While the image of a spider acting like a miniature vampire is common in popular culture, the scientific answer to whether spiders suck blood is a definitive no. Spiders are not hematophagous creatures; they do not feed on the blood of vertebrates, including humans. This misconception stems from a misunderstanding of their unique and highly specialized feeding methods. Instead of relying on blood, spiders utilize a complex biological process to consume their prey that separates them from true blood-feeding insects like mosquitoes or ticks.
Spider Diet and Primary Food Sources
Spiders are primarily obligate carnivores, meaning their diet consists almost entirely of other animals. Their main food sources are insects and various other small arthropods, which they capture using webs, ambush tactics, or active pursuit. This predatory behavior establishes them as significant natural pest controllers in nearly every terrestrial ecosystem on Earth. The prey’s contents, such as muscle tissue and internal organs, provide the complex proteins and lipids necessary for their survival and growth.
This carnivorous focus stands in direct contrast to hematophagy, the practice of feeding on blood, utilized by approximately 14,000 species of arthropods. The structure of a spider’s mouthparts and digestive system is fundamentally unsuited for consuming raw blood. One specialized exception is the East African jumping spider Evarcha culicivora, which hunts and eats female mosquitoes that have recently fed on vertebrate blood, indirectly acquiring a blood meal. However, for the vast majority of the over 50,000 known spider species, insects remain the staple diet.
The Mechanism of External Digestion
The impossibility of a spider consuming blood lies in its unique method of external digestion. Spiders possess a narrow gut that can only process liquid food, requiring them to liquefy their prey before ingestion. A spider first immobilizes its prey, often by injecting venom through its fangs, which may also contain digestive enzymes.
The spider then begins extra-oral digestion by expelling a cocktail of powerful digestive enzymes from its midgut onto or into the prey. These enzymes break down the insect’s internal tissues, turning the contents into a nutrient-rich liquid. The spider uses a muscular sucking stomach to draw this liquefied meal into its foregut.
Some species crush the prey with their chelicerae while flooding it with enzymes, while others repeatedly pump digestive fluid into the carcass and suck the resulting liquid back out. A complex filtering system ensures that only liquid material, sometimes down to particles as small as one micrometer, passes into the gut, leaving the indigestible exoskeleton behind. This reliance on pre-digested, liquefied tissue explains why raw blood is not a viable food source.
Why Spiders Bite Humans
When a spider bite occurs, it is almost always a defensive reaction, not an attempt to feed or draw blood. Spiders do not view humans as prey because humans are far too large and do not fit the spider’s specialized dietary needs. Venom is an energetically costly resource primarily reserved for subduing insect prey, making its use on a non-food threat like a human counterproductive.
Bites typically happen when the spider feels trapped or threatened, such as when it is pressed against the skin. This occurs accidentally when a person puts on clothing or shoes where a spider is hiding, or when a person rolls over on a spider in bed. The venom injected in a defensive bite is meant to deter a perceived threat, not to initiate feeding. Most alleged spider bites are actually bites or stings from other arthropods, or skin reactions mistakenly attributed to spiders.