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
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.

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

Nephila (Bird-Catching) spider

Solifugae	Whip scorpion	Scorpion from Sri-Lanka
Tarantula from Java