Saturday, May 4, 2013

MD Spiders: Maryland's Largest Spiders

Thousands of species of spiders call Maryland home. Most are so diminutive in size and discreet in habit that they go largely unnoticed, even when residing in our own gardens or homes. But some spiders are simply too large to ignore!  Spiders with impressive girths, legspans, and webs easily catch our eyes and our interest. This WMD101 post will explore some of the largest and most commonly encountered spider species in Maryland.

Most spiders are diminutive, like this tiny wolf spider. 
In the mid-eastern U.S., many spiders reach full size in late summer or autumn, and therefore the largest spiders are often encountered late in the year. However, some spiders, including some wolf and fishing spiders, overwinter as adults or subadults, and may be found at large size throughout the year

The Orb Weavers
Orbweavers are sit-and-wait hunters that construct sticky, orb-shaped webs in strategic locations.  They typically rest in the center of the web or in hideaways under leaves or bark, rushing in to capture the unlucky creatures that stumble into their webs. The webs of these large orbweavers can be two or more feet in width.

Neoscona crucifera building her web at night
Male orbweavers are often very small and difficult to spot, while the females can be quite large, dwarfing the males.  If you are staggered by the size of an orbweaver, you are almost certainly observing a female.

The largest orbweavers in Maryland are found in three genera: Argiope, Araneus, and Neoscona.

ARGIOPE
Argiopes  have long, oval- or almond-shaped abdomens that are brightly colored with yellow and black patterns.  Argiopes typically sit in their webs during the day, often in full sun.  They are sometimes called writing spiders due to the zigzag patterns they often weave into their webs.

  • Argiope aurantia (black-and-yellow argiope, yellow garden spider) - up to 1" body length; distinctive black and yellow pattern on abdomen, forming yellow spots   Photos from BugGuide
  • Argiope trifasciata (banded argiope, banded garden spider) - up to 1" body length; distinctive bands of white, yellow, and black across its abdomen   Photos from BugGuide


Argiope aurantia
ARANEUS and NEOSCONA
These two genera can be very difficult to tell apart. Both have fat, round, pumpkin-shaped abdomens and are often brownish, but may have distinctive patterns and coloration. They typically sit in their webs at night or in shaded or overcast conditions. During the day, they hide away under tree bark or eaves, or within a curled leaf.  To tell the two genera apart, look for a groove in the center of the cephalothorax ("head" body part).  In Neoscona, the groove is a line running the length of the cephalothorax.  In Araneus, it is a dimple or angled groove.

Araneus marmoreus
Araneus

  • Araneus bicentenarius (giant orbweaver, lichen spider) - up to 1.1" body length; body color beautifully patterned in shades of black and lichen gray or green; two large humps on its abdomen just after its cephalothorax; reaches full size earlier in the year than other large orbweavers; prefers habitats with lichen   Photos from BugGuide
  • Araneus marmoreus (marbled orbweaver, Halloween spider) - up to 0.7" body length; abdomen color is typically orange, yellow, reddish, or cream, marbled with brown, purplish, or black markings; legs are banded with white and black and often reddish or orange close to the body   Photos from BugGuide
  • Araneus trifolium (shamrock orbweaver) - up to 0.8" in body length; abdomen color yellow, orange, red, or cream with white spots; legs banded with white or tan and black   Photos from BugGuide
  • Araneus diadematus (cross orbweaver, European garden spider) - up to 0.8" in body length; imported to North America from Europe; brownish coloration; white spots on abdomen forming the shape of a cross; legs banded with dark brown and light brown   Photos from BugGuide


Neoscona crucifera with finger for scale.
Neoscona

  • Neoscona crucifera (Hentz's orbweaver, barn spider) - up to 0.8" in body length; brown in coloration; a light brown cross shape can often be seen on the abdomen with several dimples toward the top of the cross; legs often reddish close to the body   Photos from BugGuide
  • Neoscona domiciliorum (spotted forest orbweaver) - up to 0.6" in body length; black abdomen with a white or tan cross-shaped marking in its center and a row of white spots along its sides; legs white and black banded, but red close to the body   Photos from BugGuide



The Active Hunters
These large spiders are not only impressive for their size, but also for their speed! Wolf spiders, and their cousins, the fishing spiders, are active hunters.  They use their long, powerful legs to overtake prey. They prowl lawns, gardens, forest floors, and even the surface of water, searching for insect meals. Their wanderings often take them inside human dwellings, where the size of the largest species, as well as their tendency to stand their ground, may cause some alarm. Both wolf spiders and fishing spiders have:
  • long, sturdy legs
  • low, rubust bodies
  • camouflaging coloration (typically mottled or streaked brown or gray)
  • a covering of hairs, especially obvious on the legs
  • and two large, forward facing eyes in the middle of the face with a row of 4 smaller eyes beneath
A Tigrosa wolf spider
WOLF SPIDERS
Wolf spiders (Lycosidae) are a very large and diverse group, ranging greatly in size, with some as small as a grain of rice. The largest species however, can have a body length of 1 inch, giving them quite a formidable appearance. The largest species in Maryland are members of the Hogna and Tigrosa genera.  These wolf spiders can be identified by their large as adult size (often around 1" in body length in adult females) and by their eye arrangement.  Unlike most wolf spiders, the lower row of 4 small eyes is as wide or wider than the two large eyes in the center of the face.

Eye arrangements of common hunting spiders
Several members of Hogna and Tigrosa are found in Maryland, however. Often, they are most easily identified by getting a look at their undersides (this can be done by shooing one into a clear drinking glass). The following species are large and fairly common.

Hogna - typically has a pale stripe running the length of the cephalothorax ("head" body part), with several pale stripes radiating outwards from its center; the underside of abdomen is typically black

  • Hogna carolinensis (Carolina hogna) – largest of the Hogna species, up to 1.4" body length; entire underside black, often with dark markings on the undersides of its legs   Photos from BugGuide
  • Hogna baltimoriana (Baltimore hogna) - 0.5" to 1" body length; underside of abdomen is black, but with a pale band on its abdomen just behind its legs; undersides of legs have dark markings   Photos from BugGuide
  • Hogna lenta (field hogna) - 0.5" to 1" body length; underside of abdomen is black, with two pale patches on its abdomen just behind its legs; undersides of legs are unmarked  Photos from BugGuide


This Tigrosa is as long as a thumb.  Notice the large central eyes. 
Tigrosa - typically has a dark cephalothorax ("head" body part) with a thin, pale stripe running between its eyes, sometimes the length of the cephalothorax; underside of abdomen typically speckled

  • Tigrosa aspersa (speckled tiger wolf spider) – 0.6" to 1" body length; dark brown with a very narrow yellow stripe between eyes; underside of abdomen orangey with black speckles   Photos from BugGuide
  • Tigrosa georgicola (Geogian tiger wolf spider) - 0.4" to 0.9" body length; pale stripe running the length of the cephalothorax; underside of abdomen darkly speckled with three dark stripes starting just after the legs and converging at the tip (spinnerets)   Photos from BugGuide
  • Tigrosa helluo (gluttonous tiger wolf spider) – 0.4" to 0.8" body length; pale stripe running the length of the cephalothorax; underside of abdomen orangey with black speckles   Photos from BugGuide


FISHING SPIDERS
Fishing spiders and nursery web spiders (Pisauridae) are relatives of wolf spiders and look very similar.  Their eye arrangement and habits are a bit different.  The eyes of fishing spiders are all very close in size, whereas the two center eyes of wolf spiders are noticeably larger.  Wolf spiders often rest with their legs bent, while fishing spiders typically rest with their legs flattened against the ground.  When carrying egg sacs, wolf spiders carry their sacs by their spinnerets at the tip of their abdomens, while fishing spiders carry them in their jaws. Finally, fishing spiders tend to be found around water, while wolf spiders may be found in all habitats, though there are exceptions.

A fishing spider molt showing the eye arrangement.
Fishing spiders (Dolomedes) are very adept at travelling on the surface of water as they search for aquatic prey.  Some may even dive for prey and their large size allows them to capture small fish and frogs!  A few species, including the dark fishing spider (Dolomedes tenebrosus), commonly travels away from water and is even encountered in homes - this often causes quite a stir as this species may be Maryland's largest spider!

Dolomedes tenebrosus showing scale and flattened stance
Five species of fishing spider call Maryland home:

  • Dolomedes albineus (white fishing spider) - up to 1.2" body length; whitish-gray in color; typically found in wetlands; may travel high up trees; may travel far from water when it matures in mid- to late-summer   Photos from BugGuide
  • Dolomedes scriptus and Dolomedes vittatus (scriptured whitewater fishing spider and striped whitewater fishing spider) - up to 1.2" body length; grayish to brownish in color; white W-shaped markings on abdomen, more pronounced in D. scriptus; two narrow dark markings in the very center of the cephalothorax ("head") of D. scriptus; two broad dark markings in the very center of the cephalothorax of D. vittatus; both tend to have pale stripes running the along the sides of their bodies, but this is more common in males than females; both typically found near clear, fast-moving, rocky portions of streams, but D. scriptus prefer sunnier spots and D. vittatus prefers shadier spots   Photos of D. scriptus   Photos of D. vittatus
  • Dolomedes tenebrosus (dark fishing spider) - up to 1.5" body length (rivaled only by Hogna carolinensis); leg span may be an impressive 4"; dark gray-brown in color with black markings; four dark W-shaped markings on abdomen; black "mask" over face; distinctly banded legs; often roams far from water and into houses   Photos from BugGuide
  • Dolomedes triton (six-spotted fishing spider) - up to 0.8" long; many parallel rows of white spots on upperside of abdomen; six spots on underside; typically has two distinctive white stripes running along the sides of its body; often black and white in color, but can also be brownish, tan, or grayish; very rarely found away from water; typically seen in still ponds, often with water lilies   Photo from BugGuide


Dolomedes tenebrosus 
Are these spiders dangerous?
The short answer is NO.  While these large spiders may be physically capable of biting a human, none have venom potent enough to be considered dangerous to humans.  A bite from one of these spiders is akin to the sensation of a bee sting. However, if you are allergic to bee stings or other invertebrate stings, seek medical attention if you receive a spider bite, as you may be reactive to spider venom.

Bites from spiders are generally very rare, particularly those of orbweavers.  Orbweavers rarely travel away from their webs, much less indoors, and tend to seek refuge or drop entirely from their webs when disturbed.  Wolf spiders and fishing spiders may roam indoors and are more likely to encounter humans.  Large wolf spiders and fishing spiders typically flee from humans, but may occasionally stand their ground if disturbed.  They may even wave their arms and rush an attacker if further provoked. In general, however, all spiders only bite if actively grabbed and squeezed against the skin.

This Araneus marmoreus just wants to stay away from you. 
Spiders are important predators of many pests and are the top invertebrate predators of many ecosystems. Even in homes or backyards that harbor many spiders, the risk of a spider bite is very low, provided spiders are treated with respect and are not roughly handled. See this post for instructions on how to catch and relocate a spider that has wandered indoors.

So what is the largest spider in Maryland?
The spider that reaches the longest body length in Maryland is the dark fishing spider (Dolomedes tenebrosus) at 1.5", with the Carolina hogna (Hogna carolinensis) coming in a close second at 1.4". The giant orbweaver (Araneus bicentenarius), with its round, fat abdomen, may be the heaviest, though I was unable to find such measurements.

Dolomedes tenebrosus
 SOURCES:
"BugGuide." BugGuide.Net. Iowa State University Entomology. Web. 04 May 2013. <http://www.bugguide.net/>.

Gaddy, L. L., and Rick Kollath. Spiders of the Carolinas. Duluth, MN: Kollath-Stensaas, 2009. Print.

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