Chelicerates Group: Key Characteristics, Species, and Classifications

Chelicerates (Chelicerata) are a group of arthropods that includes harvestmen, scorpions, mites, spiders, horseshoe crabs, sea spiders, and ticks. There are about 77,000 living species of chelicerates. Chelicerates have two body segments (tagmenta) and six pairs of appendages. Four pairs of appendages are used for walking and two (the chelicerae and the pedipalps) are used as mouthparts. Chelicerates have no mandibles and no antennae.

Chelicerates are an ancient group of arthropods that first evolved about 500 million years ago. Early members of the group included the giant water scorpions which were the largest of all arthropods, measuring up to 3 meters in length. The closest living cousins to giant water scorpions are the horseshoe crabs.

Early chelicerates were predatory arthropods, but modern chelicerates have diversified to take advantage of a variety of feeding strategies. Members of this group are herbivores, detritivores, predators, parasites, and scavengers.

Most chelicerates suck liquid food from their prey. Many chelicerates (such as scorpions and spiders) are unable to eat solid food due to their narrow gut. Instead, they must expel digestive enzymes onto their prey. The prey liquifies and they can then ingest the food.

The exoskeleton of a chelicerate is a hard external structure made of chitin that protects the arthropod, prevents desiccation, and provides structural support. Since the exoskeleton is rigid, it cannot grow with the animal and must be molted periodically to allow for increases in size. After molting, a new exoskeleton is secreted by the epidermis. Muscles connect to the exoskeleton and enable the animal to control the movement of its joints.

Key Characteristics

• six pairs of appendages and two body segments
• chelicerae and pedipalps
• no mandibles and no antennae

Classification

Chelicerates are classified within the following taxonomic hierarchy:

Animals > Invertebrates > Arthropods > Chelicerates

Chelicerates are divided into the following taxonomic groups:

• Horseshoe crabs (Merostomata): There are five species of horseshoe crabs alive today. Members of this group live in shallow marine waters along the Atlantic coast of North America. Horseshoe crabs are an ancient group of chelicerates that date back to the Cambrian. Horseshoe crabs have a distinct and unsegmented carapace (hard dorsal shell) and a long telson (a spine-like tailpiece).
• Sea spiders (Pycnogonida): There are about 1300 species of sea spiders alive today. Members of this group have four pairs of very thin walking legs, a small abdomen, and elongated cephalothorax. Sea spiders are marine arthropods that feed on nutrients of other soft-bodied marine invertebrates. Sea spiders have a proboscis that enables them to obtain food from prey.
• Arachnids (Arachnida): There are more than 80,000 species of arachnids alive today (scientists estimate that there might be more than 100,00 living species). Members of this group include spiders, scorpions, whip scorpions, ticks, mites, pseudoscorpions, and harvestmen. Most arachnids feed on insects and other small invertebrates. Arachnids kill their prey using their chelicerae and pedipalps.

Sources

• Hickman C, Roberts L, Keen S. Animal Diversity. 6th ed. New York: McGraw Hill; 2012. 479 p.
• Ruppert E, Fox R, Barnes R. Invertebrate Zoology: A Functional Evolutionary Approach. 7th ed. Belmont CA: Brooks/Cole; 2004. 963 p.