The question of whether “daddy long legs” consume flies is complex because the common name refers to three entirely different creatures. Only one of these commonly known organisms is a true predator of flying insects. Disentangling the identities is the first step to understanding their varied diets and roles in the environment.
The Three Identities of “Daddy Long Legs”
The term “daddy long legs” is used to describe three distinct groups of arthropods, two of which are arachnids and one is an insect.
The most common interpretation, particularly in North America, refers to the Harvestman (Order Opiliones). These arachnids have eight legs but are not true spiders, as they lack the characteristic two-part body and venom glands.
Another common interpretation is the Cellar Spider (Family Pholcidae). These are true spiders, possessing eight legs, two main body segments, silk, and venom. Cellar spiders are often found hanging upside down in messy, irregular webs in damp corners of buildings.
The third creature sometimes called a “daddy long legs” is the Crane Fly, an insect (Order Diptera). It resembles a large mosquito but is harmless, having only six legs and a pair of wings.
The Harvestman: Diet and Scavenging Habits
The Harvestman is primarily an omnivorous scavenger, a unique feeder among its arachnid relatives. Their diet consists mainly of decaying matter, fungi, plant material, and small, soft-bodied invertebrates like aphids and mites. They consume recently dead insects, including flies, but they are not active hunters of living, flying prey.
Harvestmen lack venom, fangs, and silk, so they cannot subdue an active fly. Instead, they rely on finding food that is already dead or very slow-moving. Their role is akin to a recycler, helping to break down organic material in damp, cool environments.
The Cellar Spider: A True Fly Predator
The Cellar Spider is a skilled predator that actively consumes flies and other insects. These true spiders build messy, tangled webs, often in corners, basements, and cellars, which serve as traps for flying insects. When prey becomes entangled, the spider quickly moves in to immobilize its meal.
Cellar spiders are known for their aggressive hunting of other spiders, including species much larger than themselves. They subdue their prey by quickly wrapping it in silk and then delivering a venomous bite. Their presence in human dwellings makes them effective at controlling populations of common house flies and mosquitoes.
How They Eat: Feeding Mechanisms
The way the two arachnids consume their food highlights their biological differences. Harvestmen possess pincer-like mouthparts, known as chelicerae, which allow them to tear and ingest small pieces of solid food. Unlike most other arachnids, harvestmen can swallow solid particles, rather than relying solely on liquid food.
Cellar spiders, like all true spiders, utilize a process called extra-oral digestion. They inject venom into their prey, which liquefies the internal tissues. The spider then sucks the resulting liquid meal through its mouth, leaving behind the empty exoskeleton.