Do spiders eat dead flies? Generally, spiders do not consume dead flies or other deceased insects. Their feeding mechanisms are adapted for capturing and consuming live prey. This preference is rooted in their sensory biology, hunting strategies, and digestive processes.
Why Spiders Don’t Typically Eat Dead Flies
Spiders largely rely on detecting movement and vibrations to identify and capture their prey. Most spiders possess eyes, but many species have poor eyesight, depending instead on their highly sensitive hairs (setae) and specialized organs on their legs, known as slit sensilla, to detect subtle changes in their environment. These sensory structures allow them to perceive vibrations in their webs or on surfaces, signaling the presence of a struggling, live insect. A dead fly, lacking movement, does not trigger these sensory cues, making it largely undetectable or uninteresting to a spider.
The digestive process of spiders is externally-oriented, requiring fresh, liquefied internal contents. After capturing prey, a spider injects venom containing digestive enzymes to immobilize and break down tissues. These enzymes liquefy the prey’s insides into a “soup” that the spider then sucks up. A dead, dried-out fly would not provide the necessary fresh fluids for this external digestion, making it an unsuitable meal. Spiders’ predatory instinct is geared towards active capture and immediate pre-digestion of fresh prey.
What Spiders Prefer to Eat
Most spiders primarily eat live insects and other small arthropods. Common prey include flies, mosquitoes, moths, beetles, crickets, and other spiders. Larger species, like certain tarantulas, can prey on small vertebrates such as lizards, frogs, or even small birds and bats. This diverse diet helps spiders act as natural pest control agents in many ecosystems.
Spiders employ various hunting strategies to secure their live meals. Many are web-builders, constructing intricate silk snares to trap flying insects. Once prey is entangled, the spider quickly immobilizes it with venom and silk. Other spiders are active hunters, such as wolf spiders and jumping spiders, which pursue or ambush prey using keen eyesight and agility. Trapdoor spiders, for example, wait hidden in burrows, lunging out to capture unsuspecting insects.
Exceptional Circumstances
While spiders generally prefer live prey, rare instances exist where they might consume deceased insects. Extreme starvation can make a spider less selective, prompting it to consider recently killed prey. Some species, particularly certain tarantulas and wolf spiders, have been observed to scavenge, especially when young or in captivity. A spider might also consume very recently killed prey that is still “fresh” or shows residual nerve activity. This differs from scavenging long-dead, desiccated insects.
Spiders have a slow metabolism, allowing them to survive for extended periods, often several weeks to months, without food. This means scavenging old, dead insects is not a typical or preferred feeding behavior for most wild spiders, as their survival relies on patience and pursuing live food.