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Introduction to Arachnids

Taxonomy | Life Cycle | Sexual Dimorphism | Identification

Welcome to the Electronic Field Guide to Arachnids! While it is not exhaustive, this site will allow you to explore a variety of arachnids from Massachusetts. You can find a particular species by name, taxonomic grouping, or visual key. Below you can find information about arachnids in general. You may also want to look at the glossary or an overview of arachnid anatomy.


Arachnids, members of the class Arachnida, are arthropods. This phylum is characterized by members having a segmented body covered in a jointed, chitinous exoskeleton. They possess a complex nervous system with a dorsal brain, and have an open circulatory system. One of the major subdivisions of Arthropoda is the subphylum Chelicerata, which encompasses Merostomata (horseshoe crabs), Pycnogonida (sea spiders), and Arachnida. Chelicerates are so named for chelicerae , a modified pair of appendages that serve as jaws. Chelicerates also posess a cephalothorax and pedipalps, and lack antennae.

Arachnida is divided into 11 orders:

  • Acharina -- mites and ticks -- have a single body segment and lack spinnerets
  • Amblypygi -- whipspiders or tailless whipscorpions -- have a long, whip-like first pair of legs
  • Araneae -- spiders -- have four pairs of legs, have two distinct body segments (the cephalothorax and abdomen), have poisonous fangs on their chelicerae, generally have 8 eyes (though some have 6 and a small number of species have fewer), have spinnerets , and do not have tails
  • Opiliones -- harvestmen -- similar to spiders but have a single body segment, two eyes, and lack spinnerets
  • Palpigradi -- microwhipscorpions -- are small (> 2mm) and have a relatively long multi-segmented tail
  • Pseudoscorpionida -- pseudoscorpions -- have large front pincers (which often contain poison glands) and lack a tail
  • Ricinulida -- ricinuleids -- resemble ticks but have a moveable covering over their head and mouthparts
  • Schizomida -- schizomids -- are small (> 6mm) and eyeless, and have a short tail
  • Scorpionida -- scorpions -- have pincers and a long tail with a stinger at the end
  • Solpugida -- windscorpions -- have large jaws, lack a tail, and use their pedipalps and first pair of legs as feelers
  • Uropygida -- whipscorpions -- have pincers and a tail but lack a stinger

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Life Cycle

The spider life cycle has four stages: egg, larvae, nymph, and adult. Spiders nymphs go through several stages of molting. Each stage looks similar to an adult spider, although the markings and coloration are often different. Male spiders do not use penises to mate. Instead, they coat their palps with sperm, and search for the female. In most species, the males court the females, often with elaborate rituals. The female stores the sperm until the conditions are right for her to release her eggs.

Spider eggs are laid in a cocoon by the mother. Many species leave the eggs after laying them, although some guard them, or even carry them around. The eggs hatch into larvae, which remain inside the cocoon, receiving nourishment from the yolk. After about two weeks, the larvae molt into spiderlings, which leave the egg sac. They go through many molts--some as few as five, some as many as 40--before reaching sexual maturity after their final molt.

Nearly all spiders produce silk. Most of them use this silk to create webs. These webs are varied in shape and construction. One of the most recognizable are orb webs (see picture), which consist of crossing strands meeting in the middle and a continuous thread spiraling outward. There are also many other styles, such as cobwebs (see picture), which are an irregular mesh of silk, or funnel webs (see picture), which have a flat sheet of silk with a small funnel retreat where the spider waits for prey. Most spiders use their web to catch insects. Some spiders do not build snares for prey at all, but instead actively hunt their prey. Spiders that do not build webs to feed may still build silken retreats, in which they take shelter.

Once the prey is caught, a spider will using their chelicerae to pierce it and inject poison into the wound. Then, the spider regurgitates digestive juices into its prey to digest it. Finally, powerful muscles in the spider's stomach suck the liquefied tissue into the spider's body. It is strained by filters in the mouth, then further digested by enzymes in the midgut.

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Sexual Dimorphism

Spiders are sexually dimorphic. Male spiders are fairly easy to identify with a hand lens because their palps are larger and contain a swollen end, which often appears fuzzy. Female spiders are usually bigger. In some species the females may be more than five times the size of the males. Females usually have more rounded abdomens and may have different color patterns than males. Sex is usually impossible to determine in juveniles.

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Without a great deal of experience or at least a hand lens, it is extremely difficult to identify spiders. Coloration patterns are highly variable within a genus, and sometimes even within a species. One of the most important clues to look for is the eye pattern. If you are using this site for identification purposes, you should be aware that many of the specimens on this site were preserved in alcohol before being photographed. This alters their coloration, and can make patterns indiscernible. The alternative to preserving the spiders, however, is having them shrivel up and become even harder to identify. It is often not possible for the amateur enthusiast to identify a spider to species. Some genera have dozens of species, many of which are only distinguishable by minute characteristics of on the genetalia or on the claws on the ends of their legs, for example.

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Taxonomy | Life Cycle | Sexual Dimorphism | Identification

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Last updated 12/13/2004