Harvestmen are often mistaken for spiders due to their eight legs and similar appearance. However, these creatures are not spiders. Understanding their differences clarifies their unique biological roles and characteristics.
Understanding Harvestmen Classification
Harvestmen are arachnids, a class of arthropods that also includes spiders, scorpions, and mites. Despite this shared classification, harvestmen belong to their own distinct order called Opiliones. Spiders, in contrast, are part of the order Araneae. This means harvestmen are no more closely related to spiders than to scorpions.
The Opiliones order encompasses over 6,650 known species worldwide, with an estimated total exceeding 10,000. These creatures have a long evolutionary history, with well-preserved fossils dating back 400 million years that resemble modern harvestmen, indicating their basic body plan has remained unchanged.
Distinguishing Harvestmen from Spiders
A noticeable difference between harvestmen and spiders is their body structure. Harvestmen have a broadly joined body, where the cephalothorax (head and thorax) and abdomen appear as a single, oval-shaped unit without a distinct waist-like constriction. Spiders, conversely, possess two clearly separated body segments connected by a narrow pedicel.
Another distinguishing feature is their eyes. Harvestmen typically have two eyes, often on a raised mound called an ocularium, which are generally unable to form images and primarily sense light and dark. Spiders usually have eight eyes arranged in various patterns. Harvestmen also lack silk glands; they do not produce silk or construct webs, unlike most spiders.
Harvestmen do not possess venom glands or fangs. Their mouthparts, known as chelicerae, are small, pincer-like structures used for grasping and tearing food into small pieces. Spiders, on the other hand, typically have venom glands and fangs to subdue prey.
Harvestmen Behavior and Diet
Harvestmen are adaptable creatures found in diverse habitats, including forests, grasslands, caves, and arid regions. They often prefer moist, shaded environments, such as under rocks, logs, or leaf litter, and can be found on tree trunks or garden vegetation. Many species are nocturnal, foraging at night and resting during the day.
Their diet is varied, making them omnivores or scavengers. They feed on various organic materials, including small insects, decaying matter, fungi, and feces. Unlike spiders, which liquefy their prey, harvestmen can ingest solid food using their chewing mouthparts. When disturbed, some harvestmen release a foul-smelling fluid from scent glands as a defense. They also bob their bodies to confuse predators or detach a leg (autotomy).
Are Harvestmen Dangerous?
Harvestmen are harmless to humans. They do not possess venom glands, nor do their mouthparts have fangs capable of piercing human skin. The common urban legend that harvestmen are highly venomous but cannot bite humans is entirely false. Even if a harvestman were to attempt a bite, it would not result in any significant effects. Therefore, there is no need for concern when encountering these arachnids.