Why Spider Webs Trap Dragonflies: A Closer Look
Spiders are famous for constructing the most perfect passive insect traps the world has ever known – spider webs. Perfected over millions of years of evolution, spider webs capture and restrain a wide variety of flying insects.
On the other hand, dragonflies are relatively large, very powerful flying insects, and often share habitat with web-building spiders. Do spiders eat dragonflies?
Large orbweaver spiders prey on dragonflies. Dragonflies get caught in large, vertical spider webs. Sticky web silk trap the insects; struggles to escape worsen the dragonflies’ entanglement. Eventually, the resident spiders kill and consume the dragonflies.
However, this answer is only the start of a bigger, more interesting story. Read on for the details about the spiders that eat dragonflies.
Which spiders eat dragonflies?
Spiders classified in the orbweaver guild spin stereotypical spider webs which trap dragonflies. Most species are classified in family Araneidae, although some belong to the Nephelidae and Tetragnathidae families. Known as “orb-weavers”, these spiders construct large, sturdy, vertical capture nets out of sticky spider silk. They suspend their webs across prime dragonfly flight paths – gaps in vegetation, often near water.
Orb-weavers prey on whatever insects their webs catch. They don’t target dragonflies specifically; their webs are passive traps that catch and hold at random. However, dragonflies provide orb weaver spiders a great deal of nutrition when consumed, thanks to their large size.
Orb weaver spiders range in size from tiny to some of the largest spiders found in the southeastern United States.
Marbled Orbweaver Spider (Araneus marmoreus)
Marbled Orbweaver spiders, Araneus marmoreus, live throughout the southeastern United States. Females grow larger than males, and can reach 18 mm (0.7 in) – big enough to span an American quarter coin. Also known as Halloween spiders, these bright orange, yellow, black, and white spiders build large webs capable of capturing dragonflies.
Yellow Garden spider (Argiope aurantia)
One of the more common large spider species in the eastern United States, Yellow Garden Spiders are unmistakable.
Females are much larger than males. They grow to 28 mm (1.1 inches) in length, with very long legs and bright yellow and black abdomens. Fine gray hairs cover their cephalothoraxes like fur. Males are rarely seen.
Yellow Garden spiders build large circular orb webs up to a meter in diameter. They wait head down in the webs’ centers for insects to impact the sticky strands. Argiopes dash out and paralyze their prey by biting the backs of their heads. They often save their meals for later by wrapping the unfortunate insects in tight spider silk shrouds.
Insects other than dragonflies make up the majority of Argiope prey. These include bees, wasps, grasshoppers, beetles, butterflies and moths (Carrel and Deyrup, 2019). But these spiders are more than big enough to successfully catch and kill dragonflies.
Golden Silk Orbweaver spider (Nephila clavipes)
Nephila clavipes spiders win the prize for the biggest orbweaver species in the southeastern United States. People call these spiders Golden Silk Orbweavers, or Banana Spiders.
As with its smaller orbweaver cousins, female Golden Silk spiders grow much larger than the males – up to 10 times as large. Just a female’s body alone can reach 45 mm (1.8 in), and their legs are much, much longer. Bright and colorful, yellow spots speckle their curved orange abdomens. Distinctive tufts of black fur sprout from the joints of their yellow legs.
These gargantuan orbweaver spiders construct truly enormous webs that can span almost 2 meters (6 feet) in diameter. All orb webs consist of sticky threads that trap insects on contact. But the web strands of the Golden Silk Orbweaver’s webs are especially sticky. The web itself is strong enough to easily trap large, powerful insects like dragonflies.
How do spiders catch dragonflies?
Vertical orb webs trap dragonflies as they fly through their territories. The spiders suspend their webs in midair from non-sticky support lines. The lines attach to vegetation or other structures many feet apart. A round, flexible, open circle of sticky strands forms the capture net. The spiders spiral out from the central hub and attach the sticky strands to the webs’ support spokes. The sticky web strands trap insects on contact.
Spiders construct their insect capture webs from spider silk, and are capable of spinning several different types of silk with different characteristics at will.
Spider silk is extremely strong and stretchy. It is capable of absorbing and muting significant force without breaking, such as the impact of a large dragonfly colliding with the web. In fact, spider silk is so strong, a pencil thin strand would be strong enough to stop a 747 jet plane in midair (Ring and Carde, 2009).
The capture area of spiral webs vary significantly across different species of orb weavers. Bigger webs catch more dragonflies.
For example, female Argiope aurantias are consistently heavier than Argiope florida spider females and have longer legs. Researchers James E. Carrel and Mark Deyrup found that A. aurantia females built webs an average of 1.3 – 1.5 times wider and taller than those of A. florida spiders, resulting in a 1.9 times greater effective capture field. Of the two spider species, only the webs constructed by Argiope aurantia spiders snared dragonflies (Carrel and Deyrup, 2019).
What happens to dragonflies caught in spider webs?
Most spiders kill insects trapped in their webs by injecting venom. Small spiders may leave dangerously large prey, like dragonflies, to die of exhaustion or dehydration. But spiders eat every dragonfly their webs catch eventually.
Spiders specialize in killing and consuming insects. Orb weaver spiders specialize in killing and consuming those flying insects which become trapped in their webs. However, an entangled dragonfly may still be mobile enough to injure a spider in self-defense.
Dragonflies are strong insects, and heavier than most orb weaver spider species. Possible exceptions are smaller dragonfly species such as Eastern Amberwing dragonflies, Perithemis tenera, when compared to the larger orbweaver spiders, like Yellow Garden spiders (Argiope aurantia) and Golden Silk Orbweaver spiders (Nephila clavipes).
Dragonfly mouth parts are powerful enough to bite through spider carapaces easily, and these insects sport long legs lined with sharp spikes. Therefore, spiders must be cautious in how they go about killing dragonflies snared by their webs.
Spiders may kill dragonflies quickly
A spider may attack a dragonfly caught in its web immediately. Spiders bite with venomous fangs. The venom liquefies the insects’ inner body tissues, which spiders then suck out.
Large spiders and especially hungry spiders tend to attack and subdue their prey immediately. They may attack dragonflies held only lightly by the web quickly as well.
Spiders may mummify dragonflies
Other times, spiders confronted by dragonflies trapped in their webs may protect themselves from injury by wrapping the insects in silk. These spiders essentially mummify the hapless dragonflies in many strands of strong, thin silk, rendering them helpless.
Spiders who are relatively large in relation to the dragonfly may opt to administer the killing bite through the silk bindings immediately. Smaller individuals, and those who are less hungry, may wait for the dragonfly to die of exhaustion or dehydration before beginning to feed.
Spiders may wait for dragonflies to die
Sometimes, spiders opt to let their trapped prey die on their own.
Dragonflies caught in spider webs know they’re in mortal peril. They struggle to escape and often exhaust themselves in the attempts. Spiders rely on the strength of their webs to keep dragonflies from escaping. They can be patient while the insects struggle. Trapped dragonflies will eventually die, at which time the spiders can safely feed without risk.
Small spiders, and those who’ve eaten recently may favor this approach.
Why can’t dragonflies just avoid spider webs?
Dragonflies have superb vision across visual fields of nearly 360 degrees, and can see movement, color, polarized, and ultraviolet light (Dunkle, 2000). They are also incredibly agile in flight.
So, if spider webs are so dangerous to them, and they can both see and fly extremely well, why do dragonflies fly into webs in the first place? Why don’t they just avoid spider webs completely?
Dragonflies may fail to see spider webs because webs are transparent in visible light, blend into the background in ultraviolet light, and are often suspended in mid-air across wide expanses of space. They may also collide with webs when distracted by pursuing prey or fleeing from other predators.
Spider webs are transparent in visible light. A web is essentially invisible unless the sun shines off the strands at just the right angle, or the strands are covered with dew.
Some spiders manipulate the visibility of their webs in the ultraviolet spectrum.
For example, Argiope orb weaver spiders build their webs out of silk with low UV reflectivity, which decreases contrast and makes the webs harder to see. However, they also decorate their webs with strands that shine brightly in ultraviolet light. As many insects upon which dragonflies prey also shine in the ultraviolet spectrum, these web decorations may trick tempt dragonflies into flying dangerously close to the webs (Craig and Bernard, 1990).
Also, dragonflies fly quickly through their habitats in pursuit of prey or mates, and may not notice a web until it’s too late. Depending on the span between web strands, a small midge might fly straight through the web, while the much larger dragonfly makes contact and becomes trapped. Or the resident spider gets a smorgasbord by capturing both insects.
Finally, spiders suspend their webs in mid-air from extremely long support lines. These lines can span spaces that otherwise appear wide open and safe. With lots of empty air space above, below, and to the sides of the web, a dragonfly may fly past a web multiple times without incident through sheer luck. But a slight alteration in its flight path may put it on a collision course with the capture net.
Do spiders always eat dragonflies caught in webs?
Although most dragonflies caught in spider webs are eaten by the resident spiders, exceptions occur. Dragonflies may be caught in webs maintained by spiders which are too small to consume insects the size of dragonflies or caught in abandoned or empty webs.
Some orb weavers build capture webs that are extremely large compared to their own body size. Webs are amazingly strong structures, so tiny spiders may catch prey like dragonflies that are simply too large for them to handle.
Small orb weavers may be unable to trapped dragonflies. Dragonflies are arthropods and thus are covered in stiff exoskeletons. A small spider’s fangs may be too small or weak to pierce a dragonfly’s exoskeleton, even the relatively thin layer covering its abdomen. In that case, the spider has no choice but to ignore the dragonfly in favor of smaller insects it can actually eat.
Sometimes dragonflies are unlucky enough to be caught in empty webs. Spider silk is resistant to decay, so can endure long after the spiders who created the webs are gone. A resident spider may have been eaten by a predator or simply died off at the onset of winter, leaving its web still hanging. A dragonfly may fly into the empty web, become trapped, and die without the spider getting any benefit at all.
Can dragonflies escape from spider webs?
Dragonflies can escape spider webs but rarely. Escape is more likely when a dragonfly is only loosely in contact with the sticky web strands, entangled in the non-sticky support lines only, or caught in the lower part of the vertical web net.
First, a dragonfly is more likely to escape a web if it is entangled in one of the non-sticky support lines only, or only minimally in contact with the sticky spiral. The more points at which a dragonfly’s body contacts the sticky spiral strands of the capture web, the harder it will be for the dragonfly to escape.
Second, a dragonfly is more likely to escape a web if it collides with the lower part of the net. Gravity pulls insects toward the ground; heavy insects can sometimes be pulled completely free if only loosely held by the web. But a dragonfly that tumbles toward the ground from a point high in the capture net may contact the web again as it falls and be re-captured (Zschokke et al, 2006).
Conclusion
Spiders and dragonflies are both highly adapted and highly successful predators of flying insects. Unfortunately for many dragonflies, they are flying insects themselves, and are therefore on the menu of orb-weaver spiders who evolved the ability to weave the world’s most perfect insect trap.
References
Carrel, James E., and Deyrup, Mark. 2019. “Analysis of Body Size, Web Size, and Diet of Two Congeneric Orb-Weaving Spiders (Araneae: Araneidae) Syntopic in Florida Scrub.” The Florida Entomologist 102 (2) (06): 388-394. https://doi.org/10.1653/024.102.0215
Craig, Catherine C. and Bernard, Gary D. 1990. “Insect Attraction to Ultraviolet-Reflecting Spider Webs and Web Decorations.” Ecology 71 (02): 616-623. https://doi.org/10.2307/1940315
Dunkle, Sidney W. 2000. “Dragonflies Through Binoculars: A Field Guide to Dragonflies of North America.” Cary: Oxford University Press, Incorporated.
Resh, Vincent H., and Cardé, Ring T., eds. 2009. Encyclopedia of Insects. San Diego: Elsevier Science & Technology.
Zschokke, Samuel, Hanaut, Yann, Suresh, Benjamin P., and Garcia-Balllinas J. Alvaro. 2006. “Prey-Capture Strategies in Sympatric Web-Building Spiders.” Canadian Journal of Zoology/Revue Canadienne De Zoologie 84 (7) (07): 964-973. https://doi.org/10.1139/z06-074