Northern Minnesota phenology for August 2014: Crab Spiders

By: John Latimer

On a beautiful, sunny August day a white admiral butterfly captivates the attention of my wife Denise. It’s a bit oddly perched and the undersides of the wings are visible. Seen from below the colors are subtler and the black of the dorsal view is brown. She begins to take pictures and I can’t believe the butterfly is being so cooperative, it allows her to approach with in a foot without leaving. I lean in for a closer look and it is then that I discover the crab spider that has grasped the butterfly by the head.

Crab Spider - Frontal. CC-licensed photo, click for original.
Crab Spider – Frontal. CC-licensed photo, click for original.

Crab spiders ambush their prey and often take insects much larger than themselves. Using their back legs to maintain a firm grip on the flower they grasp the hapless victim with their long powerful front legs. Using their small fangs they quickly administer a dose of venom that paralyzes the insect. After that they can hold it with their jaws alone. The venom reduces the interior of the quarry to a liquid and the spider sucks it up.

Goldenrod crab spiders are like the chameleon. They can change their body color to blend with their background. Since they belong to the group of spiders that hunt without a web they need to camouflage themselves to ambush their prey. At times they may simply attach themselves to a plant stem and mimic a flower. Any shortsighted insect that comes to investigate will be lucky to escape.

Our local goldenrod crab spiders can, at any given time, assume one of three distinct colors. She may be bright yellow like the goldenrod of her name. She may be white or she can shift to a pale green. Regardless of the color every one I have seen has matching slashes of pink or red on either side of her abdomen.

This color shift is not achieved in seconds or minutes. It can take as long as twenty days, though most accomplish the change in three days or so. It can depend on several factors including the age of the spider and her need for protein to complete the production of eggs. The color isn’t the result of some remarkable system of chromatophores within the skin; it is an actual pigment that the spider must create. Once she has assembled the pigment it is infused into her skin cells.

Crab Spider (Thomisus sp). CC-licensed photo, click for original.
Crab Spider (Thomisus sp). CC-licensed photo, click for original.

The trigger for the color change is her vision. When scientists block a spider’s eyesight she will not attempt to blend with her background. If she is moved to another plant where she does not blend she will attempt to relocate to a plant of the appropriate color. If she is prevented from relocating she can resorb the yellow pigments for example, and sequester them in her body. She then will present as a white or perhaps cream-colored spider. Should she need to become yellow again she can re-infuse the yellow pigments she has stored or if she has excreted them she can synthesize new pigments.

You may have noticed that throughout this piece I referred to the spiders in the feminine tense. The male goldenrod spider is seldom seen. When you do find one it will be a quarter to a third the size of the female, have dark legs and twin pink stripes lengthwise down its white back. Apparently they are not able to change color.

The males mature early and will mate as soon as they can locate willing females. Once the female has bred she will attempt to procure as much protein as she can prior to laying her eggs. In August she will find a leaf, often on a milkweed plant, roll it into a shelter and lay her eggs. She then stands watch over her clutch until she dies. As the eggs hatch the spiderlings disperse and over-winter in the leaf litter. They emerge in the spring ready to ambush the next generations of pollinators attracted to the white, green, or yellow flowers.

From the phenology notebook:

August 1, 2003     The first monarch butterflies of the generation that will fly to Mexico are emerging from their chrysalises. In the wetter ditches the purple fringed orchids are starting to bloom. In the same locations the last flowers of the water hemlock are fading. Wild cucumber has spent the summer climbing over its neighbors and now is ready to flower.

 August 21, 2013     Fireweed has gone to seed. In Alaska this is a signal that the first snows will be falling in six weeks. Typically in Minnesota we will see our first snows in early October, so six weeks is not too far off as a rule of thumb. The phoebes are about to fledge their second brood. Gray goldenrod has begun to flower. The bloom is a tight finger of bright yellow with a slight crook near the top. Cicadas are singing, and so are the tree frogs. The latter appear to be influenced by the ratio of daylight to dark that mimics the actual breeding season in May.

August 31, 1999      There is a horse chestnut growing along the River Road in Grand Rapids that turns a spectacular red in the fall and today it has just begun to hint at the color that is to come. I see a buck today and his antlers are draped with tattered velvet. No doubt he has been rubbing them against some small shrub polishing them in preparation for the duels coming in the next months. The loons are molting into the drab gray that will carry them through the winter as they while away the hours on the ocean before molting back into their more familiar breeding plumage and returning to Minnesota next spring.



John Latimer is well known throughout northern Minnesota for his phenology work.  John appears weekly on KAXE radio in Grand Rapids, and audio and twitter archives are available here. We hope his work will be a frequent feature on MyMinnesotaWoods.  This article also appeared in the Duluth Senior Journal.  It is printed with the author’s permission.

The University of Minnesota Extension Forestry team.

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