Opiliones

The Opiliones are a diverse order of arachnids. They are commonly known by names like harvestmen, daddy long legs, or granddaddy long legs.

While they look like spiders, they belong to a distinct group. There are over 6,650 discovered species worldwide. Some scientists believe the total number of living species may exceed 10,000. They are found on every continent except Antarctica.

To understand a harvestman, you must look at its body structure. Unlike spiders, which have a narrow waist, harvestmen have fused body regions. Their body consists of two tagmata, or main parts. These are the anterior cephalothorax, also called the prosoma, and the posterior abdomen, or opisthosoma. Because these parts are joined broadly, the body looks like a single oval.

This is one of the easiest ways to tell them apart from spiders. Most harvestmen also have a single pair of eyes located in the middle of the cephalothorax. Spiders usually have three or four pairs of eyes around the edges of their heads.

Harvestmen have unique ways of interacting with their environment. Their eyes cannot form clear images, so they rely on touch. The second pair of legs is often much longer than the others. These legs function like antennae or feelers to explore the world.

Their feeding apparatus is called the stomotheca. It is formed by extensions of the first pair of legs and the pedipalps. Most arachnids only eat liquids, but the stomotheca allows harvestmen to swallow chunks of solid food. This allows them to be omnivores. They eat insects, plants, fungi, and even dead organisms or bird dung.

Scientists have organized the Opiliones into five distinct suborders. These groups were named in 2014: Cyphophthalmi, Eupnoi, Dyspnoi, Laniatores, and Tetrophthalmi.

Each suborder has different traits. For example, Cyphophthalmi are often small and live in soil. Many species in other groups, like the Laniatores, can grow quite large. Some species are very tiny, measuring less than 1 mm. However, the largest known species, Trogulus torosus, can reach 1 cm in length. Their leg spans can be much larger than their bodies, sometimes exceeding 10 cm in Southeast Asia.

We know that harvestmen have existed for a very long time. Fossils have been found in the Rhynie cherts of Scotland, which are 400 million years old. Other fossils were found in 305-million-year-old rocks in France.

These fossils look surprisingly modern. This tells us their basic body shape developed very early in history. Their exact place in the arachnid family tree is still being studied. They may be most closely related to camel spiders, known as Solifugae. Or, they might belong to a larger group that includes horseshoe crabs and scorpions.

Harvestmen use several methods to protect themselves from predators. Some use crypsis, which is a form of camouflage. They use patterns or colors to blend into leaves or dirt.

Others use aposematism, which means using bright colors to warn predators. Some species also have defensive scent glands called ozopores. When disturbed, these glands secrete a smelly fluid. In some species, this fluid contains noxious quinones that taste bad to predators. Some harvestmen even live in massive groups, called aggregations, to stay safe. These groups can contain more than 70,000 individuals!

Reproduction in Opiliones is also quite interesting. Most species reproduce sexually through direct copulation. Unlike most other arachnids, males have a penis.

In some species, males provide a "nuptial gift" from their chelicerae to the female. Some males even show paternal care. This means the father stays to guard the eggs and clean them. This behavior has evolved independently at least three different times in different groups. This shows how complex and varied the lives of these ancient creatures can be.