Sunday, May 27, 2012

Spider Sunday: Slender Crab Spiders (and their look-a-likes)

Many spiders are “built” to fit their habitat and lifestyle, and perhaps none are better examples than the slender crab spiders of the genus Tibellus, family Philodromidae. They hunt by lying in ambush on grasses or foliage. Their distinctive long body and vertical striped pattern camouflage them almost perfectly. They are not alone, however, as several other spiders share similar coloration and it is easy to confuse them all.

Pictured above is a Tibellus I found in North Cheyenne CaƱon, Colorado Springs, Colorado, earlier this week. Had it not been on a green leaf, I probably would have overlooked it easily. Note that this is not a particularly large spider. There are seven species of Tibellus in North America north of Mexico, and adult females collectively range from 6-11 millimeters in body length. Males are slightly smaller, 5-8 millimeters. Pictured below is a juvenile nursery web spider, Pisaurina mira. It will eventually become much larger, as adult females are 12.5 to 16.5 millimeters, males 10.5-15 millimeters. These spiders prowl the same habitats as Tibellus, and adopt a similar resting posture. Pisaurina are more robust in their overall appearance, however, a bit hairier, and have a different eye arrangement.

Another genus that may be confused with slender crab spiders is Thanatus, also in the family Philodromidae. The first two images in my ”Spring Spiders” post are of a specimen of Thanatus. Note the more robust appearance and the difference in the pattern of stripes compared to Tibellus. Both spiders are of similar size, but Thanatus hunts mostly on the ground amid grasses and weeds, whereas Tibellus is almost always found on stems, stalks, grassblades, or leaves.

Lastly, the Striped Lynx Spider, Oxyopes salticus, can sometimes be mistaken for Tibellus. Below is an example I found near Chicago, Illinois. Note the very long spines on the legs, and the more scrunched-up posture as it waits in ambush on a grass stem. Lynx spiders have a longer “face,” too, whereas slender crab spiders have an overall more flattened appearance.

Female slender crab spiders are good mothers. They spin a flattened egg case which they guard religiously until the spiderlings emerge. Here is one I found in Tucson, Arizona. I maneuvered the grassblade several times for better light exposure and she never even flinched.

Do keep an eye out for these amazing grass-mimics. You may find still other spiders, even orb weavers, with nearly identical shapes and patterns in the process of looking for Tibellus. Try using a sweep net if you can’t find one otherwise. You’d be astonished at the diversity of arthropods in a “disturbed habitat” like a vacant lot.

Sources: Balaban, John and Jane, et al. 2004. “Genus Tibellus - Slender Crab Spiders,” BugGuide.net
Kaston, B. J. 1978. How to Know the Spiders (3rd ed.). Dubuque, Iowa: Wm. C. Brown Company Publishers. 272 pp.
Ubick, D., P. Paquin, P. E. Cushing, and V. Roth, eds. 2005. Spiders of North America: an identification manual. American Arachnological Society. 377 pp.

Wednesday, May 23, 2012

Wasp Wednesday: Symmorphus

Sometimes I think I could do “Wasp Wednesday” for years just focusing on the subfamily Eumeninae (potter and mason wasps) of the Vespidae (social paper wasps, hornets, yellowjackets; and the solitary mason wasps). The diversity is so great that it seems impossible to exhaust them as subject matter. It helps that they are also amazing in terms of behavior; and common enough that virtually anyone can find them. Such is the case with the genus Symmorphus.

I had to double-check to make sure that the male wasp I recently photographed at Fountain Creek Regional Park in Colorado was indeed a species of Symmorphus. Eumenid wasps are challenging to identify even to genus. I find that most specimens of Symmorphus are small (roughly 6-10 mm), and slender compared to most other mason wasps. This is especially true of the males.

The defining characters of the genus are harder to see, however. The first dorsal (top) segment of the abdomen has a horizontal ridge (“carina” in scientific terms) about halfway down its length. That same segment also has a shallow vertical groove. Males have the last segment of the antennae simple, whereas in other genera of eumenids this last segment may be hooked or coiled.

Four species of Symmorphus occur in North America: S. albomarginatus ranges from Alaska to California, New Mexico, Quebec, and much of the eastern U.S. S. canadensis, our smallest species, is transcontinental except for Florida, Texas, New Mexico, and Arizona. S. cristatus is equally widespread, from Canada southward. S. projectus is strictly western, from the Pacific Northwest to Montana, Wyoming, and southward.

Mason wasps are our friends, for they prey on larval insects that can defoliate trees and shrubs if not culled by predators. The female wasps hunt the caterpillars and larvae and store them as paralyzed victims for their offspring to eat. Symmorphus wasps are specialists on the larvae of leaf beetles, weevils, or leaf-mining moth caterpillars.

The female wasp uses a pre-existing cavity in wood, or a hollow twig or stem for her nest. She partitions the tunnel into several cells placing a curtain of mud between each cell. She starts from the back and works forward. She first lays an egg in the cell, then begins hunting prey. Symmorphus are remarkably efficient in provisioning their nests. One 7-cell nest of S. canadensis was completed in about three days (Krombein, 1967).

The number of prey larvae per cell depends on the size of the prey, and in part on the gender of the wasp’s offspring. Female eumenid wasps can control the gender of each egg they lay. Larvae of female wasps generally require more food than male offspring.

Symmorphus often leaves an empty (“vestibular”) cell upon completion of the nest. This dummy cell may be an attempt to fool parasitic insects into thinking they have found an entirely empty nest. Some nests also have “intercalary” cells that are also vacant, between a series of provisioned cells. Despite this, nests are frequently parasitized by cuckoo wasps (family Chrysididae), eulophid wasps (genus Melittobia, family Eulophidae), a torymid wasp in the genus Monodontomerus, and satellite flies (genus Amobia, family Sarcophagidae). The Grain Itch Mite, Pyemotes ventricosus can infest nests; and scavengers like humpbacked flies (family Phoridae) and carpet beetles (family Dermestidae) may destroy wasp eggs in the process of consuming the stored prey intended for the wasp’s offspring.

Symmorphus is one of those wasps for which you can provide housing in the form of holes drilled in wooden blocks and placed in sheltered situations like the stack of firewood, or under an eave on your home or garage. They favor holes 3-4 millimetes in diameter, and probably no more than six inches deep. Try it. They will help keep your trees free of leaf beetle larvae in exchange for free rent.

Also look for these wasps around aphid colonies. Like most eumenids, and other wasps, they like the sweet, liquid waste secreted by aphids. Called “honeydew,” this sticky substance is like manna from heaven to many kinds of insects.

Sources: Buck, Matthias, Stephen A. Marshall, and David K.B. Cheung. 2008. “Identification Atlas of the Vespidae (Hymenoptera, Aculeata) of the northeastern Nearctic Region,” Canadian Journal of Arthropod Identification No. 5. Biological Survey of Canada. 492 pp. (PDF)
Budriene, Anna. 2003. “Prey of Symmorphus Wasps (Hymenoptera: Eumeninae) in Lithuania,” Acta Zoologica Lituanica 13(3): 306-310
Cumming, Jeffrey M. 1989. “Classification and Evolution of the Eumenine Wasp Genus Symmorphus Wesmael (Hymenoptera: Vespidae),” Mem. Entomol. Soc. Can. 121(148): 1-168.
Krombein, Karl V. and Paul D. Hurd, Jr., et al. 1979. Catalog of Hymenoptera in America North of Mexico (vol. 2). Washington, DC: Smithsonian Institution Press. 1199-2209.Krombein, Karl V. 1967. Trap-nesting Wasps and Bees: Life Histories, Nests, and Associates. Washington, DC: Smithsonian Press. 570 pp.

Sunday, May 20, 2012

Spider Sunday: Spitting Spiders

Heidi and I recently spent several days in South Carolina, mostly looking for dragonflies at a gathering of the Dragonfly Society of the Americas in Florence. We stayed with another couple at their home in Hemingway, and it was there that I encountered one of the strangest of all spider families: the Scytodidae, known commonly as “spitting spiders.”

The last night of our stay, I once again prowled around the home of our hosts. Along with the moths and other insects coming to the light on the back porch, I noticed a rather large, sprawling spider at the edge of a window frame. I was puzzled by its uniform beige color, and long legs. I mentally paged through my brain’s spider files. Was it a cellar spider? Not quite right. Was it one of the dangerously venomous brown spiders in the genus Loxosceles? My eyes are not what they once (ok, ever) were, so it was not until I took an image of the critter and zoomed in that it became instantly apparent what it was.

One can hardly be blamed for mistaking a spitting spider for a brown recluse or related species. The two families, Scytodidae and Sicariidae, are not that far removed from each other anyway. Spiders in both families have only six eyes, instead of the usual eight for spiders. Those eyes are arranged in a nearly identical triad pattern of three pairs. Spitting spiders frequently have dark markings on the cephalothorax, though usually not in a “violin” of “fiddle” shape. Both families are in the larger group of haplogyne spiders which possess relatively uncomplicated genital structures.

Perhaps the best way to recognize a spitting spider is by the very convex shape of its cephalothorax. This body section has to be large and domed to accommodate the arachnid’s enormous venom glands. Don’t jump to conclusions, the spitting spiders are not at all dangerous to people. However, insects, and even other spiders, should take heed.

The venom glands of scytodids are divided into two lobes, the front lobe of which houses traditional venom designed to paralyze the prey and begin the digestive process. The hind lobe of the venom glands contains large amounts of what can only be described as the arachnid equivalent of Krazy Glue.

Spitting spiders stalk potential prey cautiously, with slow, deliberate movements. When the victim is within a few centimeters, the spider lets go with a rapid-fire barrage of its sticky venom. The victim is instantly adhered to whatever surface it was sitting on. Seriously, the attack lasts a total of thirty milliseconds. No, I am not making this up. I swear, there are videos to prove it.

The “glue” is very similar to spider silk in that it solidifies into elastic threads that contract up to 50%, helping to bind the prey tightly. The threads form a zigzag pattern as a result of rapid oscillation of each jaw. The glue is shot out of each fang.

Interestingly, the long-legged species of spitting spiders also spin webs, in the traditional manner, with silk from the spinnerets at the tip of the abdomen. The webs are tangled and sheet-like, similar to those of cellar spiders (family Pholcidae). Individuals may be quite tolerant of each other, even living communally in some cases.

I found three specimens on the exterior of the house in South Carolina, and apparently several species in the genus Scytodes are often found associated with human habitations. I have not positively identified which species this is, and of the seven known species in North America, at least three are introduced from the tropics. There are also very likely more species that remain undescribed by scientists.

Female Scytodes do not build an elaborate egg sac, but simply wrap their eggs loosely in a thin sheet of silk and secure the package beneath their bodies. An average clutch varies from 20-40 eggs.

Incidentally, there are also “short-legged” species in the Scytodes genus. They tend to be found on the ground in leaf litter, or under rocks. They are strictly wandering hunters that do not spin webs.

The most widely-distributed spitting spider in North America is Scytodes thoracica, an ornately-marked animal that ranges from Canada south, west to at least St. Louis, Missouri and Ames, Iowa. This is a small-bodied spider, females measuring 4-6 millimeters, and males about 4 millimeters. The long legs make them look larger, of course. The specimens shown here were about 6-8 millimeters in body length, and may represent Scytodes longipes, one of those exotic imports mentioned earlier.

Sources: Foelix, Rainer F. 2011. Biology of Spiders (3rd Ed.). Oxford: Oxford University Press. 419 pp.
Ubick, D., P. Paquin, P. E. Cushing, and V. Roth, eds. 2005. Spiders of North America: an identification manual. American Arachnological Society. 377 pp.
Valerio, Carlos E. 1981. “Spitting Spiders (Araneae, Scytodidae, Scytodes) from Central America,” Bull. Amer. Mus. Nat. Hist. 170: 80-89.

Wednesday, May 16, 2012

Wasp Wednesday: Netelia

Surprisingly, you may find some wasps at your porch light after dark. Among the more common nocturnal hymenopterans are ichneumon wasps in the subfamily Ophioninae. They are large, gangly wasps, usually uniformly orange in color with long antennae and large ocelli (“simple eyes” arranged in a triangle at the crown of the head, between the compound eyes). The ovipositor is very short, if it is even evident at all.

If you see a slightly smaller orange ichneumon with a longer ovipositor, it is likely to be a wasp in the genus Netelia, in the subfamily Tryphoninae. There are currently 73 species in six subgenera in North America north of Mexico (Carlson, 2009).

Unlike many ichneumon wasps, the females of Netelia can sting painfully if handled carelessly. The sting is mostly used to temporarily paralyze the large caterpillar hosts of these parasites. The female then lays an egg on the stunned victim, puncturing the body wall of the caterpillar in doing so. The egg is stalked, and in newly deposited eggs the coil of the stalk is elastic. It later becomes rigid. The egg holds firmly to the flexible exoskeleton of the host larva by means of a plug or anchor.

The larval wasp that hatches from the egg remains attached to it via specialized bristles on its posterior end. The wasp larva feeds on the caterpillar as an external parasite. Parasites that do not arrest the development of their host, but allow it to grow normally instead, are called “koinobionts.” Netelia ichneumons are placed in this category.

Below is an example of an ichneumon in the subfamily Ophioninae for comparison to Netelia.


Note in particular the absence of an obvious ovipositor at the end of the abdomen. More on the Ophioninae in a later edition of “Wasp Wednesday.” Look for both species coming to outdoor lighting near you!

Sources: Carlson, Robert W. 2009. "Database of Hymenoptera in America North of Mexico: Superfamily Ichneumonoidea, Family Ichneumonidae," Discover Life
Goulet, Henri and John T. Huber, eds. 1993. Hymenoptera of the World: An identification guide to families. Ottawa, Ontario: Agriculture Canada. 668 pp.
Kasparyan, D. R. 1989. Fauna of the USSR: Hymenoptera vol. III. Brill Archive. 414 pp.