What is an Arachnid?


One of our most common identification requests are pictures of spiders and other arachnids. Nancy and I did a live video awhile back talking about arachnids. In the video we say that you can type the word “SPIDER” to receive a free PDF; however, that only works on Facebook. You can also download the PDF from here: Arachnid Bug Quiz Answers

What is an arachnid anyway? Like insects, arachnids are arthropods. Insects are a class of arthropods, and arachnids are also a class within Arthropoda. In biology we organize organisms based on their relationships (biological classification – think of Carl Linnaeus). A broad group is one that shares a lot of characters, while a more specific group shares unique character(s). Although insects and arachnids are related, they are different enough that they are placed in separate groups.

All arachnids have the following characteristics:

  • They possess 4 pairs of legs (8 legs total), chelicerae and pedipalps
, but they have no wings or antennae
  • Some groups have modified 1st pairs of legs that act like antennae, but are not technically antennae
  • Their main bodies are broken up into two segments:
    • prosoma/cephalothorax
    • opisthosoma/abdomen
  • There are some orders of arachnids in which these segments are fused (e.g. Acari, the ticks and mites) or are broadly connected (e.g. Opiliones, the harvestmen)

What is an order, you ask? Arachnids are organized into orders. Insects are also organized this way; for example, within Insecta (the name for the class of insects) there are beetles (Coleoptera), moths and butterflies (Lepidoptera), flies (Diptera), and so on. Check out Wikipedia to see a list of all arachnid orders!


Image: Big Bend Tarantula – October 2017

Bug Quiz image photo credits: Top left (A) jovengandalf, top right Daniel Llavaneras (B), bottom left Jon Richfield (C), and bottom right André Karwath (D).

About JoanieTheEntomologist

PhD candidate in entomology.
This entry was posted in Identification, Taxonomy. Bookmark the permalink.

1 Response to What is an Arachnid?

  1. Pingback: Ticks – Resources & Search Tips

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