Spiders are among the most diverse and widespread groups of terrestrial predators, with over 50,000 known species globally. They are almost universally defined by their predatory lifestyle, relying on other animals for sustenance. The vast majority of these species are carnivores, meaning their diet consists almost entirely of insects, other spiders, and sometimes small vertebrates. This specialized diet is dictated by their unique physiology and method of consuming food.
How Spiders Capture and Consume Prey
The typical spider’s feeding strategy is highly specialized for capturing and processing animal prey, which explains their natural aversion to plant material. Spiders use various methods to immobilize their meals, ranging from trapping them in intricate silk webs to actively hunting them down using venom. Once captured, the prey is subdued, often with venom injected through the fangs, which acts to paralyze and sometimes initiate the breakdown of tissues.
The crucial step in spider feeding is external digestion, a process where the spider turns the solid tissues of its prey into a liquid meal. Spiders accomplish this by pumping digestive enzymes from their midgut into the body of the captured animal. These enzymes liquefy the internal structures of the prey, essentially dissolving the tissues from the inside out.
Since a spider’s gut is too narrow to ingest solid particles, they then suck the resulting nutrient-rich liquid into their digestive system using a muscular, pumping stomach. Some species, like tarantulas, also use their chelicerae and pedipalps to crush and grind the prey while flooding it with enzymes. This process leaves behind an empty husk, confirming the spider’s reliance on a liquid, pre-digested diet derived from animal protein.
Incidental Consumption of Plant Matter
While spiders are primarily carnivores, many species supplement their diets with plant-derived materials. This consumption is often passive or opportunistic, providing necessary carbohydrates and moisture. Spiders frequently consume nectar and pollen deliberately to gain a quick source of energy, especially when insect prey is scarce.
Nectar is typically taken from extrafloral nectaries, which are sugar-secreting glands on plants separate from the flowers. Pollen and fungal spores are also ingested, particularly by orb-weaving spiders when they dismantle and recycle their silk webs. Studies have shown that for some juvenile orb-weavers, pollen can account for up to 25% of their total food intake, providing better survival rates. This plant-based supplementation helps them avoid the energy cost and risks associated with hunting, but it does not constitute a primary food source for growth and reproduction.
The One Known Herbivorous Spider
The single exception to the carnivorous rule is the jumping spider Bagheera kiplingi, the only known spider species with a truly herbivorous diet. This small, colorful arachnid, found in Central America, feeds almost exclusively on specialized plant structures. Its primary food source is the Beltian bodies, which are nutrient-rich nubs of tissue produced on the leaves of certain acacia plants.
These Beltian bodies are packed with proteins and fats, making them a highly concentrated food source for the spider. The acacia trees produce these nubs as part of a mutualistic relationship with aggressive ants, which the plant “hires” to guard against other herbivores. Bagheera kiplingi has developed a strategy of stealth and evasion, relying on its agility and excellent eyesight to actively avoid the patrolling ants and steal the nutrient packages.
In some populations, the Beltian bodies account for over 90% of the Bagheera kiplingi diet, a proportion confirmed by isotopic analysis. This species has successfully adapted to a readily available, year-round food source, distinguishing it from all other spiders. The mechanism by which this spider processes and metabolizes the solid plant material, which is high in fiber, remains a subject of ongoing research, as it defies the typical liquid-only feeding strategy of other spiders.