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Arachnids

Arachnids are one class within the group of Arthropods. Arachnids can be further broken down into sub-groups of spiders, scorpions, ticks, mites and pseudoscorpions.

All arachnids are typified by the following features: no antennae

  • two body parts: cephalothorax (fused head and thorax) and abdomen
  • four pairs of legs attached to the cephalothorax
  • pincer-like or dagger-like jaws (chelicerae)
  • breathe via trachea (tube-like breathing apparatus) or book-lungs (sets of delicate sheet-like tissues which provide maximum surface area for respiration)

The diagrams and table below show the similarities and differences between two arachnid sub-groups: spiders and scorpions.

Spiders Scorpions
Spider and Scorpion
  • no antennae
  • no antennae
  • have a cephalothorax
  • have a cephalothorax
  • have pincer-like or dagger-like jaws called chelicerae containing fangs located on the cephalothorax
  • have pincer-like jaws called chelicerae located on the cephalothorax and no fangs
  • have a pair of palps (organs of touch) on the cephalothorax
  • have a pair of palps that are enlarged into pincer-like claws on the cephalothorax
  • have 4 pairs of legs attached to the cephalothorax
  • have 4 pairs of legs attached to the cephalothorax
  • have an obvious ‘waist’ between the cephalothorax and the abdomen
  • do not have an obvious ‘waist’ between the cephalothorax and the abdomen
  • have an abdomen which has spinnerets (organs used for silk production) at the rear
  • have an extended abdomen called a telson which looks like a narrow flexible tail
  • have book-lungs for breathing
  • have book-lungs for breathing

Other examples
Harvestmen are often mis-identified as spiders, but are rarely found indoors as they prefer moist outdoor areas. Harvestmen can be recognised by the fused cephalothorax and abdomen and the segmentation of the abdomen.

Harvestman

Arachnids include solifugae and scorpions (images are not to scale).

Bird Catching spider
Nephila (Bird-Catching) spider
Solifugae
Solifugae
Whip scorpion
Whip scorpion
Scorpion
Scorpion from Sri-Lanka
Tarantula
Tarantula from Java

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