The question of whether flies consume spiders often arises. While this might seem plausible, the typical relationship is the opposite. Spiders are, in fact, often predators of flies.
Spiders: The Predators of Flies
Spiders commonly prey on flies, making them a natural form of pest control in many environments. Different spider species employ diverse strategies to capture these winged insects. Web-building spiders, such as orb-weavers, construct intricate silk traps designed to ensnare flying insects like flies. When a fly contacts the sticky threads of the web, its struggles send vibrations through the silk, alerting the spider to its presence. The spider then quickly immobilizes the prey, often by wrapping it in more silk or delivering a venomous bite.
Beyond web-builders, many spiders actively hunt their prey without relying on silk snares. Jumping spiders, for instance, possess excellent vision and stalk flies before leaping to capture them with precision. Wolf spiders are swift ground hunters that chase down their meals, while huntsman spiders use their speed and agility to ambush insects. Crab spiders are ambush predators that often hide on flowers, waiting to snatch unsuspecting flies that visit for nectar.
The Diverse Diets of Flies
Flies exhibit a wide range of dietary habits, and consuming spiders is not a common part of their natural feeding behavior. Most fly species are adapted to feed on specific types of organic matter or liquids.
House flies, for example, primarily consume decaying organic materials, such as food waste, animal feces, and sugary substances, which they liquefy with saliva before ingesting. Fruit flies are attracted to and feed on the yeast and fungi found on fermenting fruits and other sugary liquids. Blow flies specialize in decaying meat and dung, with their larvae, known as maggots, playing a significant role in decomposition. Other flies, like adult mosquitoes, feed on plant nectar, although female mosquitoes are known for consuming blood.
Many flies possess a proboscis, a specialized tube-like mouthpart, allowing them to sponge up liquids, pierce tissues, or siphon nectar. These adaptations are suited for their distinct food sources and do not typically involve subduing a spider.
Unusual Interactions: When Flies Attack Spiders?
While spiders generally prey on flies, some rare and specific instances exist where the dynamic is reversed. Robber flies (family Asilidae) are predatory insects known for their aggressive hunting of other arthropods, including other flies, bees, beetles, and occasionally, spiders. These flies are swift and can capture spiders in mid-air or snatch them from surfaces, injecting them with venom and digestive enzymes before consuming their liquefied insides.
Beyond direct predation, some flies engage in parasitic relationships with spiders. Certain species of small-headed flies (family Pipunculidae) and tachinid flies (family Tachinidae) have larvae that develop inside a spider’s body. The adult female fly lays an egg on or near a spider, and the hatched larva burrows into the spider, consuming it from the inside as it grows. These interactions are a form of parasitism rather than typical predation.