Sunday, October 28, 2007

science and spiders

Spiny Crab-like Orb Weaver

common name: spinybacked orbweaver scientific name: Gasteracantha cancriformis (Linnaeus) (Arachnida: Araneae: Araneidae)
Introduction - Systematics - Identification - Biology - Survey and Detection - Selected References
Introduction One of the more colorful spiders in Florida is the spinybacked orbweaver, Gasteracantha cancriformis (Linnaeus) 1767. Although not as large as some of the other common orb weavers (e.g.; Argiope, Levi 1968; Neoscona, Edwards 1984), the combination of color, shape, and web characteristics make G. cancriformis one of the most conspicuous of spiders. The colloquial name for this spider in parts of Florida is "crab spider," although it is not related to any of the families of spiders commonly called crab spiders, e.g., Thomisidoe. This species belongs to a pantropical genus which contains many species in the Old World. With the possible exception of the West Indian G. tetracantha (L.) (which may be only a geographic race), G. cancriformis is the only species of its genus to occur in the New World, ranging from the southern United States to northern Argentina (Levi 1978). The bite of this common species is not known to cause serious effects to humans.
SystematicsBecause of the variations in color and shape of the abdominal "spines" throughout its range, G. cancriformis has been described by numerous early scientists under a plethora of names (Levi 1978). Although Kaston (1978) continued the use of the name G. elipsoides (Walckenaer) 1841, resurrected by Chamberlin and Ivie (1944), Levi (1978) examined this species and found it to be a synonym of G. cancriformis.
IdentificationThis species can be easily distinguished from all other spiders in Florida. Females may be 5 to nearly 9 mm in length, but 10 to 13 mm wide. They have six pointed abdominal projections frequently referred to as "spines." The carapace, legs, and venter are black, with some white spots on the underside of the abdomen. The dorsum of the abdomen is, typically for Florida specimens, white with black spots and red spines. Specimens from other areas may have the abdominal dorsum yellow instead of white, may have black spines instead of red, or may be almost entirely black dorsally and ventrally. Males are much smaller than females, 2 to 3 mm long, and slightly longer than wide. Color is similar to the female, except the abdomen is gray with white spots. The large abdominal spines are lacking, although there are four or five posterior small humps (Levi 1978, Muma 1971).
BiologyMuma (1971) discussed the life cycle and web construction of G. cancriformis in Florida. Although males have been found in every month except December and January (Levi 1978), they are most common in October and November. Females, which are found as adults throughout the year, are most common from October through January. Mixed-mesophytic woodlands and citrus groves are where they are most frequently found. Males hang by single threads from the females' webs prior to mating, described by Muma (1971).
Ovate egg sacs, 20 to 25 mm long by 10 to 15 mm wide, are deposited on the undersides of leaves adjacent to the female's web from October through January. The egg mass consists of 101 to 256 eggs, with a mean of 169 (based on 15 egg masses). After the eggs are laid on a white silken sheet, they are first covered with a loose, tangled mass of fine white or yellowish silk, then several strands of dark green silk are laid along the longitudinal axis of the egg mass, followed by a net-like canopy of coarse green and yellow threads. Eggs are frequently attacked by specialized predators, primarily Phalacrotophora epeirae (Brues) (Diptera: Phoridae), and occasionally Arachnophago ferruginea Gahan (Hymenoptera: Eupelmidae) (Muma and Stone 1971). Eggs take 11 to 13 days to hatch, then spend two to three days in a pink and white deutova stage before molting to the first instar.
egg sac
After another five to seven days, the spiderlings acquire dark coloration. Spiderlings dispersed within a week later in disturbed laboratory colonies, but remained in the eggsacs an additional two to five weeks in the field. Spiderlings make tiny, inconspicious orb webs or hang from single strands. In the late summer and early fall, significant increases occur in both body and web size. The larger webs have 10 to 30 radii. The central disk where the spider rests is separated from the sticky (viscid) spirals by an open area 4 to 8 cm wide. There may be as many as 30 loops of the viscid spiral, spaced at 2 to 4 mm intervals. The catching area of the web may be 30 to 60 cm in diameter. Conspicuous tufts of silk occur on the web, primarily on the foundation lines. The function of these tufts is unknown, but one hypothesis suggests that the tufts make the webs more conspicuous to birds (Eisner and Nowicki 1983), preventing the birds from flying into and destroying the webs. The webs may be less than 1 m to more than 6 m above ground. The spiders prey on whiteflies, flies, moths, and beetles that are caught in the webs.
Survey and DetectionCitrus workers frequently encounter this species, and it may occur on trees and shrubs around houses and nurseries. Specimens may be easily collected in small vials, and are best preserved, as are all spiders, in 70 to 80% ethyl or isopropyl alcohol.
Selected References
Chamberlin RV, Ivie W. 1944. Spiders of the Georgia region of North America. Bulletin of the University of Utah 35: 1-267.
Edwards GB. 1984. Large Florida orb weavers of the genus Neoscona (Araneae: Araneidae). Florida Department of Agriculture and Consumer Services, Division of Plant Industry Entomology Circular 266: 1-2.
Eisner T, Nowicki S. 1983. Spider web protection through visual advertisement: Role of the stabilimentum. Science 219: 185-187.
Kaston BJ. 1978. How to Know the Spiders. 3rd ed. Wm. C. Brown Co., Dubuque, Iowa. 272 pp.
Levi HW. 1968. The spider genera Gea and Argiope in America (Araneae: Araneidae). Bulletin of the Museum of Comparative Zoology 136: 319-352.
Levi HW. 1978. The American orb-weaver genera Colphepeira, Microtheno and Gasteracantha North of Mexico. Bulletin of the Museum of Comparative Zoology 148: 417-442.
Muma MH. 1971. Biological and behavioral notes on Gasteracantha cancriformis (Arachnida: Araneidae). Florida Entomologist 54: 345-351.
Muma MH, Stone KJ. 1971. Predation of Gasteracantha cancriformis (Arachnida: Araneidae) eggs in Florida citrus groves by Phalacrotophora epeirae (Insecta: Phoridae) and Arachnophaga ferruginea (Insecta: Eupelmidae). Florida Entomologist 54: 305-310.
Author: G.B. Edwards, Florida Department of Agriculture and Consumer Services, Division of Plant Industry. Originally published as DPI Entomology Circular 308. Photographs: Lyle J. Buss and others, University of Florida Project Coordinator: Thomas R. Fasulo, University of Florida Publication Number: EENY-167 Publication Date: October 2000. Latest revision: December 2005. Copyright 2000-2005, University of Florida
end of source
This is one of my buddies at the farm. I have mentioned them in some earlier blogs, but this one is strictly about this rather small creature. The orb weavers generally make a rather beautiful web. They still argue about why they put the extra tufts of silk in certain locations on their webs. Let no one tell you that scientists agree about much of anything. There will always be a few contrarians even on the most settled of subjects and the ones which have no clear answers are full of rancor and disdain for the opposition. If there was ever a clear case of hypocrisy it is when scientists point fingers at religion and laugh at their disagreements. They are no better at agreeing on something as simple as the extra adornments of a web, much less the origin of life or its meaning. The author of the source admits that the early authors had a "plethora" of names for this arachnid, but only alludes to the idea that the West Indies version of this critter may only be a geographic species. Meaning, it is only a distinct species because of geography. Sorry, but the very concept of a geographic species flies in the face of the definition of a species. It is splitting hairs. The hard and fast definition that has been clung to is that species cannot interbreed. If my girlfriend is in NYC and I am in Texas that means that we cannot have sex. Duh. It does not mean we are not capable of reproduction, it means that there is no opportunity to do so. I have grown weary of scientists basing the idea of species on a few minute and insignificant traits that may be more pronounced in a particular location. The fact that this species has become isolated, for whatever reason, does not mean that it is a separate species. Perhaps I am willing to venture them as a variety, botanical or horticultural, but their ability to be interfertile with other species means that they are not by the lowest common denominator of the definition that they are a unique, separate species. Genus has a lot more to do with the inability to be fertile than species. Species has become specious and suspect. Oaks. I can give you oaks immediately. In any mixed hardwood forest there is a range of oak hybridization, a mingling of species. So much so that even consultants on judicial cases must testify that there may be in this individual or collection of trees the parentage of a few to several species of trees represented. If you ever write a contract for millions of dollars of trees, reserve the right of refusal for shipments and hire someone who knows what they are doing when they pick out adaptable trees.
Olive trees do not belong in North Texas. Whoever signed off on the contract to plant them should have had his ass kicked. What a waste of money.
Have I finished ranting yet? not sure. I meant to talk about the glob of eggs that one of these peculiar looking spiders had deposited on my farm truck windshield. It is a shade greener than the picture, but it has the same network of strands to hold it fast and the same general layout, size and form as the picture. I am determined not to cut it off if I can avoid it. I may have to if I am going to save it from the elements, particularly wind. I have been surprised that going down the highway at 60mph seems to have no deleterious effect. I can't say what a pounding rain is going to do. I might have to scrape it off and put it somewhere safe. I thought about sending an egg casing to someone as a prank. Hide it in a plant or some other form of Trojan horse and when the moment was ripe they would hatch out and be a parade of tiny spiders. But the thought of some idiot friend of mine stomping on them or potentially killing their dog/cat/bird/child/spouse in a fog of insecticide was not appealing. I think I will just stash them on the shelf with the other egg casing I had to rescue. I will find a home for them somewhere. Maybe I can turn them loose in a greenhouse somewhere or a good barn. They are hellacious fly and gnat catchers.

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