The observation of a long-legged creature suddenly engaging in rapid, chaotic movement leads many people to wonder about the purpose of this strange “dance” or vibration. This distinctive behavior is a specific biological response, but its meaning depends on which of the two commonly named “Daddy Long Legs” arthropods is being observed. The movements serve entirely different evolutionary purposes for defense, communication, and predation. Understanding the science behind the vibration requires first distinguishing between the two unrelated groups of eight-legged animals that share this common name.
Two Creatures, One Common Name
The name “Daddy Long Legs” refers primarily to two different types of arachnids: the Harvestman (order Opiliones) and the Cellar Spider (family Pholcidae). These two groups are easily distinguished by their body structure, which is the most reliable way to determine the function of their movements. The Harvestman has a single, fused body segment, meaning there is no visible waist connecting a separate head and abdomen.
In contrast, the Cellar Spider possesses the classic two-part body structure of a true spider, with a distinct cephalothorax and abdomen connected by a thin, stalk-like pedicel. This morphological difference relates directly to their lifestyle. The Cellar Spider produces silk and builds the messy, tangled webs often found in corners, while the Harvestman does not spin silk or build webs. Because their tools and habitats are different, their vibrating behaviors have evolved to perform distinct functions.
The Cellar Spider’s Web Shaking
For the Cellar Spider, the rapid, jerky movement is a whole-body action performed while the animal remains suspended in its irregular, three-dimensional web. This frantic shaking is often triggered by a disturbance and serves as a multi-functional tool for survival and hunting. A primary purpose of this behavior is defense against visual predators, such as birds or wasps. By vibrating intensely, the spider’s slender body and legs become a blur, making it difficult for an attacker to target the small creature amid the tangle of web silk.
The vibration is also a tool for enhancing prey capture when an insect initially hits the web. The sudden, intense movement causes the struggling prey to become further entangled in the sticky silk strands, making escape less likely. Furthermore, the induced vibrations travel through the web, acting as a form of communication.
Male Cellar Spiders use specific, subtle web vibrations to court females, signaling their presence and intention without triggering the female’s predatory response. This complex vibrational language allows for species recognition and courtship rituals to take place safely.
The Harvestman’s Defensive Tremor
The Harvestman’s vibration, or tremor, is a separate behavior entirely. It occurs when the animal is out of a web and usually involves a direct, physical response to being touched or disturbed. This movement is primarily a component of the Harvestman’s sophisticated chemical defense mechanism. When threatened, the Harvestman releases noxious chemical secretions from a pair of specialized scent glands, called ozopores, located on its body.
The tremor helps to quickly distribute these defensive compounds, which include odorous benzoquinones and phenols, into the air around the animal. This rapidly deployed chemical shield is highly effective at repelling small invertebrate predators like ants and some spiders. The sudden, unexpected movement of the body also acts as a startle display, briefly confusing a potential attacker while the chemical defense takes effect.
The release of these defensive chemicals is energetically costly for the Harvestman, suggesting the tremor is a serious, last-resort defense. Studies indicate that the production of these secretions depends on the animal’s overall physical condition and food availability, meaning the Harvestman must conserve this resource.