Linda Crampton is a writer and former science teacher with an honors degree in biology. She enjoys writing about science and nature.
Introduction to Joro Spiders
Female Joro spiders are large and attractive animals with yellow and blue-green stripes on their upper body and striped legs. The males are much smaller and are brown. The size and colour of the females are not their only interesting features. They create dense, three-dimensional webs that coat objects, including porches, mailboxes, and power lines. The webs are often shaped like a basket and are said to be up to three metres (ten feet) deep. The constructions are so big and common in some areas that they are a nuisance. The spiders are venomous, but their bite is weak and unlikely to cause major problems in humans.
Joro spiders are native to Asia but have spread to North America. They were first noticed in Georgia in 2014 but may have arrived in 2013. They have become especially noticeable in the state. They can also be found in part of North Carolina. According to some scientists, the spiders are likely here to stay and may increase their range. Some researchers suspect that they may have already spread to other states and haven't yet been discovered.
Spiders aren't insects. Insects belong to the phylum Arthropoda and the class Insecta. Spiders belong to the phylum Arthropoda and the class Arachnida.
Classification and Common Name Origin
The Joro spider has the scientific name Trichonephila clavata. It's an indigenous species in Japan, China, Taiwan, and Korea. The animal was officially moved to the Trichonephila genus in 2019. Readers may sometimes see it referred to by its former name of Nephila clavata.
The animal belongs to the family Araneidae, or the orb-weaver spiders. The web created by these spiders is generally a flat, spiral wheel. As mentioned above, the Joro spider's web is deep instead of flat. It's often said to have a golden colour as it reflects sunlight. This sounds attractive, but it's probably an insignificant fact for people who have a large and annoying population of the spiders in their neighbourhood.
The word "Joro" is capitalized in the spider's common name because it comes from the name of a creature called the Jorōgumo. In Japanese folklore, a Jorōgumo is a spider who can turn itself into a beautiful woman. The woman traps men in her web and then eats them. The spider that transforms itself is often said to be the Joro one.
Body Features of the Arachnids
One way to tell the difference between insects and spiders is to look at their external body structure. An insect's body has three sections: head, thorax, and abdomen. A spider's body has only two: the cephalothorax and the abdomen. In addition, an insect has six legs and a spider has eight.
Joro spiders exhibit sexual dimorphism. The term means that the males and females look different, as shown in the photo below. In the case of the spider, a person who hadn't heard about the animal before would think that the male and female were different species. The female's body size (not including her legs) is 17 to 30 mm. The male's body is 4 to 8 mm long. The males can often be seen within the females' webs during autumn, which is the mating season.
The genders differ in colour as well as size. The female has a bright yellow upper surface decorated by blue-green bands. She has the same colours on her undersurface with the addition of a red patch near the end of her abdomen. Her legs are black with yellow or yellow-orange bands. The male is generally brown, but he does have some variation in colour, including stripes on his legs. The stripes can be seen if a male's body is examined closely in the photo below.
Diet and Obtaining Food
Most spiders are carnivorous, but some are omnivores. The Joro spider is one of the carnivorous ones. Researchers have learned that there's at least one way in which the spider may be helpful for us. It traps and eats brown marmorated stink bugs. These insects are major crop pests. The spider catches additional insects, including mosquitoes and other species that bite us.
Orb weavers digest their food externally. Once the trapped prey is quiescent, the spiders release digestive enzymes into its body and then suck up the digested food. The process is repeated until an indigestible husk (if one exists) is left. The last stages of preparing the food for absorption happen inside the spider's body. Even when a male spider doesn't build a web for trapping prey, he uses the same digestive process to prepare the food that he finds for use in his body.
Joro spiders are thought to have been accidentally transported to the United States in shipping containers.
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Life Cycle of the Spider
In their native habitat in Japan, the life cycle of Joro spiders follows the steps described below. The cycle seems to be roughly the same for the North American population.
- The eggs survive in cocoons during the winter.
- The eggs hatch to produce spiderlings in June.
- The male spiderlings are mature by August.
- Females become mature in September and early October.
- After mating has occurred, the fertilized eggs are laid in late October and November.
- By late November, the adults have died.
The spinnerets of a spider are connected to internal silk glands. Each spinneret contains multiple spigots, or silk outlets. Silk emerges from the silk glands as a liquid and leaves the spinnerets as a solid. The males of many species use silk released by spigots near their gonopore (genital opening) to build a sperm web.
Reproduction in the Species
The female Joro spider's web may contain several males. More research is needed to understand all of the differences in the lives of the females and males. It seems that more attention has been paid to the female spiders. It is known that at least in North America a male can drift into a female's web attached to a line of silk that he released and that was propelled by air currents. The process is called ballooning.
A scientist interviewed in one of the references below says that like some other male orb weavers, the Joro male builds a sperm web. This is a very small web on which the spider deposits sperm. The male picks up the sperm with his pedipalps in order to insert the cells into the female. The pedipalps are a pair of short appendages near the mouth. They sometimes look like little legs, but they are used as sense organs and to manipulate objects instead of for locomotion.
Many males seem to wait until the female is distracted in some way before they insert the sperm, which reduces the chance that they will become part of the female's diet. The sperm is deposited in an opening under the female's abdomen called the epigyne. It's shown in the illustration above, as are the pedipalps.
Effects on the Environment and Human Lives
None of the references that I've examined say that Joro spiders are dangerous for humans, except perhaps in the rare case in which someone is allergic to the venom and some of the venom is able to enter their body. The reports of the sensation created by a bite vary. Some researchers say the bite is too weak to pierce the skin. Some people say that they do feel the bite, but it's more like a pinch than a bite. I've read only one report saying that a bite may feel like a bee sting.
It's not a good idea to deliberately handle the spiders or to treat them roughly if they are handled, which would increase the probability of a bite. The animals aren't aggressive, but understandably they do try to protect themselves when necessary.
The University of Georgia says that the Joro spiders don't compete with native spiders for food. In addition, they say that the fact that they eat brown marmorated stink bugs is a major advantage. The biologists do admit that the large number of Joro spiders in some areas and the copious webs that they create can be a problem for us, however. Some people talk about getting their face covered by web as they go outside on their property, which must be an unpleasant sensation.
For some reason, the population of the arachnids greatly increased in 2021. This brought the problems that the animals can cause to the forefront and suggested that we don't fully understand the factors affecting their numbers. One University of Georgia scientist says he killed 300 of the spiders on his property in 2021. He does say that the animals are not a threat to humans, even though they can be a major nuisance.
I suggest that anyone who is interested in exploring the latest news about the spiders look for reputable sources, such as articles written by scientists who study the animals or containing quotes from them. Some reports in the popular press refer to "giant venomous spiders"', which makes the animals sound more dangerous than they really are.
Future of the Spiders in North America
The spiders are sometimes said to be invasive. The word "invasive" has a connotation of "harmful". Many of the reports by scientists that I've read say that it's unlikely that the species will be harmful beyond the possible nuisance of its webs. Other reports say that we shouldn't make assumptions that the presence of the animals is harmless and should monitor the situation carefully. The species is expected to spread, like other invasive creatures. It can survive in cooler climates than the ones in its present North American habitats.
Some scientists suspect that eventually a local predator will learn that the arachnids can be a good source of food and then help to keep their population in check. Others are not so sure that this will happen. It will be interesting to see what becomes of the Joro spider population in the United States and to explore the effects of the animals over time. It's an interesting species.
- Nephila clavata newly recorded from North America by E. Richard Hoebeke et al, PeerJ journal
- Joro spiders are here to stay from the University of Georgia
- Spider invading Georgia from Newsweek
- The Joro spider in the United States (including an interview with a scientist) from How Stuff Works
- Invasive Joro spiders spining golden webs in Georgia (including an interview with the director of the Georgia Museum of Natural History) from Treehugger
- A written discussion between two scientists with opposing views about the spider from a radio station run by the University of Georgia
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2021 Linda Crampton