Spiders, members of the class Arachnida, are widespread and ecologically diverse predators. Spiders are overwhelmingly classified as carnivores. Their entire physiology, from their narrow mouthparts to their digestive process, is specialized for consuming animal matter. While this predatory nature is the rule for almost all of the roughly 50,000 known species, a few exceptions demonstrate the adaptability of life.
The Carnivorous Diet of Spiders
The primary diet of spiders consists of insects and other arthropods; larger species occasionally take small vertebrates like lizards, frogs, or birds. Spider anatomy dictates this reliance on animal protein because their feeding apparatus and foregut are too narrow to process solid food. This structural constraint necessitates a unique method of consuming prey known as external digestion.
Once prey is subdued, the spider injects a cocktail of powerful digestive enzymes into the victim’s body. These enzymes, which include proteases and lipases, quickly break down the internal tissues into a nutrient-rich liquid. The spider then uses a strong sucking stomach to pump the liquefied contents back into its body, leaving behind only the indigestible exoskeleton. This efficient process maximizes nutrient uptake while minimizing the consumption of hard, fibrous material.
Methods of Prey Capture
Spiders employ an array of behavioral strategies and specialized tools to secure their carnivorous meals. These predatory tactics divide into passive trapping methods and active hunting techniques. Passive hunters, such as orb-weavers, construct intricate webs designed to intercept flying insects.
The classic orb web features non-sticky radial threads for structural support and a sticky spiral thread for capture. This capture silk possesses elasticity and an adhesive coating, allowing it to absorb the kinetic energy of an insect and hold it fast until the spider arrives. Other web-builders, like cobweb spiders, create a three-dimensional tangle web with sticky anchor threads that ensnare walking prey, often lifting them off the ground when the thread breaks.
Active hunters rely on speed, stealth, and acute senses rather than silk traps. Wolf spiders are agile ground hunters with exceptional eyesight, particularly at night, due to a reflective layer in their eyes called a tapetum lucidum. They locate prey and then chase it down or ambush it with a rapid pounce, immobilizing it with a venomous bite. Crab spiders are masters of camouflage, positioning themselves on flowers or leaves and often changing color over several days to match the background. They wait in ambush with their long, outstretched front legs, snatching unsuspecting pollinators that land nearby and injecting venom to quickly paralyze the meal.
Rare Exceptions to the Rule
While the majority of spiders are obligate carnivores, some species incorporate plant matter into their diet as a supplement. Many species, particularly in warmer climates, drink nectar from flowers or extrafloral nectaries, which provides simple sugars and water. They may also consume pollen accidentally while recycling silk or deliberately pierce pollen grains to access protein-rich contents. This behavior classifies them as facultative omnivores, meaning they opportunistically feed on plants, but animal prey remains their primary food source.
The most notable exception is the Central American jumping spider Bagheera kiplingi, which is the only known spider to be predominantly herbivorous. This tiny arachnid feeds primarily on specialized plant structures called Beltian bodies, which are protein and fat-rich nubs found on acacia tree leaves. In some populations, these plant parts constitute over 90% of the spider’s diet, confirmed by analyzing chemical isotopes in the spider’s tissues. Bagheera kiplingi must actively evade the aggressive ants that guard the Beltian bodies, demonstrating a specialized foraging strategy to maintain its plant-based sustenance.