25 September 2011

Argiope Spiders

Late summer/early fall seems to be the season of spiders.  This time of year, it is difficult to walk through the woods, fields, or wetlands without wearing webs on your face and pants.  Some of the most common and conspicuous spiders that I notice this time of year are those in the genus Argiope.  Often called "garden spiders," these ~2" long (the bodies of the males are much smaller) yellow, black, and silvery white orb-weavers can be quite intimidating for those with even the slightest bit of arachnophobia, but unless you have six legs and a pair of antennae, you have nothing to fear.  A severly harassed female with an egg sac may bite a human, but the bite is said to be no worse than a bee sting.

Female Black-and-yellow Argiope.  Be sure to click on this photo to enlarge it so that you can see the characteristic zig-zag stabilimentum that she weaves into her web.

An interesting feature of the argiope spiders is that they weave a ribbon of silk called a stabilimentum into their webs; this stabilimentum is often in a zig-zag pattern.  Although entomologists cannot agree on the true purpose of the stabilimentum, there are several theories as to why argiope spiders expend energy to create them.  One possibility is that it adds to the stability of the web.  Another idea is that the stabilimentum is easily seen by birds and mammals that as a result avoid the web and therefore don't destroy it.  Others think that the structure helps to camouflage the spider, while some think that the stabilimentum may actually attract potential prey.  Regardless of its purpose, each species of argiope spider produces a distinctive stabilimentum that helps biologists distinguish between species by only looking at the webs.

There are five species of Argiope in North America, with a sixth (Bruennich's Argiope, Argiope bruennichi) potentially present in Alaska.  Of the five species in the continental United States, three have fairly restricted ranges.  The Silver Argiope (Argiope argentata) is primarily known from the southern parts of California, Texas, and Florida, as well as from Arizona.  A similar species, Argiope blanda, is known in the United States only from the southern tip of Texas.  Florida Argiope (Argiope florida) can be found in the southeastern United States, from North Carolina to Florida and west to Louisiana; it may also be found in Arizona.  The other two species are widespread.  The one I see most frequently is the Black-and-yellow Argiope (Argiope aurantia).

Female Black-and-yellow Argiope, dorsal side.
 Black-and-yellow Argiopes have a characteristic black and yellow pattern on the back of their abdomen, whereas the carapace is covered in silvery hairs.  The legs are mostly black but often have reddish or orangish coloration at the proximal end.  Black-and-yellow Argiopes also have distinctive yellow and black coloration on their undersides.  You can find this species of spider in old fields, wet meadows, riparian areas, and gardens.

Female Black-and-yellow Argiope, ventral side.
 The other widespread species is the Banded Argiope (Argiope trifasciata).

Female Banded Argiope, dorsal side.
 Banded Argiopes are identified by the yellow, black, and silvery-white stripes on the back of their abdomens.  Like the previous species, the carapace of this species is covered with silvery hairs.  The legs of the Banded Argiope are striped black and orangish-brown.  The underside of the abdomen of the Banded Argiope is also distinctively patterned with black and yellow.  You can find the Banded Argiope in grassy and shrubby areas that are usually a bit drier than where you find the Black-and-yellow Argiope, but the two can be found in the same area.

Female Banded Argiope, ventral side.
 One of my observational differences in the females of these two species is that the Black-and-yellow Argiope seems to drop from her web fairly quickly when I bump into it, whereas the Banded Argiope seems to hold her ground.

If you are one of those people who is deathly afraid of spiders, I hope that this at least gives you a better respect for the argiopes.  If you are one who likes spiders, be sure to get out in the next couple of weeks before it gets too cold to admire these large black and yellow arachnids.

18 September 2011

Blue Curls Resurfaces

On a recent work trip in Muskegon County, Michigan, Linda VanAndel and I saw the State Threatened Blue Curls (Trichostema dichotomum) in bloom.  This annual mint has flowers that are small (only about 1 cm long) but that are quite interesting, with arching hair-like stamens (Trichostema in fact means "hair-like stamens") curled over the single lower lip.  Looking at the form of the flowers, you can see how the lower, purple-spotted lip serves as a nice landing pad for insects in search of nectar, while the curled stamens deposit pollen on the back of the oblivious feeding insect.  The style is also curled and arching, meaning that the next Blue Curls flower that the insect visits will be pollinated when the pollen on the insect's back rubs up against the receptive stigma.  This is an excellent pollination strategy, considering that bee species characterstically move from flower to flower of plants of the same species in search of nectar.

Blue Curls is known from many states and provinces in the eastern half of North America.  It grows in coarse, acidic soils in open areas including prairies, savannas, open woodlands, sandhills, and pine flatwoods.  Blue Curls relies on fire or other disturbance to persist as part of a local flora, as it is not tolerant of competition from other plant species.

Stable over its North American range, Blue Curls is also a species of concern in Indiana (State Rare).  The site where we saw it in Michigan was excavated a few years ago, creating disturbed, open soil that is ideal.  I have seen it at a few sites in Indiana, including one location in Porter County that historically was mined for sand and another along a recently excavated roadside berm in St. Joseph County (both, again, disturbed sandy soil).  The yet to be answered question is: where does this species come from when there is no apparent seed source and sandy soil is excavated?  Have the seeds been covered by years of sand and organic accumulation, just waiting for a disturbance to bring them back to the surface so that they can germinate?  Are the seeds brought in by excavating equipment?  My guess is the former, but then you have to wonder how long the seeds can remain viable.  Surely, it takes hundreds, if not thousands, of years for a foot or more of soil to accumulate, meaning that the seeds may remain viable for that long.  Nature is resilient.

05 September 2011

Small Mammals That Excite Even A Wildlife Biologist

I apologize in advance for the poor quality of the two photos in this post, but I couldn't resist including them. Within a week's time, I saw two mammal species that made even esteemed Cardno JFNew wildlife biologist Jeremy Sheets giddy.

At a mitigation wetland at a landfill in Lake County, Indiana, I was sampling vegetation quadrats when I heard a Mallard smacking its wing against the water. I've seen Killdeer on many the occasion perform the "broken wing" routine when trying to keep a predator away from a nest, but I'd never heard of a Mallard displaying this behavior. Still, that was all that I could imagine was going on. The wing smacking continued, so I eventually walked in the direction of the noise, but the Mallard didn't leave the spot. As I got closer and the smacking stopped, I saw that the back of the Mallard's neck was covered in blood. Then I saw a small, dark mammal bound and scurry away through the several inches of water. I'd witnessed a kill! I followed in the direction that I'd seen the mammal go but I couldn't find it. Reluctantly, I went back to sampling without seeing the predator. Soon after, I heard rustling of vegetaton from the location of the dead Mallard. I snuck back over and again saw the dark mammal scurry away. I decided this time to stay close to the dead Mallard and wait to see if the culprit returned. Less than a minute later, I saw the killer, shown below.

This was my second ever live encounter with a Mink (Mustela vision), but my first encounter when I wasn't in a car. They live in areas where there is water, such as borders of lakes, marshes, streams, ditches, and ponds. Mink are omnivores, but nearly all of their food consists of other animals, and their preferred prey is Common Muskrat (Ondatra zibethicus). These dark-colored weasel relatives are also known to inhabit Common Muskrat dens and usually eventually drive these slightly larger mammals away. At this site, there were numerous Muskrats two years ago, but this year I saw only abandoned dens. Because of their typically nocturnal habits, Mink are often not observed by humans. Mink are found throughout much of North America, with the exceptions of parts of northern Canada and the southwestern United States.

My second mammal encounter occurred at a mitigation site in Superior, Wisconsin, while I was meeting with representatives from the U.S. Army Corps of Engineers and the Wisconsin Department of Natural Resources. As we were talking, the Corps of Engineers biologist saw a squirrel come out from under a parked vehicle.

This was my first ever encounter with a Franklin's Ground Squirrel (Spermophilus franklinii), a rare diurnal species restricted in range to east-central Alberta, southern Saskatchewan, and southern Manitoba south to northern Kansas, northern Illinois, and northwestern Indiana. At first, we thought we may be looking at a Gray Squirrel (Sciurus carolinensis), but this individual was smaller than a Gray Squirrel and had a gray head and gray tail with a brown body, and we later saw its burrow in the loose soil of an exposed mound of dirt. Franklin's Ground Squirrels feed on a mix of plants and animals, with green clover leaves being the primary vegetation and caterpillars and ants being the primary animals. They are found in dense grassy areas, often along railroads and on roadsides.