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.

SENSORY SYSTEM

Sight

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.

Hairs

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 air currents.

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.

BODY DIVISIONS

A tarantula’s body is divided into two parts:

Prosoma (Cephalothrox)
Opisthosoma (Abdomen)

Prosoma

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 glands.

Opisthosoma

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.


APPENDAGES

Pedipalps

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.

Legs

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.

Chelicerae

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.

Fangs

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.

 

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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.

Nervous system

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.

Respiratory system

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.

Circulatory system

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.

Digestive system

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.

Silk glands

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.

Reproductive system

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.