In this video we are going to look at some of the characteristics that define the different
Arachnid orders.
There are 11 extant orders of arachnids.
All Arachnids consist of the same basic anatomy.
They are made up of two body segments called tagmata; the cephalothorax (also known as
the prosoma) and the abdomen (also known as the opisthosoma).
They possess 4 pairs of legs (8 legs in total) in the adult form.
They also have 2 pairs of appendages found at the front of the cephalothorax, the chelicerae
and the pedipalps.
They all lack wings and antennae.
Despite all orders possessing these same features, there are also features that help define each
order.
The first of these orders is the Opiliones, commonly known as the harvestmen.
Members of this order have a broad connection between the cephalothorax and the abdomen,
which gives this order its distinctive oval body shape.
They also usually possess long legs.
Like several other arachnid orders, the second pair of legs is longer then the rest as they
function as antennae or feeler like structures instead of for locomotion.
They also have a single pair of eyes which are located in the middle of the cephalothorax.
The second order is the Scorpiones, which are the Scorpions.
Members of this order also have a broad connection between the cephalothorax and the abdomen,
giving the appearance of one long body part.
All four pairs of legs function in walking.
The pedipalps terminate in claws or pincers called chelae, that are used for grasping.
They have two eyes located on the top of the cephalothorax.
They also have 2 to 5 pairs of lateral eyes located along the front side parts of the
cephalothorax.
The most characteristic feature of the scorpions is that the abdomen extends at the end to
form a tail, that is often held in a curve over the back.
This tail terminates in a stinger also known as the telson, that contain venom glands.
They also have a thick exoskeleton.
The next order is the Pseudoscorpiones, they are often known as Pseudoscorpions but are
also called false scorpions or book scorpions.
Members of this order are small and the abdomen is rounded giving the overall body a pear-like
shape.
Similar to scorpions the pedipalps terminate in pincers challed chelae, were venom glands
are found in the mobile finger portion of the pincer.
The amount of eyes vary from none, two or four.
They make disk-shaped cocoons for themselves by spinning silk made in a gland found on
their jaws.
The next order is the Solifugae, which are commonly known as the camel spiders, wind
scorpions or sun spiders.
Members of this order have some but not a lot of constriction between the cephalothorax
and the abdomen.
The first pair of legs are reduced and are smaller and thinner than the other pairs of
legs.
They have long pedipalps that look like they are the first pair of legs.
The most defining feature of this order is the presence of large chelicerae, made of
They usually have large central eyes and lack lateral eyes.
The next order is the Acari, which are the mites and the ticks.
Members of this order are small or minute, and the cephalothorax and abdomen are fused
creating a single round body call the idiosoma.
They also have an anterior region of the body called the gnathosoma or capitulum that includes
chelicerae and the pedipalps.
This anterior region is joined to the idiosoma by the circumcapitular furrow.
Most have four pairs of legs, but some have less.
Most lack eyes, but not all.
The next order is the Ricinulei, which are the hooded tickspiders.
Members of this order are small and have a narrow waist called the pedicel attaching
the cephalothorax to the abdomen.
This is not as obvious as it is in spiders as the front of the abdomen tucks into a fold
in the back of the carapace of the cephalothorax.
A thick exoskeleton covers the body and the legs.
Like in several other arachnid orders, the second pair of legs is longer then the rest
as they function as antennae or feeler like structures instead of for locomotion.
The most defining feature of this order is the presence of the cucullus which is a hood
like structure that covers the head that they can raise and lower to expose the mouth and
chelicerae.
The pedipalps teminate in pincers, but these pincers are small.
They also have no eyes.
The next order is the Palpigradi, which are the microwhip scorpions.
Members of this order are tiny and have a narrowing of the waist.
The are usually pale in colour.
The most distinctive feature of this order is that the abdomen terminates in a whip-like
flagellum that is covered in bristles . Members of this order also have the first
set of legs used for sensory purposes instead of locomotion.
They have long pedipalps that are used in locomotion, making it look as if they have
five pairs of legs.
They also have no eyes.
The next order is the Araneae, commonly known as the spiders.
Members of this order have a distinct separation between the cephalothorax and abdomen, these
are connected by the pedicel.
This allows the abdomen to move independent of the cephalothorax when they make silk.
They possess spinnerets and silk glands at the end of the abdomen.
The chelicerae terminate in usually venomous fangs.
They often have four pairs of eyes.
The next order is the Amblypygi, which are the whip spiders and tailless whip scorpions.
Members of this order are large and have bodies that are flattened.
The first set of legs in this order are much thinner and longer than the other legs, they
are not used in walking, but as sensory organs The most defining feature of this order is
the large raptorial pedipalps, that are modified to allow them to grab prey.
They usually have four pairs of eyes, one at the front and the other 3 pairs on the
side of the cephalothorax.
The next order is the Thelyphonida, which are the whip scorpions or vinegaroons.
Members of this order also have the first set of legs used for sensory purposes instead
of locomotion.
The pedipalps form large pincers.
The abdomen terminates in a whip-like tail.
They have glands at the end of the abdomen that produce and spray a mixture of acetic
acid and caprylic acid.
They have four pairs of eyes, one at the front and the other 3 pairs on the side of the cephalothorax.
The next order is the Schizomida, which are the short-tailed whip scorpions.
Members of this order have soft bodies.
They have a clear constriction or waist between the cephalothorax and the abdomen where the
first abdominal segment forms a pedicel connecting the two.
The abdomen terminates in a short whip-like flagellum or tail.
The first two legs are modified into sensory organs.
The pedipalps form large pincers.
They also do not have any eyes.
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