How do spiders differ from insects?

While spiders and insects are both arthropods (animals with an exoskeleton) and have a lot in common, there are some important differences that separate them. Spiders are actually arachnids, a group that includes mites, ticks, scorpions and harvestmen, among others. The most obvious feature that distinguishes arachnids from insects is the number of legs. Arachnids have eight, while insects only have six.

Unlike insects, spiders don't have antennae (feelers). The main part of the spider's body is also different from an insect's - while an insect has a head, thorax and abdomen, the spider has the head and thorax fused into one structure called a cephalothorax.

How many kinds of spiders are there?

About 38,000 species of spiders belonging to 3526 genera of 109 families are known to science. Many spider species are not yet known to science, particularly in the tropics. Some of the most diverse and abundant spider families include jumping spiders (Family Salticidae, 4889 species), the sheet-web spiders (Family Linyphiidae, 4214 species) the orb weaving spiders ( Family Araneidae, 2817 species), the comb-footed spiders (Family Theridiidae, 2199 species), the wolf spiders (Family Lycosidae, 2261 species), and the crab spiders (Family Thomisidae, 2023 species).

Where can I find spiders?

Spiders are found all over the world in all sorts of habitats from the sea shore to the dessert - on the ground, under rocks, on plants, in trees, in caves, on water, in human dwellings. Each type of spider tends to be found in a habitat to which it has become specifically adapted.

What body parts do all spiders have?

A spider's body consists of a cephalothorax with eyes, mouthparts - a pair of jaws and a pair of pedipalps, and four pairs of jointed legs; and an abdomen connected to the cephalothorax by a narrow pedicel. The entire body is encased by a tough protective exoskeleton and much of the body has sensory hairs growing from the skin.

How many eyes do spiders have?

Most spiders have eight simple eyes. The two main eyes of a spider each have a simple lens, and a retina which is made up of light sensitive cells whose surfaces point toward he light as it enters the eye. These main eyes have a small field of vision with high resolution. They are especially well developed in jumping spiders. A spider's secondary eyes also have a lens but the light sensitive cells of these eyes point away from the light as do the similar cells in a human eye. The secondary eyes detect shadows and the difference between light and dark. A few spider families are characterized by only six eyes.

Do spiders have claws?

Yes. Spiders have claws at the end of each leg. Spiders' legs are segmented and each leg has 7 segments: a coxa (attached to the cephalothorax), trochanter, femur, patella, tibia, metatarsus, and finally a tarsus which may end in two or three small claws. Web-building spiders typically have three claws on the end of each leg. The middle claw and a small tuft of hairs help the spider cling on to its web.

Do spiders have blood?

Yes. Spiders have an open, blood circulatory system. This sort of blood system has a heart, arteries and veins but no capillaries. The heart is tubular with a single cavity and with valves to maintain the flow of blood always in the same direction. The spider's blood is pale blue due to the presence of haemocyanin dissolved in the lymph. There are some blood cells but they are for wound-healing and defense against infection. There are no special blood cells like the human red cells which carry oxygen around the body.

How do spiders breathe?

Spiders have lungs. There are two sorts of lungs neither of which is like a human lung. Some spiders have book lungs. A book lung has a stack of soft plates called lamellae. Oxygen in the air passing between the lamellae diffuses through the tissue into the blood. Other spiders have tracheae which are breathing tubes held open by rings of chitin. The tracheae open to the outside and the opening is called a spiracle. There appears to be no active, muscular breathing mechanism. Air seems to pass in and out of the book lung or the tracheae in a passive manner.

What and how do spiders eat?

All spiders are carnivorous. Most of them eat insects but a few of the larger species are big enough to prey on small vertebrate animals like mice or small birds. Most spiders' jaws work from side to side. They have toothed edges used in breaking up the prey during feeding. Digestion starts before any of the prey (the food) is swallowed. Some spiders inject digestive enzymes into the prey before they start breaking it up; others secrete digestive fluids as they are breaking the food up with their jaws. The partially digested food is sucked into the spider's alimentary canal (gut).

Do spiders bite?

Yes. Spiders have two small jaws (chelicerae) that end in fangs below the eyes on head end of the cephalothorax. Venom (poison) is produced in glands behind the jaws and empties along ducts in the fangs to paralyze or kill prey. Relatively few spiders bite people because they are not able to pierce the skin with their fangs. The majority of those spiders that can bite people have venom that is harmless to people. Two notable exceptions are the Brown Recluse spider and the Black Widow spider which are very uncommon in the tropics.

How many spiders have poisonous venom that affects humans?

While there are only a few spiders in India that are really dangerous to humans, there are significant numbers around the world. The black widow spider found throughout the United States and recluse spiders have very toxic venom that can be life threatening to humans. Some wolf spiders in South America, and some running spiders (family Chiracanthium) worldwide have venom that causes painful symptoms in humans. Pigeon spiders of West Africa give very painful bites.

Do all poisonous spider bites have the same effect?

No. When dealing with the effect of spider venom on humans there are two types. Some spider venom is neurotoxic, i.e. it affects the human nervous system beyond the site of the bite. The black widow venom is neurotoxic. A principal component of this venom is a-latrotoxin. A black widow bite causes rigidity , cramp, and paralysis of the sympathetic systems. Occasionally it causes death. Other spider venom is necrotic and causes damage to the tissues surrounding the site of the bite. The brown recluse spiders have necrotic venom. The damage usually results in skin blisters, ulcers and blackening of the local tissues.

How do spiders catch their prey?

Spiders capture their prey in usually four methods.

i) Sedentary spiders living in silk-lined burrows leap out to capture passing insects.
ii) Some, normally active spiders, lie in ambush on plants, tree bark, on the ground or under stones.
iii) Hunting spiders go in search of their prey.
iv) Many spiders spin webs to entrap their prey.

Do all spiders spin webs?

No. Nonetheless, spiders that do not spin webs do produce silk. Some spiders living in burrows line the burrows with the silk from their silk glands. Young spiders ride the wind on long silk threads, a process called ballooning. Most spiders put their eggs into silk sacs.

What is spider silk made of?

Spider silk is a protein that is formed as a liquid in the silk glands and squeezed out of spinnerets. The liquid thread hardens as it leaves the spinneret and some types of such thread become stronger than a steel thread of the same diameter. Most of the silk threads in a spider web are multiple strands of fine silk lying alongside each other. Spiders produce several types of silk from different types of spinning glands. One type of silk formed by all spiders is the type used for wrapping prey. Another type of silk is used to make the egg sac, and yet other is a sticky type often used as part of a web.

What is ballooning?

This is a method by which the spiderlings of some species such as katipo and nursery web spiders use to colonise new areas. The spiderling will climb to a point (such as the end of a branch) where it is exposed to air currents. It will then produce a small quantity of silk called gossamer. The wind catches the gossamer and carries it aloft, taking the spiderling with it. If conditions are right, the spider may not land for hundreds of kilometres, although a journey of only a few kilometres would be more usual.