Spiders are eight-legged arthropods, often mistaken for insects. For the vast majority of the over 50,000 described species, the answer to whether they eat plants is no. Almost all spiders are predators, subsisting on a diet of animals they hunt or trap. However, a small number of species have developed specialized dietary habits that incorporate or rely heavily on plant matter, presenting a fascinating exception.
The Predatory Nature of Spiders
The typical spider diet consists primarily of insects and other small invertebrates, which provide the food necessary for growth and reproduction. Spiders employ diverse methods to secure their prey, ranging from the intricate, sticky threads of orb-weavers to the swift attacks of ambush hunters like crab spiders. Hunting spiders, such as wolf spiders, actively stalk and pounce on their targets, while others use silk trip-lines to alert them to a passing meal.
Once captured, prey is subdued with a venomous bite that immobilizes or kills the victim. Spiders cannot ingest solid food because their gut is too narrow, a biological constraint that dictates their entire feeding strategy. Instead, they use a process of external digestion, where they inject or regurgitate enzyme-rich digestive fluids onto the prey.
These enzymes break down the prey’s internal tissues, effectively liquefying the insides. The spider then uses a muscular, pumping stomach to suck up the resulting nutrient-rich liquid, leaving behind only the indigestible hard parts like the exoskeleton. This reliance on external liquefaction is why the consumption of solid plant material is so rare.
The Rare Cases of Omnivory
While the foundational requirement of a spider’s diet is animal protein, a small number of species have evolved to supplement their meals with plant materials, a behavior called omnivory. The most striking example is the Central American jumping spider, Bagheera kiplingi, which is the only known spider to be predominantly herbivorous. This species feeds on specialized nubs called Beltian bodies that grow on the tips of acacia plant leaflets.
In some parts of Mexico, Beltian bodies can account for over 90% of the B. kiplingi diet. They access this solid plant matter by piercing it and injecting digestive fluids to break it down, similar to how they process insect prey. The advantage of this diet is year-round food availability, circumventing periods of insect scarcity.
Other spiders, particularly those in the jumping spider family, consume plant fluids. Over 30 species of jumping spiders have been observed intentionally drinking nectar from flowers and extrafloral nectaries (sweet-producing glands on plants). Nectar provides carbohydrates, which can be an energy supplement when hunting insects is less successful.
Pollen is another plant-based food source, often ingested by web-building spiders when they recycle their old silk. As they consume their own webs to reuse the protein, they also swallow any tiny particles stuck in the strands, including calorie-rich pollen and fungal spores. Additionally, spiders may consume:
- Sugary internal sap, accessed by piercing plant leaves with chelicerae.
- Honeydew, a sugary excretion produced by plant-feeding insects like aphids.
Why Spiders Are Often Found on Vegetation
The common sight of spiders on plants typically relates to their predatory lifestyle. Vegetation provides the ideal platform for most spiders to hunt the insects that constitute their primary food source. Plants act as a natural magnet for insects, creating a concentrated feeding ground for the resident arachnids.
Plants also offer spiders a stable environment for web construction. The numerous branches and leaves provide multiple anchor points to support large, intricate webs designed to intercept prey. For hunting spiders, the dense foliage offers excellent natural camouflage, allowing species like the crab spider to blend in and ambush insects visiting flowers.
The plant environment also offers shelter from predators and temperature extremes. Dense leaves and shaded areas protect spiders from direct sunlight. Spiders, which get most of their water from their prey, can also absorb moisture from the environment, making damp vegetation a source of hydration in the form of dew or water droplets.