Photographing Orbs

I was hanging around in a leather shop in Laurel. If you must know, I was dropping off a damaged strap, bridle, girth or something ... for a horse.  The owner was showing everybody photos of orbs he had taken near Gettysburg one night. According to him, orbs are the floating spirit energies of dead soldiers or other existentially challenged folks, not the flash or other light reflecting from dust or bugs. To be fair, I suppose that both could be true: aerial flotsam could contain the spirits of dead soldiers; cavalry soldiers could be riding the dust. (How many orb soldiers can fit on a dust speck?) Anyway, I have a question: if the orb hunters really thought that orbs were the spirits of dead soldiers, why would they treat them with such disrespect, interrupting their ghostly business with blinding flashes and inevitable hooting and hollering? Thank you for your service!

Look at this YouTube video for an explanation of what orbs really are.

Now you know

I photographed different orbs: the orb webs of spiders. Orb webs have threads radiating from a central point that support a sticky spiral thread that captures flying insects. Until very recently, arachnologists thought that orb webs evolved once in a group of spiders aptly called the Orbiculariae. However, it now appears that the orb web is more ancient than we thought and that most nontarantula-like spiders had ancestors with orb webs (Paper). Orb webs are great for capturing flying insects, but not always so good for crawling insects and they may be energetically expensive to make. For whatever reason, there's been a tendency for spiders to switch to sheet webs or cob webs or to dump the whole web thing altogether.

Superfamily Araneoidea

Family Araneidae: Common Orbweavers

Araneid orbweavers are very diverse and abundant, and this diversity increases to a crescendo in late summer and early fall. They generally build vertical orb webs: spoke-like radial strands attaching to a frame lines and bearing a spiral with tiny gluey droplets.

Left: Black and Yellow Garden Spider, Argiope aurantia. Charlotte, a recent mascot of the Dept of Entomology, University of Maryland. RIP. Center: Banded Garden Spider, Argiope trifasciata, upper surface. Right: A. trifasciata, ventral surface.

The genus Argiope has about 80 species distributed throughout the world, including five in the contiguous 48 states and two in Maryland. They tend to be large spiders (the females, anyway) and spin large webs that are usually and variably decorated with a central pattern of silken bands called a stabilimentum, a structure produced by many different orbweavers. Arachnologists have researched and debated the function of stabilimenta for decades and Argiope species, with their many variations in design, have been frequent experimental subjects. Stabilimenta may serve as a signal to birds to prevent them from flying into and destroying the web, as a way of breaking up the outline of the spider as a sort of camouflage, and attracting insects with reflected UV light. These functions may vary between species and perhaps could even differ within a species depending on the ecological or behavioral context.

Left: A newish web at the University of Maryland in some ornamental shrubs. Right: An older web in in rural southern Frederick County on a wooden fence.

These are Cyclosa turbinata, the Humped Trashline Orbweaver, which is a small and fairly common orbweaver. A second trashline spider, Cyclosa conica, also occurs in Maryland. The “trashline” is a vertical row of silk-packaged remains of previous meals, egg sacs as well as the spider itself.  The spider occupies the center of the orb near the center of the trashline. This is a kind of camouflage and, unlike many other araneids that will run away or take other evasive action upon detecting an approaching person, Cyclosa will tend to just hang in there. I suppose this gives some weight to the stabilimentum as "hiding place" idea.

Left: Spined Micrathena, Micrathena gracilis. Center: White Micrathena, Micrathena mitrata. Right: Arrowhead Spider, Verrucosa arenata

I found these three spiders in my backyard one day in September. Micrathena is a genus of small to medium-sized orbweavers that make their webs at about the level of an adult human's face. There are times during the summer when you can't walk through the woods around here without getting a face full of their webs. The near-vertical webs are hard to see, even when you are looking out for them. The spiders will generally try to run off the web to a supporting branch or twig as you approach, but it often gets wrapped around your head or shoulders along with the web, anyway. They don't bite and their abdomen is so hardened that you can pick them up with your fingers without hurting them and place them on a twig or leaf to resume their spidery business. There are three species in Maryland, the two pictured above and the Arrowshaped Micrathena, M. sagittata.

The third spider, Verrucosa arenata, is larger than Micrathena. Sometimes the triangular portion on its back is white, sometimes yellow. The species' main claim to arachnological fame is that it's one of the few orbweavers that rests face-up in the web rather than face down. Why?  I doubt anybody knows.

Two of many color variations of the common tree or barn spider, Neoscona crucifera

Spiders in the genus Neoscona are often called Spotted Orbweavers, apparently due to the pair of light spots on the underside of their bellies, as in the first image of this blog. The "crucifera" part of N. crucifera means "cross bearing", because the top of the abdomen often has a mid-longitudinal line and a transverse line at the front end that together create a cross. Neither of the specimens depicted here show an obvious cross, although you can kind of see it in the one depicted on the right. This is a very common spider in summer and autumn in Maryland and throughout much of the eastern U.S. They build relatively large webs, up to 2 feet across, in trees, bushes, fences, railings, etc. They are largely nocturnal, although they will hang out in the middle of their webs for a while during the day if it's cloudy or shady, but they eventually take the spiral of their web down by eating it and then rebuild it in the evening.

Tetragnathidae: Long-jawed Orbweavers

Like the araneid orbweavers, the tetragnathids build webs with a gluey spiral, but the webs are usually oriented in a more horizontal plane. There aren't as many tetragnathid species in the eastern U.S. as there are araneids, but some species are common. One of the most recognizable genera is Tetragnatha, which have long legs, long abdomens and long chelicerae or "jaws". I didn't happen to run across any this summer, but you can check them out at either the Maryland Biodiversity Project or Bug Guide.

Orchard Spider, Leucauge venusta. Note spirit orbs caught in the orb web on the left.

Leucauge venusta is one of the most common orbweavers in the eastern states. It builds a near-horizontal web with an open hub in low bushes and shrubs. It is easy to recognized by greenish legs and an abdomen patterned with silvery white, black, yellow and green, sometimes with an orange mark on the under surface.

Superfamily Deinopoidea

The nontarantula-like spiders (that is, the spiders we are most likely to encounter in eastern North America) were once classified into two groups, cribellate and ecribellate, which differ in the kind of silk they produce. The cribellate spiders have a special "spinning plate" near the spinnerets called a cribellum that produces lots of fine silken strands, and the fourth leg has a special comb, the calamistrum, that is used to manipulate this silk. The cribellate silk ensnares insects by mechanical entanglement rather than by a chemical sticky glue. Back in the day, arachnologists figured that both cribellate and ecribellate groups had orb weavers, sheet weavers, ground spiders, tube makers, etc. and that this was an example of parallel evolution. Later, it was shown that the cribellate silk was actually the ancestral kind of silk for all these spiders and that lots of spiders lost it, resulting in a complicated mix of cribellate and ecribellate spiders. So, there are cribellate spiders that look like our familiar true ecribellate wolf spiders and they are actually closely related.

The superfamily gets its name from the genus Deinopis (Family Deinopidae), which means "terrible or fearsome eyes". Their common name is ogre-faced or gladiator spider, and they live in tropical and subtropical areas. They build a web of mostly cribellate silk that they hold with the first two leg pairs, while clinging to a silken support with their other legs. They use the web like a net to capture crawling or flying insects. (See video)  Obviously, they need good eyes to accomplish this.

 Family Uloboridae: Hackleband Orbweavers

Octonoba sinensis

The uloborids are the cribellate orbweavers. There are very few species of uloborids in eastern North America, although they have a variety of web designs. For example, Hyptiotes, the triangle spider, builds a triangular web that looks like a pizza slice of an orb web, but the pointy end is held taught by the spider via a single long thread. The spider releases the thread when prey hits the triangle and the web collapses on the insect. (see Video). 

The uloborid shown here, Octonoba sinesis, builds a fairly typical horizontal orb with a doily-like stabilimentum. The species appears to have been introduced accidentally into the United States from East Asia. I found these living the vertical shaft of a grated outside drain on a horse stable in Frederick Co., Maryland.  The high density of webs is probably made possible by the continual emergence of flies from the drain. To me, they looked like galaxies suspended in the smelly void of space. (In space, no one can hear you wretch.)