January 21st, 2007
Recently, I had to scare up some photos of Goldenrod Crab Spiders (Misumena vatia) for a photo researcher. That got me thinking that I probably haven’t written anything much about these spiders in the past. This seems like as good a time as any to remedy the situation, so I’ve picked out several photos to illustrate this piece (click on each image to see a larger version). My advance apologies to the arachnophobes among you. I promise to do a nice safe piece about a recent hike and will post that tomorrow.
Goldenrod Crab Spiders are referred to as such because they are frequently found hanging out on goldenrod (Solidago) flowers. This species is a member of the Crab Spider family (Thomisidae). As you can see from the above photo, the front two pairs of legs are long and powerful, much like those of a crab. Adult females can become reasonably large, having a rounded abdomen about the size of a pea. The background colour of the female spider ranges from white to bright yellow and can change from one colour to the other over a period of days as the spider moves from white to yellow flowers, or yellow to white. The upper side of the abdomen has two wavy pink lateral markings. The area around the eyes is yellow and mask-like. Males are much smaller and have a dark cephalothorax and legs, with a small, pale abdomen with dark stripes. I do have a couple of photos of males but can’t remember where they are so will have to save them for another time.
Edit: Pamela from Thomasburg Walks has a couple of nice shots of a male M. Vatia in this post — see second and third photos from top.
These spiders do not spin webs, but instead hunt by locating a suitable flower and then hiding in the bloom with the powerful front pairs of legs raised in readiness to grasp any insect that comes within reach. As I make the rounds of the gardens and fields here at the farm, I often spot these spiders and try to revisit them more than once in a day. It’s quite surprising how long they will stay motionless on one flower. For example, the above spider on the lavender coloured bloom was first spotted in the morning. I checked on her again around noon and she was still in position. I had to go to town to do a few things, so I left for the afternoon, only to return and find her still in position. It didn’t look like she was having much luck capturing a meal. I did wonder about her choice of flowers as it was some type of ornamental plant in my herb garden, and it didn’t seem to attract many insects, at least at times when I was around. By the next day, the spider was gone — most likely having moved on to better hunting grounds. As one might guess, a suitable flower is very important to the success and survival of these spiders. There have been several studies on flower selection by this species. After a very quick look, I found this this abstract for a study of young M. vatia spiderlings and Ambush bug (Phymata) nymphs that found they didn’t seem to be quite as discriminating as adult females. It seems that older females are selective of flowers as they must expend a lot of energy to move from one to the next. I don’t doubt that as these large female spiders are ponderously slow when moving about.
Sometimes, I find mature Misumena vatia in odd places where there are no flowers. I found the above spider in the center of a small Red Oak sapling in our garden. However, I’m not entirely sure she was hunting. At the time when I shot this photo, I thought that might be the case – and perhaps she was – but after examing my photos, I noticed some threads of webbing just below a nearby leaf. I’m guessing that she had secured her egg case to the underside of a leaf and was actually engaged in guarding behaviour. That just goes to show that I should probably be wearing reading glasses while wandering around in the garden — who knows what other good things I’ve been missing.
Almost a year ago, I posted a piece entitled Watchful Spiders, and gave several illustrated examples of egg case guarding by female spiders. Of one spider, I wrote:
And then there was this Goldenrod Crab Spider (Misumena vatia) that guarded her egg case, sealed inside a rugosa rose leaf, in my garden for about a month. Each day, I stopped to visit her as she tightly gripped the carefully sealed leaf packet — watched while she became increasing desiccated, abdomen gradually collapsing, until eventually, the morning after a hard frost, she was gone.
That is typical behaviour for female Crab spiders. They will guard their egg cases until they expire. You can see a photo of the desiccating spider here. The spider in the photo directly above this text is a yellow phase spider that is guarding an egg case concealed within the folded edge of a milkweed leaf.
I should probably say something about the coloration of these spiders. Although it’s an area that I don’t know a great deal about – at least of the physiological part of the operation – I did find a brief reference on wikipedia, referenced to a paper by Oxford, G.S. & Gillespie, R.G. (1998). It states:
The color change is made possible by secreting a liquid yellow pigment into the outer cell layer of the body. On a white base, this pigment is transported into lower layers, so that inner glands, filled with white guanin, become visible. If the spider dwells longer on a white plant, the yellow pigment is often excreted. It will then take the spider much longer to change to yellow, because it will have to produce the yellow pigment first. The color change is induced by visual feedback; spiders with painted eyes were found to have lost this ability.
The color change from white to yellow takes between 10 to 25 days, the reverse about six days. The yellow pigments have been identified as kynurenine and 3-hydroxykynurenine
Fascinating stuff, isn’t it?
There are other interesting things about about the coloration of these spiders. When photographing them, I’ve noticed that the underside of the cephalothorax has a yellow spot that is visible when the spider is raised up with front legs in the hunting position — the above photo shows this quite well (click on image for larger view). It seems as though that spot must resemble the coloured center of many kinds of flowers that attract insects. Imagine how well that spot would guide potential prey right into the arms of the waiting spider!
Regarding the wavy pink markings on the abdomen, back in April, when I wrote a post about spider eyes, John Caddy of the Morning Earth website left an interesting comment that I haven’t done any follow-up on, but that makes sense:
Possible hint about your flower spider in tree. Recently, the red “chevrons” have been identified as “bee guide” mimicry. So when a bee seems them in the UV spectrum, it responds as it does to the true bee guides that center many flowers and point the bee toward both nectar and pollen. The flower spiders are imitating ultraviolet-visible patterns that flowers evolved so that bees would help them make seeds–an excellent co-evolution.
Also explains why/how the bee does not see that huge white spider lurking there.
So, it seems that those wavy, bright pink markings may have a function that contributes to the success of this spider.
I’ve saved this photo to the end so as not to scare off the arachnophobes among you. This is how I often find these spiders — in the middle of feeding on prey. It looks like this spider has been quite successful, as she has both the fly she’s feeding on, and the remains of what looks to be a small Robber fly off to the left. Like most native species of spiders, M. vatia does not have strong jaws with which to dissect prey. Instead, these spiders must kill their prey by making a small bite and injecting paralyzing venom. Once the prey is subdued, the spider injects digestive enzymes that will liquefy the inside of the prey’s body so that it can be sucked out. The outer body of the prey is left looking relatively undamaged. If you find intact dead flies, bees or similar on flowers, most likely they’ve had a deadly encounter with a spider, an Ambush or Assassin bug, or one of the other predators that feed in this way.
(Edit: In the comments below, Wayne brought up an interesting point about how spiders might use their jaws. I realize that I did not explain that very well in the above paragraph. From what I know, some species of spiders can use their jaws (chelicerae) to crush and kill, but many kill by puncturing and injecting venom through fangs located near the tip of the chelicerae. However, all spiders feed by sucking the liquefied internal organs of their prey after it has been subdued and killed. If you’re interested in taking a good look at spider jaws and how spiders feed, you might like to visit this page on the website of Ed Nieuwenhuys. There are some great close-ups of spider jaws along with explanations of how the spider feeds, etc…).
These spiders are said to have good eyesight, and I believe that to be true. It seems likely that they would need good vision in order to capture approaching insects as they come within grasping range. I’ve noticed that they react to my presence or that of my cameras, either moving to the safe shelter of a flower petal, or taking a defensive stance with arms raised.
I believe that’s about all I was going to write about these spiders today. If you’re interested in seeing more examples of Crab Spiders, you can visit this one of my online photo galleries. If you’re interested in more information on M. vatia, check out this species page on the Nearctic Spider Database.