Arachnids

Arachnids are invertebrates (do not have a back bone) and belong to the phylum (group) arthropoda. Arthropod means 'jointed legs' and the group also includes insects, crabs, millipedes and centipedes. Approximately 900,000 species of arthropods have been identified so far. They live all over the world in the sea, in rivers and lakes and on land. Arthropods have a tough outer skin made of chitin called the cuticle. This is a water proof layer is like a protective coat of armour which supports their body like a skeleton. Because this layer is on the outside of the animal we call it an 'exoskeleton'.

Arachnids are all carnivores, they have eight legs, no antennae and no wings; this group includes spiders, scorpions, whip spiders, whip scorpions, pseudoscorpions, harvestman, camel spiders, ticks and mites.

Tarantulas

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Introduction
Classification
Distribution & Habitat
Diet
Venom
Self Defence
Predators
Senses
Moulting
Lifespan
Reproduction

Introduction	There are around 35,000 species of spider. Spiders have two body segments; a fused head and thorax (cephalothorax) and an abdomen. The eight legs, jaws (chelicerae), eight eyes and two palps (feelers and male reproductive organs) are positioned on the cephalothorax. Spiders spin silk from spinnerets on the abdomen although not all spiders make webs. They kill their prey by injecting venom through their hollow chelicerae that digests the internal tissues, and then suck the contents out. All spiders are venomous but very few are harmful to humans. back to top
Classification	Tarantulas are primitive orthognath spiders and have backward facing fangs which move like index and middle fingers. This group also includes trapdoor, funnel web, purse web and folding door spiders. Less primitive (true) spiders have fangs which pinch together like a finger and thumb. Like most primitive spiders, tarantulas are generally larger and have less highly developed senses and behaviours. A tarantula is actually a large spider from southern Italy with a very painful bite. This word was used by explorers to describe large spiders when they were found all over the world. Some Europeans also call these spiders 'bird eating' but they are technically best described as theraphosids. back to top
Distribution & Habitat	Tarantulas live on every continent of the world in areas with tropical, subtropical or hot-temperate climates. They inhabit a huge range of habitats from rainforest canopies to desert burrows. Most ground living tarantulas use an abandoned burrow or rocks to hide under. Individual territories are marked out using a carpet or net of silk. back to top
Diet	Diet varies depending on species but most tarantulas eat insects, frogs, lizards and small mammals. Birds are rarely eaten because they are very difficult to catch. They can often go for months without eating, especially hibernating species. They are excellent ambush predators, sitting motionless until suitable prey moves into just the right position for the tarantula to grab it. The prey may be drained, partially eaten or wrapped in silk and saved for later. Water is mainly absorbed from their food or from the atmosphere. back to top
Venom	All spiders have venom which they can inject using their fangs, but this is not usually dangerous to humans. There are two types of venom produced by spiders; neurotoxins and cytotoxins. Neurotoxins affect the nervous system and have a paralysing effect on the prey. Black widow and Sydney funnel web spiders have neurotoxic venom. Cytotoxins affect the tissues of the body and may cause them to swell or decay. Brown recluse spiders use cytotoxins. Tarantulas have a very low-grade neurotoxin which is not life threatening to humans unless the person is allergic. Venom is very valuable to the spider and is normally only injected when the prey is particularly large or strong. back to top
Self Defence	Different tarantulas behave differently but most species will avoid coming into contact with a large animal like a human and will normally try to run away. They really are more afraid of you than you are of them. If they cannot escape they will try to scare the predator by standing on tip toes and raising the front legs to make themselves look bigger. If they are still under threat they will rub their legs against their abdomen, spraying bristles into the air. The bristles are very itchy and can cause an allergic reaction in some people. They can cause coughing and sneezing if breathed in. If this hasn't put the predator off the spider may attempt a bite. However, tarantulas cannot eat a human and do not want to waste venom. They are most likely to attempt a dry bite where the skin is punctured but no venom is injected. If the tarantula does inject venom it is not likely to be lethal. Most bites produce an ache or tingling in the area, some redness and possibly mild swelling. It is always advisable to seek medical advice if you are bitten by an unusual spider or have any signs of a reaction to it. There are less than a dozen commonly dangerous species of tarantula worldwide. back to top
Predators	The most common predators of tarantulas are spider-hunting wasps. At only one tenth the size of the tarantula these solitary wasps inject the spider with paralysing venom. Then the tarantula is dragged to a burrow, a wasp egg is laid on top and the burrow is sealed. The unfortunate tarantula will then be fed on by the wasp larvae when it hatches. back to top
Senses	Tarantulas have group of eight small, simple eyes (ocelli) on the top of the cephalothorax just behind the chelicerae (fangs) but their sense of sight is poor. The bristles covering their body are very sensitive to vibrations and are used to detect and capture their prey and to avoid predators. Whether spiders can hear, taste or smell is still debatable. They do avoid eating some insects which are known to taste bad. back to top
Moulting	The tough exoskeleton of all arthropods prevents them from growing continuously like humans. Arthropods have to shed their outer skin to allow them to increase in size and repair damage to this outer layer. This process is called shedding, moulting or ecdysis and is controlled by hormones. A new skin develops underneath the old one, the old one is removed and the new skin is inflated until it hardens. Tarantulas lie on their back, the upper covering of the cephalothorax splits and the old skin is pushed up with the legs and thrown to one side. This is usually done in the safety of night and takes several hours before the spider is fully dried and hardened. Most tarantulas shed once a year or more depending on the rate of growth. All external body parts, including eyes, legs and fangs, are still attached to the shed skin (exuvium). These sheds are often mistaken for dead spiders. During this process broken limbs can be regenerated, although it may take several moults before a whole leg is re-grown. back to top
Lifespan	Male and female tarantulas look very similar until they are four to seven years old. The male undergoes his maturing moult and develops hooks on the front pair of legs, the pedipalps are enlarged and they become increasingly active and keen to wander about (looking for females). This is the first tick of the time bomb and males will die within the next 18 months. Female tarantulas continue to grow and are generally larger because they live for longer, up to 30 years in some species. back to top
Reproduction	Male tarantulas use a secondary sexual organ to pass sperm to the female. The enlarged pedipalps are filled with sperm either directly from the opening under the abdomen or by making a tent of silk. The male lies upside down under the tent, places a drop of sperm on the roof of the tent then fills the pedipalps from this drop. Tarantulas have a delicate courtship where the male signals to the female, with taps and touches, so that she doesn't mistake him for prey and attack him. The males' front legs then reach over the females head, the new hooks hold onto her fangs and he lifts up the female, bending her backwards. He then transfers sperm to the female from underneath. Females do not always eat the male but spiders are cannibalistic and may attack the male if they are hungry. The female can delay production and fertilisation of eggs for several months. When the time is right she spins a cocoon-like web to hold her eggs called an egg sac or an ootheca. Up to 1,000 tiny eggs are produced. The female tarantula is unusual in tending and protecting her eggs and she may become aggressive. Incubation time can vary depending on the temperature and humidity. The tiny spiderlings are only a few millimetres long, they leave their mother after about a week. If they stay around too long the adult female or their siblings may eat them. back to top