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9.13: Arachnids

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    Is a spider a type of insect?

    Although spiders and insects are both arthropods, a spider is not an insect. One key difference is that insects have six legs, while spiders have eight legs.


    Arachnids are a class of joint-legged invertebrates in the subphylum Chelicerata. They live mainly on land but are also found in fresh water and in all marine environments, except for the open ocean. There are over 100,000 named species, including many species of spiders, scorpions, daddy-long-legs, ticks, and mites (Figure below). There may be up to 600,000 species in total, including unknown ones.

    A spider and a tick, which are both arachnids
    Figure \(\PageIndex{1}\): (left) A daddy-long-legs spider. (right) Various diseases are caused by bacteria that are spread to humans by arachnids, like the tick shown here.

    Characteristics of Arachnids

    Arachnids have the following characteristics:

    1. Four pairs of legs (eight total). You can tell the difference between an arachnid and an insect because insects have three pairs of legs (six total).
    2. Arachnids also have two additional pairs of appendages. The first pair, the chelicerae, serve in feeding and defense. The next pair, the pedipalps, help the organisms feed, move, and reproduce.
    3. Arachnids do not have antennae or wings.
    4. The arachnid body is organized into the cephalothorax, a fusion of the head and thorax, and the abdomen.
    5. To adapt to living on land, arachnids have internal breathing systems, like a trachea or a book lung.
    6. Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals.
    7. Several groups are venomous. They release the venom from specialized glands to kill prey or enemies.
    8. Several mites are parasitic, and some of those are carriers of disease.
    9. Arachnids usually lay eggs, which hatch into immature arachnids that are similar to adults. Scorpions, however, give birth to live young.

    Arachnid Subgroups

    The arachnids are divided into eleven subgroups. Below are the four most familiar subgroups, with a description of each (Table below).

    Subgroup Representative Organisms Approximate Number of Species Characteristics
    Araneae Spiders 40,000
    • Found all over the world, ranging from tropics to the Arctic, some in extreme environments.
    • All produce silk, which is used for trapping insects in webs, aiding in climbing, producing egg sacs, and wrapping prey.
    • Nearly all spiders inject venom to protect themselves or to kill prey. Only about 200 species have bites that can be harmful to humans.
    Opiliones Daddy-long-legs 6,300
    • Known for extremely long walking legs. No silk nor poison glands.
    • Many are omnivores, eating small insects, plant material, and fungi. Some are scavengers, eating decaying animal and other matter.
    • Mostly nocturnal (come out at night) and colored in hues of brown. A number of diurnal (come out during the day) species have vivid patterns of yellow, green, and black.
    Scorpiones Scorpions 2,000
    • Characterized by a tail with six segments, the last bearing a pair of venom glands and a venom-injecting barb.
    • Predators of small arthropods and insects. They use pincers to catch prey. Then they either crush it or inject it with a fast-acting venom, which is used to kill or paralyze the prey. Only a few species are harmful to humans.
    • Nocturnal; during the day, they find shelter in holes or under rocks.
    • Unlike the majority of arachnids, scorpions produce live young. The young are carried about on the mother’s back until they have molted at least once. They reach an age of between four to 25 years.
    Acarina Mites and ticks 30,000
    • Most are small (no more than 1.0 mm in length), but some ticks, and one species of mite, may grow to be 10-20 mm in length.
    • Live in nearly every habitat, including aquatic and terrestrial.
    • Many are parasitic, affecting both invertebrates and vertebrates. They may transmit diseases to humans and other mammals. Those that feed on plants may damage crops.


    • Arachnids have four pairs of legs, specialized appendages, and a fused head and thorax.
    • Arachnids include spiders, daddy-long-legs, scorpions, and ticks.

    Explore More

    Use the resource below to answer the questions that follow.

    1. How do spiders differ from insects?
    2. What are some of the uses of their silk?
    3. How do spiders keep from becoming stuck in their own webs?
    4. What role do spiders play in their ecosystem?


    1. List three types of arachnids.
    2. What are two key features of arachnids?
    3. Arachnids also have two additional pairs of appendages. Describe these two pairs.
    4. List two specific features of spiders.
    5. List two specific features of scorpions.

    This page titled 9.13: Arachnids is shared under a CC BY-NC license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to the style and standards of the LibreTexts platform.