying insect and stops the web from breaking.  Silver-orb Spiders are fairly large and consequently so are their webs.  They are reasonably common in the area so it does not take too much effort to find them.

Hidden Death Dealer in the Flowerheads

Sometime ago I noticed a butterfly behaving in a very strange way on one of the Lantana flowerheads.  Further investigation revealed the butterfly was dead and the unusual movement was being caused by it being manipulated within the legs of a small but efficient killer, a Crab-spider.

Crab spiders are small squat spiders and receive their name due to their ability to walk sideways as well as forwards and backwards.  They are ambush predators.  Some have the capability of changing color over a period of weeks to match that of the flower where they are sitting and waiting for a prey item to land.  In this case the victim was a White-banded Fatima (Anartia fatima).

Rainforest Crab Spider. Unidentified Thomisidae sp Costa Rica.

Unidentified Crab Spider captures, kills and eats White-banded Fatima

The butterfly would have been totally unaware of the lethal assassin’s presence.  As soon as it landed to take a feed of nectar, the longer and stronger front two pairs of spider legs would have grabbed it.  The butterfly’s wings would have been beating frantically but taking flight was no longer possible.  At the same time, a powerful venom would have been injected through the spiders fangs into the butterfly’s body killing it almost immediately.  The spider holds on until the venom has done its work.  The wings stop beating and the now lifeless butterfly is motionless.  Digestive enzymes injected with the venom liquidize the butterfly’s innards which the spider will then suck out as a soup.  It will then let the dry and drained husk of the victim fall to the ground.

To find crab spiders you have to search diligently among the flower heads.  Sitting patiently without moving and having the same color as their ambush position makes them difficult to spot.  Sometimes the only clue to their presence is the telltale sign of strange behavioral anomalies of their victims.

Rainforest Crab spider. Thomisidae Costa Rica. Rainforest animals. Rainforest Spiders.

Death hidden by beauty. Crab Spider waits at tip of a flower for a meal to arrive

Philip Davison Is a Biologist, Writer and Photographer Based in Costa Rica

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2 responses to “Spiders: The Eighth Level of Diversity

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  1. Thank you so much for this blog, I have been reading it in anticipation of visiting Bosque del Cabo on the 23rd. I am so excited! I’m hoping the rain slows down a little bit before we get there, but it won’t keep me inside all day! Looking forward to meeting you and the rest of the staff, see you soon!

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