The Gluttonous Crab Spider

The following post was written by Evan Barrientos, of our two Hubbard Fellows for this year.  Evan is a talented writer and photographer, and if you enjoy this post, I encourage you to check out his personal blog as well.  …For the record, I did not in any way encourage Evan to photograph or write about crab spiders.  This is despite my well-known affinity for them and my personal inability to walk past one without taking its portrait.

Over this summer I’ve tried to focus on photographing the prairies with a wide angle in order to show what the landscape looks like, but sometimes it’s just impossible to resist delving into the tiny details and dramas of prairie microfauna. One morning in June I was admiring the spiderwort flowers when I stopped to photograph a crab spider (Thomisidae sp.) sucking the juices out of a hoverfly (Syrphidae sp.) that she had caught.


If you’ve been reading this blog for a while, you’ll know that many crab spiders hide on flowers and ambush insects as they stop for a drink of nectar (similar to ambush bugs). Some crab spiders can even change their color to yellow or white in order to match the flower they are on, but this spider didn’t seem to need that trick. To my great surprise, a second hoverfly was brave, hungry, or stupid enough to land on the same flower while I was taking photos. Maybe it thought the spider would be content with the hoverfly already in her fangs. It was wrong.


The spider quickly honed in on the intrepid visitor, even though her fangs were already more than full. What she planned to do with the second hoverfly, I don’t really know, but  watch what happened when the clueless hoverfly strayed a little too close:


Apparently, this hoverfly liked adrenaline, because it continued to gorge itself on delicious spiderwort nectar for a few more moments and eventually flew off to safety. Thinking the spectacle had ended, I started to pack up my camera gear, but before I could, another fly landed on the flower!


As you can see from the photo sequence, the spider was once again too slow to catch the visitor. I watched her for a little longer, but eventually left to photograph a bird singing nearby. I came back a few minutes later to check on the spider and found yet another surprise.


In the five minutes I was away, the crab spider had finished eating the first hoverfly and caught another one. I don’t know if spiderworts are irresistible to flies, or if hoverflies are terrible at spotting white crab spiders on purple flowers, but this spider sure was lucky that morning!

Encounters like this remind me how important it is to pause every once in a while and notice the little details. When I do this I’m often amazed by how much is going on around me and how much I would have missed it if I hadn’t stopped. Wide-angle views certainly have their place too, but to fall in love with prairies you really need to stick your nose in them at times.

Photo of the Week – May 31, 2013

I started my annual plant community monitoring this week.  That work consists mainly of inventorying the plant species within small sampling plots.  Forcing myself to walk regularly spaced transects and stare at a square meter of prairie at a time is a great way to find creatures and sights I might miss if I was just wandering aimlessly.  This week, for example, I scared up a couple jackrabbits and found a quail nest within a few minutes of each other, and found a number of pretty neat insects.  But in that particular prairie, the star of the show was Tradescantia bracteata (bracted spiderwort), which was scattered across the site in patches about the size of a small car.

A close-up look at a patch of bracted spiderwort, with prairie ragwort (Senecio plattensis) in the background.  The Nature Conservancy's Platte River Prairies, Nebraska.

A close-up look at a patch of bracted spiderwort, with prairie ragwort (Senecio plattensis) in the background. The Nature Conservancy’s Platte River Prairies, Nebraska.  You can click on the photo to see a larger and sharper version.

These spiderworts were blooming in a prairie we planted back in 2000.  It has become of our most colorful sites – loaded with wildflowers of all kinds.  I didn’t see much spiderwort during the first 5-7  years of the prairie’s establishment (most of which were drought years).  Eventually, I started finding a lone plant here and there.  Those scattered plants have now formed colonies that radiate outward every year.

If you look closely, you can see that several of the spiderwort plants in this photo have been grazed.  They are blooming in a burned portion of the prairie, which is where cattle are focusing most of their attention (within our patch-burn grazing system).  Cattle really like to eat spiderwort, so grazing will probably impact the 2013 growth and seed production of the plants in this photo.  However, we just finished building a temporary electric fence to exclude cattle from about half of this same prairie for the rest of this growing season, so all the spiderwort patches in that exclosure should have a good year.  Next year, the patch of flowers pictured here will get a break from grazing too.

Although grazing can keep spiderwort plants short and decrease seed production, most of this species’ reproduction happens through rhizomes (underground stems), so annual seed production is not critical for its survival or spread.  In addition, periodic grazing helps open up space among the grasses and provides opportunities for spiderwort to continue its spread.  In fact, areas of our prairies that get little or no grazing tend to have fewer and smaller patches of spiderwort (though the individual plants often grow taller).