Spiders are a diverse group of arachnids found across nearly every continent, defined by a singular biological trait: they are predators. Almost all of the over 50,000 known species rely on a diet of other animals, primarily insects and other arthropods. This carnivorous behavior is fundamental to their biology, and their digestive system is adapted solely for the consumption of animal matter. Any deviation from this rule is a notable biological exception.
The Predatory Nature of Spiders
The typical spider diet consists overwhelmingly of invertebrates, such as flies, beetles, and crickets. Spiders do not possess the mandibles or chewing mouthparts needed to break down solid food. Instead, they rely on external digestion, a unique physiological process.
Upon subduing their prey, often through venom injected via their chelicerae, the spider liquefies the victim’s internal tissues. This is accomplished by pumping a cocktail of powerful digestive enzymes into the prey’s body. The soft inner parts of the insect are broken down into a nutrient-rich fluid outside the spider’s body.
The spider then uses a sucking stomach to draw this liquid meal into its narrow gut, leaving behind the indigestible husk of the exoskeleton. This mechanism means that spiders are physiologically incapable of consuming the solid, fibrous materials found in fruits, vegetables, or common plant matter. Their digestive tract is not designed to process the cellulose walls of plant cells.
The Notable Exception of Plant-Eating Spiders
Despite the carnivorous nature of most species, one remarkable exception exists among the jumping spiders: Bagheera kiplingi, found in Central America. This tiny species is the only spider known to have a predominantly herbivorous diet, actively consuming solid plant structures. Their primary food source is specialized, nutrient-rich nodules called Beltian bodies.
Beltian bodies are produced by acacia plants as part of a mutualistic relationship with aggressive ants. These nubs grow on the tips of leaflets and are packed with proteins and fats, serving as a meal for the ants that guard the tree. B. kiplingi exploits this system, stealthily navigating the acacia branches to steal the nutritious bodies while avoiding the patrolling ant guards.
In populations studied in Mexico, these plant-based structures can constitute over 90% of the spider’s diet, a proportion confirmed by analyzing the spider’s tissues for isotopic signatures. The chemical makeup of the spider’s body closely resembles that of herbivores, indicating that most of its nutrition comes from plants. While they occasionally consume ant larvae or nectar, this species relies on a reliable, year-round source of solid plant material.
Supplemental Liquid Diets
While Bagheera kiplingi is unique for consuming solid plant matter, many other spider species supplement their carnivorous diets with various plant-derived liquids. This practice, known as nectivory, is common, particularly among mobile jumping spiders. They are often observed drinking nectar directly from flowers or from extrafloral nectaries, which are sugar-secreting glands on other parts of a plant.
This liquid intake provides a source of sugar and water, offering an energy boost when insect prey is scarce. Spiders have also been documented consuming tree sap and honeydew, the sugary excretion of plant-feeding insects like aphids. These liquids offer quick energy and hydration but do not provide the complete nutritional profile necessary for growth and reproduction.
This liquid consumption is significantly different from true herbivory or fruit consumption. The spider is merely imbibing a pre-liquefied sugar solution rather than breaking down solid plant tissue. While spiders are adaptable and may sip sweet liquids, the vast majority cannot process solid materials, meaning common fruit and vegetable matter remain off the menu.