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Spider anatomy

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Title: Spider anatomy  
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Spider anatomy

Basic characteristics of arachnids include four pairs of legs (1) and a body divided into two segments: the cephalothorax (2) and the abdomen (3).

The anatomy of spiders includes many characteristics shared with other arachnids. These characteristics include bodies divided into two segments, eight jointed legs, no wings or antennae, the presence of chelicerae and pedipalps, simple eyes, and an exoskeleton, which is periodically shed.

Spiders also have several adaptations that distinguish them from other arachnids. All spiders are capable of producing silk of various types, which many species use to build webs to ensnare prey. Most spiders possess venom, which is injected into prey (or defensively, when the spider feels threatened) through the fangs of the chelicerae. Male spiders have specialized pedipalps that are used to transfer sperm to the female during mating. Many species of spiders exhibit a great deal of sexual dimorphism.[1]

Contents

  • External anatomy 1
    • Cephalothorax 1.1
      • Appendages 1.1.1
      • Eyes, vision, and sense organs 1.1.2
    • Abdomen 1.2
      • Spinnerets 1.2.1
  • Internal anatomy 2
    • Circulation 2.1
    • Breathing 2.2
    • Digestion 2.3
  • Reproductive system 3
  • Glossary 4
  • References 5
  • Bibliography 6
  • External links 7

External anatomy

The underside and head of a spider
The ventral side of a Brown Widow spider. The epigastric plates and furrow are visible, as well as the hourglass marking on the underside of the abdomen, which is a characteristic feature of widow spiders.

Spiders, unlike insects, have only two tagmata instead of three: a fused head and thorax (called a cephalothorax or prosoma) and an abdomen (also called the opisthosoma). The exception to this rule are the assassin spiders, whose cephalothorax is divided into two parts by an elongated "neck". Except for a few species of very primitive spiders (family Liphistiidae, also called segmented spiders), the abdomen is not externally segmented. The abdomen and cephalothorax are connected with a thin waist called the pedicle or the pregenital somite, a trait that allows the spider to move the abdomen in all directions. This waist is actually the last segment (somite) of the cephalothorax and is lost in most other members of the Arachnida (in scorpions it is only detectable in the embryos). Unlike insects, spiders have an endoskeleton in addition to their exoskeleton.[2]

Cephalothorax

The cephalothorax is composed of two primary surfaces: a dorsal carapace and a ventral sternum. Most external appendages on the spider are attached to the cephalothorax, including the legs, eyes, chelicerae and other mouthparts, and pedipalps.

Like other Arachnids, spiders are unable to chew their food, so they have a mouth part shaped like a short drinking straw that they use to suck up the liquified insides of their prey. However, they are able to eat their own silk to recycle proteins needed in the production of new spider webs.[3] Some spiders, such as the dewdrop spiders (Argyrodes), even eat the silk of other spider species.[4]

Appendages

Spiders typically have eight walking legs (insects have six). They do not have antennae; the frontmost pair of appendages are the pedipalps (or just palps), at the base of which are coxae or maxillae next to their mouth that aid in ingesting food. Since they do not have antennae, spiders use specialised and sensitive setae on their legs to pick up scent, sounds, vibrations and air currents.

Spiders' legs are made up of seven segments. Starting from the body end, these are the coxa, trochanter, femur, patella, tibia, metatarsus and tarsus. The tip of the tarsus bears claws, which vary in number and size. Spiders that spin webs typically have three claws, the middle one being small; hunting spiders typically have only two claws. The pedipalps have only six segments: the metatarsus is missing. In adult males, the tarsus of each palp is modified to carry an elaborate and often species-specific structure used for mating (variously called a


External links

Bibliography

  1. ^
  2. ^ Foelix (1996), p. 37.
  3. ^
  4. ^
  5. ^ Roberts (1995), pp. 12–16.
  6. ^
  7. ^ Comstock (1920), pp. 128–132.
  8. ^ Foelix (1996), pp. 53–60.
  9. ^ Comstock (1920), pp. 148–152.
  10. ^ Foelix (1996), p. 52.
  11. ^

References

Spider anatomy acronyms
acronym meaning
ALE anterior lateral eyes
ALS anterior lateral spinnerets
AME anterior median eyes
DTA dorsal tegular apophysis
DTiA dorsal tibial apophysis
LTA lateral tegular apophysis
MOQ median ocular quadrangle
PLE posterior lateral eyes
PLS posterior lateral spinnerets
PME posterior median eyes
PMS posterior median spinnerets
RCF retrolateral cymbial fold
RTA retrolateral tibial apophysis
VTA ventral tegular apophysis
VTiA ventral tibial apophysis

This is an incomplete list of abbreviations that often appear in scientific descriptions of spider specimens.

Glossary

Almost all spiders reproduce sexually. They are unusual in that they do not transfer sperm directly, for example via a penis. Instead the males transfer it to specialized structures (palpal bulbs) on the pedipalps and then meander about to search for a mate.[11] These palps are then introduced into the female's epigyne. This was first described in 1678 by Martin Lister. In 1843 it was revealed that males build a nuptial web into which they deposit a drop of semen, which is then taken up by the copulatory apparatus (the palpal bulb) in the pedipalp. The structure of the copulatory apparatus varies significantly between males of different species. While the widened palpal tarsus of Filistata hibernalis (Filistatidae) only forms a simple bulb containing the coiled blind duct, members of the genus Argiope have a highly complex structure.

Reproductive system

Mesothelae and the Orthognata, two pairs of coxal glands open onto the posterior side of the first and third coxae. They release a fluid only during feeding and play an important role in ion and water balance.[10] Digestive fluids dissolve the prey's internal tissues. Then the spider feeds by sucking the partially digested fluids out. Other spiders with more powerfully built chelicerae masticate the entire body of their prey and leave behind only a relatively small amount of indigestible materials. Spiders consume only liquid foods. Many spiders will store prey temporarily. Web weaving spiders that have made a shroud of silk to quiet their envenomed prey's death struggles will generally leave them in these shrouds and then consume them at their leisure.

Argiope aurantia feeding on silk-wrapped grasshopper.

Digestion

Among smaller araneomorph spiders we can find species who have evolved also the anterior pair of book lungs into trachea, or the remaining book lungs are simply reduced or missing, and in a very few the book lungs have developed deep channels, apparently signs of evolution into tracheae. Some very small spiders in moist and sheltered habitats don't have any breathing organs at all, as they are breathing directly through their body surface. In the tracheal system oxygen interchange is much more efficient, enabling cursorial hunting (hunting involving extended pursuit) and other advanced characteristics as having a smaller heart and the ability to live in drier habitats.

Spiders have developed several different respiratory anatomies, based either on desiccation. The trachea were originally connected to the surroundings through a pair of spiracles, but in the majority of spiders this pair of spiracles has fused into a single one in the middle, and migrated posterior close to the spinnerets.

Spider book lungs (cross section)

Breathing

The heart is located in the abdomen a short distance within the middle line of the dorsal body-wall, and above the intestine. Unlike in insects, the heart is not divided into chambers, but consists of a simple tube. The aorta, which supplies haemolymph to the cephalothorax, extends from the anterior end of the heart. Smaller arteries extend from sides and posterior end of the heart. A thin-walled sac, known as the pericardium, completely surrounds the heart.[9]

Spiders, like most hemocyanin, a respiratory protein similar in function to hemoglobin. Hemocyanin contains two copper atoms, tinting the haemolymph with a faint blue color.[8]

Circulation

The abdomen has no appendages except from one to four (usually three) modified pairs of movable telescoping organs called spinnerets, which produce silk. Originally, the common ancestor of spiders had four pairs of spinnerets, with two pairs on the tenth body segment and two pairs on the eleventh body segment, located in the middle on the ventral side of the abdomen. The suborder

Spinnerets

On the ventral side of the abdomen are two hardened plates covering the book lungs. These are called the epigastric plates. A fold, known as the epigastric furrow, separates the region of the book lungs and epigyne from the more posterior part of the abdomen. In the middle of this furrow is the opening of the oviduct (in females) and at either end are the lung slits.[7]

Abdomen

However, most spiders that lurk on flowers, webs, and other fixed locations waiting for prey tend to have very poor eyesight; instead they possess an extreme sensitivity to vibrations, which aids in prey capture. Vibration sensitive spiders can sense vibrations from such various mediums as the water surface, the soil or their silk threads. Changes in the air pressure can also be detected in search of prey.

Net-casting spiders have enormous, compound lenses that give a wide field of view and gather available light very efficiently. The lenses have an F number of 0.58, which means they can concentrate available light more efficiently than a cat (F 0.9) or an owl (F 1.1). Each night a large area of light sensitive membrane is manufactured within these eyes (and rapidly destroyed again at dawn).[6]

Spiders have up to eight eyes, made up of single lenses rather than compound., The specific arrangement of the eyes can be used to taxonomically classify different species. Most species of the Haplogynae have six eyes, although some have eight (Plectreuridae), four (e.g., Tetrablemma) or even two (most Caponiidae) eyes. Sometimes one pair of eyes is better developed than the rest, or even, in some cave species, there are no eyes at all. Several families of hunting spiders, such as jumping spiders and wolf spiders, have fair to excellent vision. The main pair of eyes in jumping spiders even sees in color.

Multiple eyes of a jumping spider

Eyes, vision, and sense organs

[5]

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