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, or other body parts might grow big enough to look like extra legs.

Almost all living arachnids live on land, but some can be found in fresh water, and a few even live in the ocean, except in the open deepest parts. There are more than 110,000 known kinds of arachnids, and about 51,000 of these are different types of spiders. 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, which is different from insects that have six legs. Arachnids also have two extra pairs of body parts used for eating, protecting themselves, and sensing their environment. 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, while in others they are used to catch prey. Some arachnids even look like they have ten legs because their pedipalps are very leg-like.

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 and some have special internal support structures.

Locomotion

See also: Arachnid locomotion

Most arachnids do not have special muscles to straighten their legs. Spiders and whip scorpions use fluid pressure from their blood to move their legs. Some creatures like solifuges and certain harvestmen use stretchy parts in their joints to bend their knees. Scorpions, pseudoscorpions, and some harvestmen have special muscles that help them move two parts of their legs at the same time. The same joints in scorpions' mouthparts move by using stored stretchiness.

Physiology

See also: Hemolymph

Arachnids have special features that help them live on land. They have structures for breathing, like tubes or modified organs that let them trade gases with the air. They also have body parts that help them move better on land, and ways to keep water inside their bodies.

Arachnids have glands that help remove waste. Their blood can change based on how they breathe. Some have a special substance in their blood that helps carry things, similar to what humans have. The heart in arachnids is found in the front part of their body, and some small creatures don’t even have a heart.

Diet and digestive system

Arachnids mostly eat insects and other small animals, but some, like ticks and mites, feed on blood or tiny particles. Some arachnids, such as harvestmen, eat both plants and animals.

Arachnids produce special juices in their bodies to break down their food. They pour these juices on their prey, turning it into a liquid they can drink. This liquid travels through their bodies, providing them with nutrients.

Senses

Arachnids have two kinds of eyes: the lateral and median ocelli. Most arachnids have lateral eyes that evolved from compound eyes, and some can have a special layer called a tapetum that helps them see better in the dark. The median eyes develop from a fold in the ectoderm.

Besides eyes, arachnids also have many fine sensory hairs on their bodies that help them feel their surroundings. Some have even more complex touch sensors called trichobothria. They also have special slit-shaped pits with tiny hairs inside that help them sense movements and possibly sounds. These pits are known as slit sense organs and are believed to help with proprioception, which is the ability to know 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 and are found in the middle part of their bodies. Most arachnids have the males give sperm to the females in a special package. Some arachnids, like certain mites and harvestmen, have males with a special organ to transfer sperm.

Most 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, but harvestmen are one of the few exceptions.

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 through careful study and analysis.

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, similar to some very ancient arachnids.