Arachnid

Arachnids are creatures that belong to a special group called the class Arachnida. They are part of a larger family of animals known as arthropods. This big group includes many different kinds of creatures, such as spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders, and vinegaroons.

Most adult arachnids have eight legs that are attached to their upper body part, called the cephalothorax. In some types, the front pair of legs might change to help them sense their surroundings.

Almost all living arachnids live on land, but some can be found in fresh water, and a few even live in the ocean. The word "arachnid" comes from an ancient Greek word for "spider," which itself has a story from mythology about a human weaver named Arachne who was turned into a spider.

Morphology

Most adult arachnids have eight legs. This is different from insects, which have six legs. Arachnids have two extra pairs of body parts for eating, protecting themselves, and sensing. One pair, called chelicerae, helps with eating and defense. The other pair, called pedipalps, can help with eating, moving, or reproduction. In some arachnids like scorpions, these pedipalps end in pinchers.

Arachnids differ from insects because they don’t have antennae or wings. Their bodies are divided into two main parts: the prosoma (upper body) and opisthosoma (lower body), sometimes called the cephalothorax and abdomen. The upper body is usually covered by a single shell. The lower body can be segmented or fused together depending on the type of arachnid. Most arachnids also have a hard outer shell.

Locomotion

See also: Arachnid locomotion

Most arachnids do not have special muscles to lift their legs. Spiders and whip scorpions use pressure from their blood to move their legs. Some creatures like solifuges and certain harvestmen use bendy parts in their joints to move. Scorpions, pseudoscorpions, and some harvestmen have special muscles that help them move their legs. The same joints in scorpions' mouthparts move using stored stretchiness.

Physiology

See also: Hemolymph

Arachnids have special features that help them live on land. They have ways to breathe, like tubes or special parts that let them get air. They also have body parts that help them move better and keep water inside.

Arachnids have glands that help get rid of waste. Their blood can change depending on how they breathe. Some have a special thing in their blood that helps carry things, like humans do. The heart in arachnids is in the front part of their body, and some tiny creatures don’t even have a heart.

Diet and digestive system

Arachnids eat insects and small animals. Some, like ticks and mites, drink blood or eat tiny particles. A few, such as harvestmen, eat both plants and animals.

Arachnids make special juices to break down their food. They pour these juices on their prey, turning it into a liquid they can drink. This liquid gives them nutrients.

Senses

Arachnids have two kinds of eyes: the lateral and median ocelli. Most arachnids have lateral eyes that came from compound eyes. Some have a special layer called a tapetum that helps them see in the dark. The median eyes grow from a fold in the ectoderm.

Besides eyes, arachnids have many fine hairs on their bodies that help them feel their surroundings. Some have more complex touch sensors called trichobothria. They also have special pits with tiny hairs inside that help them sense movements. These pits are called slit sense organs and help with proprioception, which is knowing where their body parts are.

Reproduction

See also: Spider § Reproduction and life cycle, and Scorpion § Reproduction

Arachnids have special parts in their bodies for having babies. These parts are called gonads. Most arachnids have the males give sperm to the females in a special package. Some arachnids lay eggs with extra food in them, and these eggs hatch into babies that look like small adults. However, scorpions and some mites can have babies that develop inside the mother and are born alive. Usually, only the mother takes care of the babies.

Taxonomy and evolution

The relationships among different groups of arthropods have been studied for many years. Living arthropods are divided into three main groups: chelicerates (which include arachnids), pancrustaceans (which include crustaceans and insects), and myriapods (such as centipedes and millipedes).

Chelicerates include sea spiders, horseshoe crabs, and arachnids. Studying the relationships within arachnids has been challenging. Recent studies help us understand these connections better. Arachnids include spiders, scorpions, mites, ticks, and many other groups. Scientists continue to learn more about how these animals are related.

The Uraraneida are an extinct group of spider-like arachnids from long ago. A fossil found in amber shows a creature with spinnerets for making silk and a tail.