HOME: www.hiltonpond.org

GENERAL INFO

CONTENTS

RESEARCH

EDUCATION

PUBLICATIONS

MISCELLANY

"COME ONTO MY FLOWER,"
SAID THE SPIDER TO
THE (HOVER)FLY

Almost every afternoon for the past week we've had a serious threat of rain at Hilton Pond Center. After a month-long "mini-drought," the good news is that we got 3.2" of precipitation in the seven-day period--including a 90-minute two-inch frogstrangler on the morning of 13 June. The bad news is that wet weather and an off-site workshop on the only sunny day prevented our running mist nets to catch birds. All week we had to schedule our trail work and nature observations for early morning before the thunderclouds built, and even then occasional light drizzle threatened our camera equipment when we wanted to take photos. Fortunately, the clouds thinned a bit around noon on 14 June, so we strolled down near Hilton Pond to check out some Elderberry shrubs, Sambucus canadensis, blooming at water's edge.

All text & photos © Hilton Pond Center

We've been fascinated this spring with the variety of pollinators visiting plant such as our Pickerelweed and Trumpet Creeper (see links to the previous two installments of "This Week at Hilton Pond"), so we expected NOT to be surprised by whatever insects we might find on Elderberry. Surprised we were, however, when we came across one flower head with a large, inch-long Hoverfly hanging in the shade beneath it (above). At first we thought the fly was lapping up nectar from the Elderberry's plentiful white flowers, but soon we realized it was dead and that none of its six legs clung to the plant. As we inched closer to the flower cluster, we discovered not only why the fly hadn't fallen earthward but also what had killed it: Amid pristine white blooms of the elderberry a much smaller Crab Spider was holding tightly to the hapless Hoverfly--all the while sucking out the prey's nutritious bodily fluids.

PHOTO #1: Natural Lighting

To be honest, this Crab Spider was none too easy to find. Some Crab Spiders--so called because their long, spindly legs are configured roughly like those of marine crabs (which, by the way have ten legs, not eight)--stake out a hunting ground on a large inflorescence and are called "Flower Spiders." They typically exhibit the same color as their host blossom--be it white, yellow, or pink--a perfect camouflage. Crab Spiders and Flower Spiders are classified in the closely related Thomisidae (the ambushers) or Philodromidae (the chasers), two rather large families with wide distribution; about 200 species occur in North America, and another 1,800 elsewhere.

Like all Crab Spiders, Flower Spiders do not spin elaborate webs (although males may wrap prospective mates in a silken veil--likely as a hedge against cannibalism). Instead, Flower Spiders blend in with blossoms where they wait with two pairs of forelegs (below) spread wide to ambush unsuspecting prey.

When a potential meal wanders close enough, the Flower Spider's eight tiny eyes detect motion and it quickly reaches out with long front legs to pull in the prey (below left). Simultaneously, the spider plunges its chelicerae (poison jaws or fangs) into the prey, almost immediately paralyzing it--pretty important since the quarry is often larger than the spider. The venom, which is also a digestive enzyme, breaks down the prey's internal organs as the spider slurps up the resulting slurry and sucks its victim dry. These spiders aren't too particular about who they grab; we've seen butterflies, bees, and even other spiders fall victim to a Flower Spider's lightning reflexes.

The individual we found on the Elderberry blossom appeared to be a Goldenrod Spider, Misumena vatia, named for its prevalence in summer and fall on various species of yellow Solidago. A newly hatched Goldenrod Spider appears white because it lacks pigment inside its exoskeleton. Some individuals take on odd colors if they consume heavily pigmented prey items, but females actually change color to match their surroundings. If an adult female Goldenrod Spider remains on a yellow flower for several days, the reflected yellow of the flower stimulates the spider to produce yellow pigment in her hypodermis. Initially her abdomen turns yellow, then her cephalothorax and legs, until her color matches that of the host flower almost perfectly. We're not sure whether a wandering Crab Spider we saw several years ago at Hilton Pond Center could ever duplicate the delicate lavender hues of the Passion-flower in the photo above right.

After finding the white Goldenrod Spider this week near Hilton Pond, we sauntered back to the Center's office to get our tripod, digital camera, and close-up lens. We weren't in any particular hurry because we rightly figured the spider would take quite a while to consume the sizeable fly. When we returned to the Elderberry shrub about ten minutes later, the spider had shifted its position very slightly, but its jaws were still plunged deeply into the Hoverfly's abdomen (below); in fact, the curved fangs appeared to be the only grip the spider had on its relatively heavy prey, since all eight of its walking legs were clasping nearby flowers.

We set up the tripod and moved in close to our subject, shooting several photos with various combinations of lens apertures and shutter speeds. Since the wind was blowing erractically, photography wasn't as easy a task as it could have been, and a number of exposures would end up being too blurry to use. Thus, we decided to go back to the office for one more piece of equipment, a special two-lamp electronic flash that attaches to the front of the close-up lens. We use available light for nearly all our nature photography, but decided to try the flash; it provides more light and helps combat the wind's flower-shaking by allowing use of a faster shutter. We shot numerous additional exposures with the flash--again adjusting the aperture and shutter speed and trying out varying depths of field.

Some results of our photographic efforts are on this page. After looking at what the camera had captured, we couldn't decide whether to use our available light photo of the spider and fly or the one taken with electronic flash. Thus, we ended up posting both with the idea we would ask our Web site visitors which they liked better. Below you'll find a poll through which you can choose between the two photos, but please don't make your choice based on the photo's layout or other attributes. Instead, just consider which lighting regimen you find more pleasing, the natural light or the artificial flash. We hope you'll take a minute to vote, if for no other reason than to satisfy our curiosity about which lighting appeals more to your aesthetic eye. (Besides, your vote may affect the kinds of photos we run in future installments.)

Thanks for voting, and for dropping in "This Week at Hilton Pond" for a close look at how the camouflaged and efficient Flower Spider handled a Hoverfly that flew too close to its Elderberry home.