The spider

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Chapters

Introduction	Web and silk
The body	Sex and reproduction
The jaws and poison	Spider enemies
Blood circulation, the lungs and moulting	Literature and acknowledgements
The nerve system, sensory organs and legs	Dutch version

Introduction


Sesia apiformis, wasp butterfly, an insect	Dolomedes fimbriatus, a spider

Many people confuse spiders with insects. The easy way to recognize the spider from an insect is that spider has 4 pairs of legs and an insect has 3 pairs. One major difference is that insects have compound eyes whereas the spider has singular eyes with lenses. Unlike insects, spiders do not have antennas.

There are also many similarities. Both have an external skeleton (exoskeleton). The hard part of the body is on the outside while mammals have their skeleton (bones) inside the body. The heart is located on their back. Breathing is performed with trachea and / or book lungs. The oxygen transport protein is hemocyanin and not the mammalian transport protein hemoglobin.

In 1758-59 Carolus Linnaeus published the 10^th edition of Systema Naturae in which he classified animals. Over the years the biologists improved the systematic study of the Animal Kingdom. According to this study we have single cell animals at the bottom and at the top are humans with very complex cell systems. With the aid of DNA analysis the positioning of each animal has become even more precise than before. It is called a taxonomical classification of the Animal Kingdom.

It consists of several divisions. A division is called phylum.

Phylum Arthropoda consists of animals with exoskeleton (hardened exterior) that have segmented bodies and jointed appendages. The segments are fused to form body parts. The first part is the head, followed by thorax and the hind part is abdomen. There are appendages on these segments, which are specialized to perform specific functions such as walking, jumping, eating and lots of other activities. Phylum Arthropoda is divided into 5 major classes.

Class	Crustacea	Arachnida	Diplopoda	Chilopoda	Insecta
Examples	Lobsters, crabs	Spiders	Millipedes	Centipedes	Flies, maggots
Body parts	Cephalothorax and abdomen	Cephalothorax and abdomen	Head and body	Head and body	Head, thorax and abdomen
Pair of legs	Many, usually 5 pairs	4 pairs	Many, 2 pairs per segment	Many, 1 pair per segment	3 pairs, 1 on each thoracic segment
Antennae	2 pairs	None	1 pair	1 pair	1 pair
Described number of species	25,000	70.000	5000?	5000?	800.000

Class Arachnida is divided into 10 orders.

1. Araneae	2. Scorpiones	3. Pseudo-scorpiones	4. Solfiguae	5. Schizomida
Spiders	Scorpions	Pseudo-scorpions	Solifugids	Tartarida
6. Amblypygi and Uropygi	7. Palpigradi	8. Ricinulei	9. Acari	10. Opiliones
Whip scorpions	Mini whip scorpions	Rinucleids	Mites en ticks	Harvestmen

Accordingly the spider is placed in phylum Arthropoda, class (classis) Arachnida, order (ordo) Araneae. This order is further divided into 3 sub-orders. The Mygalomorphae (the primitive spiders), the Aranaeomorphae (the modern spiders) and the Mesothelae, with one family of spiders the Liphistiidae. Every spider belongs to a family, which is further divided into genus, followed by species. The European garden spider belongs to the family Araneidae, the genus Araneus en the species diadematus. As a rule genus and species are printed in italics.

In our world around 70000 species of the class Arachnida are described. 90% of these species belong to the order Acarina (mites and ticks) and the order Araneae. In the order Araneae 1960 primitive spiders and 40000 modern spiders are known.

One can find spiders in much larger numbers then expected. A study in Great Britain counted in a meadow 130.8 spiders per square meter. An average spider consumes 0.089 g insect per day. After some calculations we can conclude that in the Netherlands, with an area of 36150 square kilometers and 15 million human habitants, there are 5000 billion spider habitants. These spiders could consume all Dutchmen in three days. Lucky for us that our spiders do not eat us.

Spiders mostly prey on insects. Most insects are useful, although some may be annoying. Therefore, it is difficult to say if a spider is valuable or not. However, spiders do control the insect population to some extent which makes them helpful, at least, in maintaining the right balance in our eco system.

Scorpion, Euscorpius italicus

We will have a close look at the near relatives of the spiders in Europe. Around the Mediterranean we can find the scorpion (order Scorpiones). Its body has a large head-breast part (prosoma) and a segmented abdomen (opisthosoma) to which a tail is connected. At the end of the tail there is a stinger. Connected to the prosoma are four pairs of sturdy legs, one pair of feelers and one pair of scissors.
A closer look reveals two tiny black eyes, although, some varieties of scorpions have more than one pair of eyes. The sting with poison is mainly used for protection and for killing large prey. The scorpion sting is unpleasant for humans and can result in possible death.
A rather unknown scorpion order is the pseudoscorpion (order Pseudoscorpiones). This small creature is a few millimeter long and lives between detritus like leaves, bark, moss, mole- and bird nests. They have relatively long scissors that can be as long as the rest of the body for the males. They do not have tails or stingers.


Tick, Ixodes ricinus	Velvet mite, Trombidium holosericeum

Even smaller than the pseudoscorpion are the ticks and the mites.

Mites are found in a great variety of forms in all kinds of habitats like deserts, in water, between rocks, in flour and in carpets, et cetera. A species of mites, commonly known as house dust mites, can be found in our houses and in our beds in huge numbers where they feed on human dander. Mites can be detected even in human hair sacs and sweat glands. The excretions of these mites are a major cause of asthma and allergy.

An easy to spot mite is the red velvet mite and it can be found in the garden.

Lately, ticks have been in the news because some may cause Lyme disease.

Harvestmen (order Opiliones) are often mistaken for spiders. Unlike spiders, the two parts of the body (cephalothorax and abdomen) of a harvestman are fused together into one part. They also have eight legs, feelers and mouth parts that work like a pair of scissors. Most species have short legs but some of them may have very long legs. Opiliones do not have any poison glands. At the top of the body there is one pair of eyes that are pointed side ways. They have no silk glands or spinners. Harvestmen eat everything (omnivorous). They catch small insects or eat the decays of any dead animal, animal dung, bird droppings, other fecal material, and all kind of plant material and fungi. The harvestman is mostly nocturnal, being active during night.


Harvestman, Mitopus morio	Harvestman, Phalangium opilio

Ed Nieuwenhuys, Jan 2006

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