Volume 52, Number 2, September 2005:
Checklist of Kansas Orbweaving Spiders

Text-only version

ISSUE HOME PAGE

ABOUT THIS ISSUE
- about KSN
- about the author

IN THIS ISSUE
- introduction
- 1-6
- 7-16
- the orb web, prey, and economic importance
- 17-22
- 23-28
- predators, parasites and spider defenses
- Kansas orb weavers
- argiopidae
- tetragnathidae
- references
- acknowledgements

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Checklist of Kansas Orbweaving Spiders
by Hank Guarisco

p. 6 caption - Figure 1. The orb web with frame (rim), radii (spokes) and sticky spiral. Mounted web provided by Bruce Cutter.

THE ORB WEB, PREY, AND ECONOMIC IMPORTANCE
The orbweavers of Kansas show a large diversity in body color, shape, size and pattern. The marbled orbweaver (Araneus marmoreus), one of the most attractive spiders in Kansas, builds large webs in wooded areas during late summer and early fall. Unlike many spiders which sit at the hub of the web, the marbled orbweaver remains hidden in a cone-shaped retreat made of leaves and silk situated at the edge of the web. When prey strikes the web, the spider detects the movement, rushes to the hub and determines its location, then proceeds rapidly to the prey and wraps it in silk. After a couple of quick bites, the spider continues to encase the prey in a silken swath. The prey bundle is then either left in place or carried back to the retreat to be eaten.

The spider is able to run rapidly upon the web without being entangled because it moves along the nonsticky radii and does not touch the sticky spiral, which contains glue droplets.

The construction of an orb web usually begins with the spider attaching silk to the substrate, then letting go. As gravity pulls her toward the ground, a silk thread, called a dragline, is pulled from the spinnerets. At some point, the spider produces a second silk thread which is lengthened by the wind (10). If the second line sticks to a solid object, the spider will strengthen it by adding silk as she traverses the line. Then she descends to a lower branch while pulling the loose thread, forming a Y. She returns to the center of the Y, anchors a silk thread, then climbs up a leg of the Y while carrying the loose thread. She climbs down the branch and anchors the thread to the branch. This process is continued until all the radii and frame threads are in place. At this point, the incomplete web resembles a wheel, with a hub (center), radii (spokes), and frame (rim). Beginning at the hub, the spider lays down temporary spiral scaffolding. Next, she proceeds from the edge of the web toward the center, laying down the sticky spiral and simultaneously removing the temporary scaffold, which she consumes (Fig. 1). At this point, some orbweavers (Micrathena sp.) remove silk from the hub. This allows the owner to pass easily from one side of the web to the other (34).

Members of the genera Argiope and Cyclosa add silk ornamentation in a zigzag or linear pattern to their webs (Fig. 2). This structure is called a stabilimentum because it was originally believed to provide strength and stability. This is unlikely, however, due to its location in the web. Experiments revealed that blue jays more strongly avoided webs of garden spiders containing stabilimenta than those that lacked these structures (21). Spiders in webs having stabilimenta were also better able to survive the attacks of mud dauber wasps, another group of spider predators (3). Interestingly, webs of Cyclosa conica with stabilimenta trapped more insects than undecorated webs (51). Detailed field and laboratory studies revealed that insects are attracted by the UV-reflecting quality of silk. Webs of the tropical garden spider, Argiope argentata, have stabilimenta which strongly reflect UV light (7). Therefore, these structures may have several functions, including prey attraction and defense against predators.

Most spiders, including orb weavers, are general predators which capture a wide diversity of prey. One unusual prey record involves the capture of a juvenile broad-headed skink (Eumeces laticeps) by the black and yellow garden spider (6). However, insects comprise the majority of a garden spider's diet. The European garden spider (Argiope bruennichi) is a voracious predator known to consume as many as 7 grasshoppers per day, and can reach high population densities - up to 6 per sq meter - in grassland during August and September. Theoretically, as many as 420,000 grasshoppers per hectare could be consumed in one day (43).

Although their prey often includes insect pests, assessing the importance of spiders as agents of biological control has only recently been attempted. In 1999, an international symposium on this topic concluded that spiders can be effective in suppressing pest species and improving the productivity of crops, and have been used in integrated pest control programs, for example, in China to control the brown planthopper and other insects (14). Leaving hedgerows, mulching and using other measures to increase the structural complexity of crop fields helps maintain higher spider numbers and diversity (50). The importance of orbweavers as predators of insect pests of cotton, such as the cotton aphid and cotton leafhopper, was studied in eastern Texas (44).