1. EXTERNAL ANATOMY
Tarantula’s have a hard cuticle or body shell called an exoskeleton. The exoskeleton covers
the cephalothorax and legs and prevents the spider from losing moisture and drying out.
The exoskeleton also provides the tarantula with structural support. The exoskeleton is
shed as the tarantula grows.
Tarantulas can sense their environment to a degree through their eyes.
Tarantula’s have 8 eyes and can detect polarised light which helps them to orientate
themselves. The eyes of a tarantula are simple eyes, meaning there is just a single lens to
each eye. Tarantula eyes are arranged in two rows of four, just back from the chelicerae.
Another way that they sense their environment is through touch. This is achieved through a
number of modified hairs covering the entire body, all with specific functions. These hairs
are connected to nerves internally.
Hairs on the lower limbs called trichobothria help with orientation, detecting the faintest of
Hairs on the feet called scopulae help to hold on to a surfaces like glass.
New-world tarantulas (found in North and South America) are equipped with
urticating hairs on
their abdomen. Tarantulas throw these hairs, which are barbed, as a first line of defence.
They do this by rubbing their legs against their abdomen. These hairs irritate sensitive areas
of the body, such as the mucous membrane if inhaled. Some species have urticating hairs
that can penetrate surfaces such as the cornea of the eye. Care needs to be taken when
dealing with these tarantulas.
A tarantula’s body is divided into two parts:
Externally the prosoma is comprised of the eight legs, the two chelicerae with their hollow
fangs, the eyes and the pedipalps. Internally the prosoma is comprised of the nervous centre and venom
The opisthosoma or abdomen internally contains the respiratory
organs (book lungs), reproductive organs, silk glands, the heart
and part of the digestive tract. The abdomen is also covered
externally by urticating hairs.
Tarantulas have two appendages near their mouths called pedipalps which are used to
manipulate their prey while feeding. The pedipalps of immature males are expanded and look
like boxing gloves. As the male matures the pedipalps are transformed into highly complex
organs that are used to inseminate females. The female’s pedipalps are slender.
We all know that spiders have 8 legs. Each leg is divided into 7 segments. The seven
segments of a tarantula’s leg make them very flexible. Tarantulas have muscles that bend
the legs closer to the body but not away from the body. Each time a tarantula needs to
stretch a leg back out it must pump fluid into that leg.
To bend the leg back, pressure is relaxed and the fluid flows out of the leg as the muscles do
their work. This resembles a hose getting stiff as it fills with water and the pressure builds;
going limp when the pressure drops and water is released.
So spiders extend their legs by changes in body fluid pressures. This is why when a spider
doesn’t receive enough fluid that its legs fold in toward the body.
The chelicerae are two single segment appendages that are located just below the eyes and
directly forward of the mouth. The chelicerae contain the venom glands that vent through
the fangs. The chelicerae of tarantulas completely contain the venom glands and the
muscles that surround them.
The fangs are hollow extensions of the chelicerae that inject venom into prey that the
tarantula bites. The fangs are also used to masticate (chew). The fangs are positioned so
that they can extend downward and outward in preparation to bite. They can also fold back
toward the chelicerae.
2. INTERNAL ANATOMY
Tarantulas also have an internal skeleton that is actually an extension of the external
exoskeleton. This internal skeleton serves as a surface for muscle attachment.
The Tarantula’s nervous centre is found in the prosoma or cephalorthorax. It essentially is
made up of two collections of nerve cell bodies (ganglia). Nerve fibres branch off from these
ganglia and connect to the various organs internally. The ganglia act as relay message
centres between the organs and the body.
Respiration in tarantulas is achieved through book lungs. Tarantulas have 4 book lungs.
Each lung consists of 15 or more thin sheets of folded tissue arranged like the pages of a
book in a cavity. These sheets of tissue are supplied by blood vessels. Air enters the cavity
through a tiny slit on each side of and near the front of the abdomen. As air enters each
lung, oxygen is taken into the blood stream through the blood vessels in the lungs. Much
needed moisture may also be absorbed from humid air in the same fashion.
A tarantula’s circulatory system is very different to that of a human. Tarantula’s blood is
also very different. A tarantula’s blood is a liquid called hemolymph. The tarantula’s heart
is a long slender tube that is located along the top of the opisthosoma or abdomen. The
heart is neurogenic and so nerve cells initiate and coordinate the heart. The heart pumps
hemolymph to all parts of the body through open passages called sinuses. If the exoskeleton
is damaged, loss of hemolymph will kill the tarantula unless the wound is small enough that
the hemolymph can dry and close the wound.
The mouth is located on the under part of the prosoma, under the chelicerae. The mouth is
a small straw-shaped opening that can only suck. This means that any food taken in needs
to be in liquid form. The chelicerae secrete digestive juices through openings that aid in the
break down (pre-digestion) of the prey.
The stomach of a tarantula runs the entire length of
the body, however, what is called the sucking stomach is located in the prosoma. Muscles
surround the sucking stomach and when these muscles contract they cause a sucking
action and the tarantula is able to ingest the liquefied prey which enters through the mouth
and then moves to the intestines. The liquefied prey is further broken down in the
intestines and passed into the hemolymph where it is distributed to the entire body. The
remains of the liquefied prey are formed into a little ball which the tarantula will discard in a
particular area. This should be removed regularly to prevent mould and fungus forming.
Excrement is voided through the anus.
At least three different silk glands can be found in the tarantula's abdomen. The silk is liquid
inside the gland. When the silk is excreted through the spinnerets, it changes into a solid.
Tarantulas typically have 2 or 4 spinnerets.
Both testes and ovaries are paired organs found in the abdomen of the tarantula. Male
tarantulas have testes and female tarantulas have ovaries. The external opening that leads
to and from the testes and ovaries is called the genital opening or epigastric furrow. Females
receive sperm from the male through the epigastric furrow.