Biologists classify animals and plants according to characteristics that they have in common.
Organisms that move from place to place of their own accord and that do not manufacture their food from raw materials in their environment are classified as animals.
Organisms that cannot move from place to place of their own accord (but can be moved by forces of nature or by animals) and that can manufacture their own food from raw materials in their environment, are called plants. Within the huge group called animals are many that have no backbones. These are called invertebrates. The invertebrates include mostly small animals. Some live in water and some on land. Some move around by wriggling, some walk, and some fly. One major group or phylum of invertebrates is called the Arthropoda. Biologists include insects and spiders in this group.
1. Spiders are not insects. Why not?
Because they have some characteristics that are different from insects.
Amongst the vast numbers of invertebrate animals in the Phylum Arthopoda, more than a million different kinds have bodies with three main parts--head, thorax, and abdomen. The head has eyes, antennae and mouthparts. The thorax has three pairs of legs. The entire body is protected by a tough outer covering called an exoskeleton. Animals that share these characteristics are called insects. The group to which they belong is called the Insecta.
Another, smaller, group of invertebrate animals has only two main body parts. The body consists of a combined head and thorax called the cephalothorax, and the abdomen. The cephalothorax has the eyes, mouthparts (no antennae) and four pairs of legs. Animals that share these characteristics include ticks, mites, scorpions and spiders. The group is called the Arachnida.
2. Are all of the Arachnida alike?
No. Within the Arachnida, spiders are classified into a special group called the Araneae that separates the spiders from the ticks, mites, and scorpions. A distinguishing characteristic of spiders (Araneae) is that they have a very slender waist or pedicel separating the cephalothorax from the abdomen. The cephalothorax is covered by a tough carapace. Spiders are believed to have existed for more than 300 million years.
3. What body parts do all spiders have?
A spider's body consists of:
a) a cephalothorax with eyes, mouthparts - a pair of jaws and a pair of pedipalps, and four pairs of jointed legs and
b) 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.
4. Do spiders have claws?
Yes. They 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.
5. Do spiders have muscles?
Yes. The four pairs of legs are powered by muscles. And, muscles attached to the spider's gut and to the inside wall of the cephalothorax can contract to create a strong sucking action during feeding.
6. 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.
7. 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.
8. Can spiders bite?
Yes. Below the eyes on head end of a spider's cephalothorax are two small jaws (chelicerae) that end in fangs. 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 in the United States are the Brown Recluse, and the Black Widow.
9. 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; that is, 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 recluse spiders have necrotic venom. The damage usually results in skin blisters, ulcers and blackening of the local tissues.
10. Are all animals affected by spider bites?
No, probably not. Of course, insects are seriously affected by the venom since insects are the staple of most spiders' diet. Spiders inject venom into the body of a captured insect to paralyze it. Cats are very susceptible to black widow venom as are horses but dogs are relatively resistant, and sheep and rabbits are apparently immune.
11. How many spiders have poisonous venom that affects humans?
While there are only a few spiders in the United States that are really dangerous to humans, there are significant numbers around the world. The black widow 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. In Australia there are between 50 and 100 venomous species at least two of which can be life threatening to humans, the red-back and the Sydney funnel-spider.
12. How do spiders catch their prey (food)?
There are four main methods that spiders use to capture prey.
a) Sedentary spiders living in silk-lined burrows leap out to capture passing insects.
b) Some, normally active spiders, lie in ambush on plants, tree bark, on the ground or under stones.
c) Some spiders, like hunters, go in search of their prey.
d) Many spiders spin webs to entrap their prey.
13. What - and how - do spiders eat?
All spiders are carnivorous, Most spiders 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).
14. How many eyes do spiders have?
Most spiders have eight simple eyes. [Insects on the other hand have large, compound 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.
15. What are spider webs made of?
Spider webs are made of continuous strands of spider silk produced from about six silk glands beneath the abdomen. A web-spinning spider first constructs a web framework attached to parts of plants or other firm supports. This framework has threads radiating out from the center. Next, the spider will work from the edge of the web toward the center laying down a spiral of sticky threads. A good description of how a spider constructs a web can be found in The Book of Spiders by Paul Hillyard.
16. What is spider silk made of?
Spider silk is a protein that is formed as a liquid by silk glands and squeezed out of spinnerets like toothpaste from a tube. 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.
17. Do all spiders spin webs?
No. However, spiders that do not spin webs do produce silk. Some live in burrows which they line with the silk from their silk glands. Young spiders ride the wind on long silk threads in a process that we call ballooning. Most spiders put their eggs into silk sacs.
18. Where do spiders live?
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. Each type of spider tends to be found in a habitat to which it has become specifically adapted.
19. How do spiders travel?
All spiders, of course, travel around their immediate territory on foot. But small and young spiders can travel for many miles being carried through the air by a breeze. This method of travel is called ballooning. It depends upon the spider being in the right position so that the breeze draws out a line of silk from one of the spinnerets on its abdomen. The long, delicate strand of silk is carried high up into the air with the tiny spider suspended at its end.
20. How many different types of spiders are there?
There are now about 35,000 named species of spiders worldwide. This includes about 3,000 species in North America. Biologists and naturalists believe that there are many more species of spiders still to be identified and named. The 35,000 species of spiders in the Order Araneae are grouped into three sub-orders and seventy different families. In the following examples, except for the so-called tarantulas, we have given only family names and have not indicated to which sub-orders they belong.
SPIDER TYPES - a selection
21. Tarantulas (Order Araneae, Family Theraphosidae)
These are the largest spiders. The name tarantula really belongs to members of the family Lycosidae but in the USA the Theraphosidae are commonly called tarantulas. Their hairy bodies may be 10 cm long with a leg span of nearly 15 cm. Some South American types have venom that is dangerous to humans but the bite of a North American tarantula is no more dangerous than a wasp or bee sting. Tarantulas typically hide during the day in burrows or dark holes. They may line their burrow with silk. They do not spin webs. Their jaws - unlike other spiders - move up and down and they feed on large insects and even small vertebrates such as mice or birds. The male tarantula lives only a few years but in captivity a female can live for 35 years.
22. Wolf Spiders (Order Araneae, Family Lycosidae)
These spiders live and hunt on the ground amongst dead leaves and debris, on sand dunes, or on the muddy surface of marshes. Most do not spin webs but dig burrows or have no home at all. Female wolf spiders carry their egg sacs around with them until the eggs hatch and then carry the spiderlings on their backs until they disperse. Burrowing members of this family are true tarantulas and belong to the genus Lycos. These large wolf spiders live in burrows with trapdoors and are known to bite humans causing tissue necrosis.
23. Trap-door spiders (Order Araneae, Family Ctenizidae)
These quite large spiders have spines on their jaws that they use to dig burrows in the ground. They seal the opening with a hinged lid or trapdoor. Some types have a delicate silken trapdoor while others have a thick, tightly fitting lid made of earth and debris. The spiders wait in their burrows until they sense that a suitable prey is close by. Then they rush out, seize the victim, inject it with venom and drag it into the burrow.
24. Jumping Spiders (Order Araneae, Family Salticidae)
This is the world's largest family of spiders. These small to medium sized spiders (usually well under 10mm long) are called jumping spiders because of the spectacular leaps they make as they pounce on their prey. Their leaps can be up to forty times their body length. They have quite large eyes and better than usual spider eyesight and are excellent hunters. As a jumping spider leaps onto its victim silk streams from its spinnerets and serves as a life-line. Some species of jumping spiders are beautifully colored and patterned, sometimes with irridescent hairs.
25. Nursery Web Spiders (Order Araneae, Family Pisauridae)
These spiders spin webs not to catch prey but to care for their young. The web is constructed in low plants or shrubs and the female guards the eggs until they hatch and the tiny, perfectly formed spiderlings emerge and disperse. Some members of this family are called fishing spiders. These spiders can run over the surface of quiet water and can creep below the surface for as long as 30 minutes to catch a small fish or tadpole.
26. Spitting Spiders (Order Araneae, Family Scytotidae)
A spitting spider has special glands in its cephalothorax which produce a sticky substance that is squirted onto its prey. As the spider spits out the gum it turns it head from side to side so that the gum falls in a zigzag pattern and holds down the prey so that the spider can give it a paralyzing bite. Once the prey is dead the spider frees it from its sticky bonds, pours digestive juices over it and starts feeding.
27. Crab Spiders (Order Araneae, Family Thomisidae)
The Crab spiders are a large family distributed worldwide. These spiders have a sideways scuttling movement similar to that of crabs. A few members of this family habitually rest on flower heads waiting for unwary insects. These spiders tend to be colored to match the flowers that they visit. Most members of the family though are drab colored and rest on vegetation or on the ground.
28. Orb Web Spiders (Order Araneae, Family Araneidae)
The members of this large family vary greatly in size, shape and color. Orb weavers spin webs with non-sticky radiating strands and spiraling sticky threads. The spider often rests near the center of the web waiting for an insect to become caught . But others like the barn spider wait above the web during the day connected to it by a silk signal line and is thus alerted when an unsuspecting insect disturbs the web. The Garden spider eats its old web and spins a new one each day. The Bola spider does not spin a web. Instead it dangles a silk thread with a sticky globule on the end and swings it to capture a passing moth.
29. Funnel-Web Spiders (Order Araneae, Family Agelenidae)
These small to medium sized spiders - under 20mm long - spin sheet webs of non-sticky silk with a funnel extending from the center to one edge and a barrier web over the top to catch insects. The spider sits in the funnel. When a flying insect hits the barrier and falls onto the sheet web the spider rushes out of the funnel, bites the victim to paralyze it, and drags it into the funnel to feed on it.
An unusual member of this family is the water spider (Argyroneta). This spider feeds on small water organisms and spends its life below the surface of lakes and ponds living in a bubble of air held in the water by a silken diving bell or thimble-like structure. A good description of how the spider constructs this astonishing structure is given in Spiders of the World by Ron & Ken Preston-Mafham.
30. Daddy Long-legs (Order Opiliones, Family Phalangiidae)
Sometimes thought to be spiders, Daddy Long-legs are not spiders. They are not included in the Order Araneae.
They have a very superficial similarity to spiders in that they have 4 pairs of legs but their legs are stilt-like and are kept bent with the body close to the ground. The daddy long-legs eats insects and other invertebrates as well as the tender gills of fungi and soft decaying matter.
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