Arthropods at the Cofrin Center for Biodiversity

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Horseshoe Crabs, Sea Spiders, Spiders and their kin

Phylum: Chelicerata (Kuh-liss-uh-rah'-tuh)

The superphylum Arthropoda is divided into two sub-groups. The phylum Mandibulata includes Hexapoda (Insects) and Myriapods (centipedes, millipedes, and their kin). These animals all have mouthparts called mandibles, which are jaw-like structures that work against one another (although these mandibles have evolved into bizarre and very un-jawlike structures in some groups). The other subphylum, Chelicerata, have mouthparts called chelicerae, which are either fang-like or pincer-like in form. The Chelicerata includes 3 classes of animals:

Class Merostomata

As their classification indicates, horseshoe crabs are not related to "true" crabs at all. There are only 4 living species of horseshoe crabs living in the world's oceans, and they have not changed much in form in the last 350-400 million years. Curiously, they are now found only in the waters off eastern North America, Southeast Asia, and Indonesia.

Class Pycnogonida

Like Arachnids, most sea spiders have 4 pairs of long walking legs, although some have 5 or 6 pairs. The abdomen is reduced to a short stump. These animals are not closely related to spiders at all. All 1,000 or so species are marine, with a fossil record extending back about 500 million years. They are often parasitic on cnidarians (jellyfish, corals, and their relatives) as young, and predators of slow-moving animals as adults.

Class Arachnida

Arachnida Arachnids all share a distinctive body plan that unites them and separates them from the other two groups of chelicerates. All Arachnids have 6 pairs of appendages. The first pair of appendages is the fang or pincerlike chelicerae. These are followed by a pair of short leglike appendages near the mouth, called pedipalps. Behind these are 4 pairs of walking legs, although the first pair is used like a pair of antennae or feelers by some mites and other groups. Most arachnids have a body divided into 2 body parts: The prosoma carries these appendages along with eyes (if any) and the brain. The opisthosoma (or abdomen) carries the reproductive organs along with many of the other internal organs such as the heart, digestive sacs, and excretory (kidney-like) organs. Most arachnids are terrestrial, although some mites live in freshwater and a few other arachnids live in the intertidal zone of oceans. There are about 11 orders of arachnids alive today, containing some 74,000 described species. Some of the orders are rather unfamiliar, at least to people from temperate regions.

Five Most Commonly Encountered Arachnid Orders

Tick Order Acari (Mites and ticks). Although only 30,000 species of Acari are known, there are so many undescribed species that Acari is almost certainly the most diverse group of arachnids. They occur in almost every habitat, including Antarctica, and many are important parasites of plants and animals. Other species are predators, fungal feeders, or scavengers. Although large blood-feeding ticks are the most familiar mites, most mites are less than 1 mm in body length. Mites are the smallest chelicerates.

Order Opiliones (harvestmen or daddy-longlegs). These animals resemble mites, but their legs are always longer than their body. These animals feed on smaller animals or are scavengers on fruit, vegetables, and especially on dead insects. It is a widespread myth that daddy-longlegs contain a powerful venom but are too small to bite humans…in reality, Opiliones have no venom at all. They also differ from spiders in lacking the ability to make silk. There are about 5,000 described Opiliones species.

Order Scorpiones (Scorpions). Scorpions have large pincerlike pedipalps and are unique among arachnids in having the abdomen drawn out into a long flexible tail armed with a venomous sting. There are about 1,500 species of scorpions worldwide, and all are predators. They are most diverse in deserts and tropical forests. There are no scorpions in northeast Wisconsin, although there are representatives of the other 4 orders mentioned here.

Order Pseudoscorpiones (Pseudoscorpions). These resemble tiny scorpions, with relatively large pincerlike pedipalps. However, pseudoscorpions lack the abdominal sting of true scorpions. Like scorpions, they are all predators, feeding on the body juices of other invertebrates. There are about 2,000 species of these small arachnids, which are only a few mm long. They often inhabit soil environments, and many travel from place to place by clinging onto flying insects, a behavior known as phoresy.

Order Araneae (Spiders)

References and Web Links

Margulis L, Schwartz KV (1998) Five kingdoms: an illustrated guide to the phyla of life on earth. 3rd ed WH Freeman, New York.

Pechenik, J. A. 2000. Biology of the Invertebrates, 4th ed. McGraw-Hill, New York.

The Arachnology Homepage has information on all the orders in class Arachnida.

Biodiversity Topics: Introduction . Plants . Animals . Mammals . Birds . Reptiles & Amphibians . Arthropods . Spiders . Insects