Acacias, Ants and Vegetarian Spiders

by

Dr Bug

I love to walk through a tropical rainforest anytime of the day or night.  If it’s dark and I can’t see much but the forest is alive with sound, much of it due to croaking frogs and loud insects.  And, with a flashlight there are plenty of glowing frogeyes to see!  If you have a lantern you are a bug’s best friend!  They will quickly fly or crawl to the warmth of your lantern.  Place your lantern on a white sheet to see the extraordinary beauty and diversity of your new insect friends.  I would try to carefully and slowly kiss the big ones but that’s me; don’t try it unless you know what you’re doing.

While walking one day in a rainforest in Costa Rica, I came upon a clearing in the forest; a rare site in a place where there are plants growing in every available spot and things, like slime molds and jelly fungus, growing on top of everything green and brown.  Not to mention, epiphytes - plants like many orchids and ferns that grow on tree branches.  After a minute or so, I knew for sure that I was near an ant acacia (Acacia cornigera) because lots of pseudomyrmex ants also known as acacia ants were busy eating the tips of all of the plants growing under and around the acacia.   This is one of the most well known examples of mutually beneficial symbiosis in the world; every college biology textbook describes it that way.  Mutualism is when two different species help one another survive, both benefit from the relationship.

So, let’s get back to the scene at hand.  Some of the ants were collecting little brown nut like ovals also known as Beltian bodies from the tips of the acacia leaves.  These little morsels are rich in protein, a macronutrient (molecularly big nutrient required in larger amounts than the micros) important to all living things that eat other living things to survive.  The ants chow-down Beltian bodies to get the protein they need.  Other ants were drinking nectar from the extra-floral (outside of the flower) nectaries produced by the acacia just for them.  I also saw ants traveling through little ant-size holes in large thorns at the base of each leaf.  The ants live in the thorns and I read somewhere that the ants actually produce a plant hormone (possibly gibberellic acid) that enlarges the thorns.  Imagine moving into a studio apartment and being able to convert it to five bedrooms!

I wanted to do an on-the-spot experiment.  I hypothesized that if it was truly an ant acacia and I walked under it, the ants would start bailing-out of the acacia to land on me and sting me.   Although it’s not good scientific practice, sometimes you just must become a part of your own experiment and this was that time for me.  I walked under the acacia and guess what happened?    You got it!  Ants started landing on my clothes and I made a mad-dash to get out of there.  If a browsing, leaf-eating animal (herbivore) passes under an ant acacia, the ants will bail out in mass and sting the creature so it leaves in a hurry.

The story doesn’t end here.  Enter the vegetarian spider; the first and only known vegetarian spider, I might add.  Apparently, these cute little spiders (see photo) live on dying acacia leaves still attached to the tree where they won’t get harassed by ants.  When the time is right, they make a mad-dash to healthy leaves and collect as many Beltian bodies as they can before making a mad dash back to their dry perch.  Unfortunately, I didn’t observe the acacia long enough to see the vegetarian spider, but you can bet that I will the next time I’m near an ant acacia in the dry to wet rainforests of Mexico and Central America.