When most of us picture a spider, we picture it hanging suspended in mid-air, waiting for a hapless fly to collide with the sticky strands of its invisible web. And most of us understand that spiders eat flies and other insects; spiders are the predators and insects are the victims. But have you ever wondered if the reverse if ever true? Do insects ever eat spiders?
Some species of insects prey on spiders, including certain species of flies, mantisflies, antlions, assassin bugs, tiger beetles, giant damselflies, and many species of wasps. Spiders are dangerous to insects; only a small number of insect species evolved to attack spiders effectively.
Depending on the species, insects may attack spiders from the air or from the ground and as larvae or adults. This post is the second in a two part series about insects that eat spiders and focuses on those species that attack from the ground.
To learn about those insects that attack spiders from the air, please check out this other Now I Wonder post “Do insects ever eat spiders? Part 1: Attacks from the air”.
Spiders are dangerous prey for an insect to tackle. All of the 49,000 spiders species known worldwide are predators and insects represent spiders’ favorite prey.
Spiders usually have good vision, both for movement and distance discrimination, so they are hard to surprise. Ground-based spiders can generally move very fast in any direction on their eight legs when fleeing, attacking, or pouncing from ambush. Web-based spiders are protected from most insects by their strong, sticky webs, across which they can travel easily but which trap insects.
Perhaps more importantly for insects that attack spiders, spiders are armed with sharp, venomous fangs, which evolved over millions of years as superb tools for killing insects.
Usually spiders kill insects, not the other way around.
Insects that can attack spiders from the ground successfully use a variety of tactics and natural skills to overcome spiders’ natural weaponry and defenses. These include faster speed, larger size, joined forces, the element of surprise, and careful timing.
Tiger beetles are predatory insects classified in order Coleoptera, suborder Adephaga, family Cicindelidae and are strong and fast enough to attack spiders encountered on the ground. Their hunting prowess relies on three things: stereoscopic vision, extremely fast running speed, and huge, powerful, serrated mandibles.
Tiger beetle eyes are bulbous and protrude from the sides of their heads so far their heads are wider than their bodies. Their eyes are also compound, which means each eye is made up of many tiny facets, each with its own lens. This gives tiger beetles wide fields of view, excellent ability to detect motion, and three-dimensional vision. Tiger beetles spot prey by scanning their environments for movement (Pearson et al. 2005).
Tiger beetles run extremely fast – so fast they outrun their own vision and have to pause periodically in order to see where they are going and to scan for movement (Pearson et al. 2005). When they spot prey, they first turn to face it, then use their incredible speed to run it down. The combination of tiger beetles’ excellent vision and speed add up to an effective surprise attack strategy.
Tiger beetles can cover ground so quickly that prey is either taken by surprise or run down in short order. Very few spiders are fast enough to either flee or ready their defense before an attacking tiger beetle closes the distance.
Tiger beetles kill their spider prey by biting with their long, sickle-shaped jaws then chew the spiders to pulp to soften them enough to eat.
Assassin bugs are another notorious insect predator which are strong and powerful enough to attack spiders. Assassin bugs rely on speed to attack like tiger beetles, but in a different way. Classified in the “true bug” order Hemiptera, assassin bugs move slowly through their environment, searching for prey. Once they locate a likely victim, they rush to stab it to death with their long, sharp beaks.
Few assassin bugs specialize in attacking spiders; most are more generalist predators, with fellow insects making up a larger part of their diet than spiders.
One exception is the thread-legged assassin bug (Emasaya spp.) (Cranshaw et al. 2017). The front legs of these insects are adapted for grasping, almost like those of mantids in order Mantodea, family Mantidae. These legs grip the spiders they attack and prevent them from writhing around enough to spike the assassin bugs with their venomous fangs. Additionally, thread-legged assassin bugs can walk safely across spider webs without becoming entangled and thus can attack spiders even in their own webs.
Some ants will attack and kill spiders, although mostly in defense of their nest, eggs, and larvae when needed. Worker ants are armed with chewing mouth parts and many species also sting. An individual ant can only do so much against a spider but the real power of ants over spiders is overwhelming force of numbers.
Many spiders are much larger than an individual ant. But individual ants rarely act alone for long; A colony of ants lives by the code “One for all and all for one”. When threatened, an ant sends a chemical distress signal that triggers immediate response from the hundreds or even thousands of fellow worker ants that shares its colony.
The members of an enraged ant colony will boil out of their nest and swiftly engulf a spider. Even if some ants die during the fight, many more remain to press the attack until the spider eventually succumbs. For this reason, many spiders avoid attacking ants that may be aggressive (Bradley 2012).
Praying mantises are powerful ambush predators that are capable of attacking spiders, although they do not specialize on them as prey.
Mantises are classified in order Mantodea, and are true ambush predators; they rely on their magnificent camouflage to blend into vegetation and on their excellent vision to spot prey and gauge attack distance.
A mantid attacks by snapping its sharply spined forelegs forward at lightning speed to impale its prey. The movement happens so fast that an unwary spider who ventures too close to a lurking mantid will likely be dead before it can even register the attack.
Small-headed flies are a group of flies classified in order Diptera, family Acrocerida. This family contains 550 species worldwide across 55 genera (Borkent et al. 2016) with several species found in North America. Although most North American species are found west of the Rockies, one species, Eulonchus marialiciae Brimley, is known in the eastern United States.
Also called “spider flies”, adults usually eat nectar and are important flowering plant pollinators. But their larvae are all internal spider parasites (known as “endoparasitoid”); they eat their spider victims from the inside out until they are are ready to pupate into adult flies (Rose 2022).
Endoparasitoids always kill their hosts in the end, which makes them different from true parasites, which don’t. In this way, they are similar to spider wasps that attack spiders from the air. However, small-headed fly females lay their eggs near spider burrows instead of directly on or in the spiders themselves.
(For detailed information about spider wasps, check out this other Now I Wonder post “Do insects ever eat spiders? – Part 1 Attacks from the air”)
The fly eggs hatch into legless “planidia” (Pinto 2009), which inch around on the ground until they bump into a spider. At this point, they chew their way inside, metamorphose into the next instar, and feed on the spiders’ internal organs until ready to pupate.
Eulonchus marialiciae Brimley attacks sensing web weaver spiders (family Euctenizidae) and folding door spiders (family Antrodiaetidae) (Borkent et al. 2016). These spiders live in burrows camouflaged with hinged or collapsible doors so are excellent prey for ground-based small-headed fly parasitoids.
Antlions are insects that resemble dragonflies and damselflies but are classified in their own family Myrmeleontidae. Antlion larvae do not specialize on spiders but can and do attack them when opportunities present.
With the exception of species within the genus Myrmeleon, which excavates burrows, antlion larvae lie in wait just beneath the soil surface (Eaton and Kaufman date unknown). When prey ventures close, they lunge out and attack, relying on the element of surprise to avoid the spiders’ venomous fangs.
Mantisflies are insects that resemble mantids like the praying mantis (order Mantodea) but are classified in their own family Mantispidae.
Mantisfly larvae prey on spider eggs, although some species will feed on the blood of female spider hosts while waiting for the spiders to lay their egg sacs. When the spider eggs are laid, the mantisfly larvae chew through the egg casings and eat the spiderlings. Once the larvae has eaten the contents of the spider eggs, they spin a cocoon within the spider egg casings; the casings protect the developing mantisflies until they are ready to emerge (Hanson 2016).
Spiders are formidable opponents; only a very few insects evolved to attack them and even these sometimes die in the attempts. It takes a specialized kind of insect predator to avoid or overwhelm spiders’ formidable defenses. But insects have existed on earth for more than 400 million years; natural selection has had plenty of time to adapt at least some of the approximately one million known species of insects to the fine art of spider hunting.
Related Now I Wonder Posts
For more information about the insects that attack spiders, check out this other Now I Wonder post “Do insects ever eat spiders? Part 1: Attacks from the air.“
For more information about what makes a fly a “fly”, check out this other Now I Wonder post “Is a dragonfly a fly?“
For information about insects in general, check out these other Now I Wonder posts:
For more information about spiders, check out these other Now I Wonder posts:
- “What are wolf spiders?“
- “Jumping Spiders #1 – An Introduction“
- “Jumping Spiders #2 – A look at their incredible vision“
- “Jumping Spiders #3 – A detailed look at a special skill: Jumping“
- “Jumping Spiders #4 – As Predators“
- “Jumping Spiders #5 – As Prey“
For more information about a spider cousin, check out this other Now I Wonder post “What is a daddy long legs?“
“Assassin Bugs.” In Garden Insects of North America, edited by Whitney Cranshaw, and David Shetlar. 2nd ed. Princeton University Press, 2017.
Borkent CJ, Gillung J, Winterton SL. 2016. Jewelled spider flies of north america: A revision and phylogeny of Eulonchus gerstaecker (diptera, acroceridae). ZooKeys. 619:103-46.
Bradley, Richard A.. 2012. Common Spiders of North America. Berkeley: University of California Press.
Eaton ER, Kaufman K. Date unknown. Kaufman field guide to insects of North America. Boston (MA): Mariner Books
Hanson, P. 2016. Insects and other arthropods of tropical America, Cornell University Press.
Pearson, DL., Knisley CB. and Kazilek, CJ. 2005. Field Guide to the Tiger Beetles of the United States and Canada : Identification, Natural History, and Distribution of the Cicindelidae. Cary (NC): Oxford University Press.
Pinto JD. “Hypermetamorphosis” In Resh, Vincent H., and Carde, Ring T., eds. 2009. Encyclopedia of Insects. San Diego (CA): Elsevier Science & Technology.
Rose S. 2022. Princeton field guides: spiders of North America. Princeton (NJ): Princeton University Press.