Fall is for pumpkin spiders

You have probably noticed a lot of these critters around town lately; they seem to be just about everywhere this time of year. These eye-catching little beasts go by many names, including “Cross Orb-Weaver,” “European Garden Spider,” and “Pumpkin Spider,” among others,1,2 but their proper scientific name is Araneus diadematus. (Note: common names like “Garden Spider” or “Pumpkin Spider” may refer to a number of different spider species, which is understandable given that there are an estimated 47,000 (or more)3 spider species on earth! Since we have not come up with 47,000 unique common names for all of these species, it is best to use scientific names to avoid confusion.)

Araneus diadematus are tan, brownish or orange, with yellow, white and black markings and a distinctive “T” or cross pattern on the back of their spade-shaped abdomen. Adults can range from about 6-20 mm in body length, not including the legs.4 Females are much larger than males and have been known to cannibalize hopeful suitors who approach them to mate.5 You are most likely to find these spiders on their characteristic “orb webs”: those beautifully geometric, Charlotte’s Web-style constructions of spiral-shaped silk supported by spokes radiating from a central hub. This web shape is typical of the spider family Araneidae (“orb weavers”).

Although A. diadematus can be found in the Bay Area throughout the year, their numbers and visibility peak in the fall because this is when they reach maturity. They are not native to this region, they originated in Europe, but their range now extends across the U.S. and into many parts of Canada.1 A. diadematus can occur at remarkably high densities in both natural areas (like woods and parks) and urban environments. Whether they truly qualify as “invasive” (which, by definition, would require having a negative impact on the environment) is debatable, but one thing is for sure: they are enormously successful here.

So what are they doing on those lovely spiral webs? Mostly lying in wait for prey, which consists of insects like flies, beetles and aphids.6 The spiral on the web is made of sticky silk, and when unwitting insects blunder into this structure, they get stuck. The insects struggle, creating vibrations through the web. The spiders, although they have poor eyesight, are incredibly sensitive to vibrations, and by resting in the center of the web – where the “spokes” converge – the spiders can detect exactly where to find the prey based on which silk strands are vibrating.7 The spider then uses a combination of biting and silk-wrapping to immobilize the hapless prey before settling down to eat. Since this whole process damages the web, A. diadematus builds a fresh web each day, eating the old one to recycle the silk.

Some people may find A. diadematus creepy or frightening, and these spiders do look formidable when they are fully grown (not to mention that we usually see a lot of them around Halloween!). However, there is no reason to fear these autumnal creepy crawlies: they are completely harmless. Although they do possess venom – as do almost all spiders – the venom’s function is to paralyze insects, not to hurt large vertebrates like us. And while a bite from one of these guys might be painful and cause a small localized reaction8 (much like a bee sting), the chances of being bitten are so slim that it is a non-issue. These spiders are not the least bit aggressive; they like to keep to their webs and mind their own business, and would only bite as a last resort (i.e. if they were about to be squished). So next time you walk through a huge web that a resident A. diadematus thoughtfully constructed right in front of your door, do not panic – the spider is running for the hills as we speak. If you are lucky, your little 8-legged buddy might stumble into your hands and let you take a closer look!

by Susan Kennedy

See also common Bay Area spiders.

References

  1. “Cross Spider.” Washington NatureMapping Program, naturemappingfoundation.org/natmap/facts/cross_spider_712.html.
  2. Bowline, Mariah. “Pumpkin Spider – Araneus diadematus.” BugGuide, Iowa State University Department of Entomology, 2005, bugguide.net/node/view/33907/bgpage.
  3. “World Spider Catalog (2017).” World Spider Catalog. Natural History Museum Bern, wsc.nmbe.ch, version 18.5, accessed on 12 September 2017. doi: 10.24436/2
  4. Araneus diadematus (Cross Orbweaver).” Spiders.us, 2017, spiders.us/species/araneus-diadematus/.
  5. Elgar, M. A. and D. R. Nash. 1988. Sexual cannibalism in the garden spider. Animal Behaviour 36(5): 1511-1517.
  6. Ludy, C. 2007. Prey selection of orb-web spiders (Araneidae) on field margins. Agriculture, Ecosystems & Environment 119(3): 368-372.
  7. Landolfa, M. A. and F. G. Barth. 1996. Vibrations in the orb web of the spider Nephila clavipes: Cues for discrimination and orientation. Journal of Comparative Physiology A: Sensory Neural and Behavioral Physiology 179(4): 493-508.
  8. McKeown, N., R. S. Vetter, and R. G. Hendrickson. 2014. Verified spider bites in Oregon (USA) with the intent to assess hobo spider venom toxicity. Toxicon 84: 51-55.

 

Sulawesi Expedition 2016

The island of Sulawesi (aka. Celebes) is nestled between Borneo, Papua, and the Philippines in a region of Indonesia known as Wallacea. It was formed by a collision of the Asia plate, Australian plate, and a system of island arcs, each contributing their fauna and flora. The resulting complex biogeographical patterns were a fascination to Alfred Russel Wallace and continue to be for modern biologists. Thanks to an NSF Biodiversity: Discovery & Analysis grant (DEB 1457845) led by Jim McGuire of the Museum of Vertebrate Zoology (MVZ), and co-PIs Rauri Bowie (MVZ), Rosemary Gillespie (Essig), and Susan Perkins (AMNH), a multi-taxon team of biologists will survey an elevation gradient across nine volcanoes distributed throughout the island.

The first expedition in July-August 2016 was to Latimojong in the northern part of South Sulawesi. Six researchers from the Berkeley Natural History Museums: Peter Oboyski (Essig), Jim McGuire, Rauri Bowie, Luke Bloch, Alexander Stubbs, Jeff Frederick (MVZ),  along with Heidi Rockney (SF State) met with colleagues from Australia and the Indonesia Research Center for Biology in Bogor, West Java to organize the inaugural trip. Permitting and other paperwork took two weeks onsite, and this was the expedited processing! Fortunately we were able to visit the Museum Zoologicum Bogoriensis and the Bogor Botanical Garden, and buy field supplies while we waited. The entomology collection is extensive with butterflies and other macrolepidoptera, beetles, bugs, bees, and flies well represented. Not so well represented are microlepidoptera and spiders – two of the targets of this expedition. The facilities are modern with good climate control. The well-educated and knowledgeable curators and staff prepare and database accessions, conduct research, and train students. Although there were some language barriers, there were enough people from each team that spoke the others’ language, which greatly facilitated our collaboration.

Two weeks later, over 20 biologists flew from Jakarta (Java) to Makassar (Sulawesi) to purchase final supplies and make the ten hour drive to the town of Belopa where we spent the night. The next morning we made our final push, by 4WD vehicles to the village of Gamaru – our first field camp. The village, at 1350m elevation, included ~20 simple wooden houses within a matrix of coffee plantations, small gardens, ponds, and secondary native forest. Our home for the first few days was the house of one of the village leaders, while a team of local villagers begin building our field camp and porting gear and supplies.

The first week was rainy and muddy, but the new moon was optimal for UV light collecting. On the first night, a sheet hung under the house with a view into the valley below was one of the best nights of moth collecting.

The next camp was at 1730m along a trail leading from Gamaru village. More secondary forest with a lot of invasive plant species along the trail, but native forest off the trail. We set up pitfall traps, malaise tents, and sifted leaf litter to put in winkler funnels at 100m intervals from 1800 to 2400m, but did not acquire many specimens (possibly due to rain or poor timing). Hand collecting for spiders proved very productive. The highest field camp (2800m) was in a dwarf forest / bog along the upper ridge of the mountain – a distinctly different habitat with a unique insect fauna.

The last few days of the three-week field operation was spent in another village at 800m, which was in the process of being cleared for agriculture and development. Remnants of native forest remained and collecting was generally good. But it was clear that this area would soon be completely converted to agriculture, like many rural areas. After returning to Bogor and the museum, it was time to count specimens for export permits and pack our bags.

More images of Sulawesi Lepidoptera can be found at our Flickr site. We can use your help in identifying these species.

  • by Peter Oboyski