Spider anatomy
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The anatomy of spiders is in some aspects similar, but also different from that of other arthropods. The following characterstics are common to all spiders: A body with two segments, eight legs, spinnerets, no chewing parts, no wings, and the presence of chelicerae, which spiders use to hold prey, and in most cases, inject venom. Spiders have non-compound eyes, with most species having eight; the spiders known as Haplogynae may have six or fewer, and certain cave-dwelling spiders may have none at all.
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. Like all arthropods, spiders periodically moult, or shed their exoskeletons. Many species of spiders exhibit a great deal of sexual dimorphism.[1]
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[edit] External anatomy
Spiders, unlike insects, have only two tagmata instead of three: a fused head and thorax (called a cephalothorax or prosoma) and an abdomen (called the opisthosoma). The exception to this rule are the assassin spiders, whose cephalothorax seems to be secondarily almost divided into two independent units. 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]
[edit] Cephalothorax
Most external appendages on the spider are attached to the cephalothorax, including the legs, eyes, chelicerae and other mouthparts, and pedipalps.
Spiders typically have eight legs (insects have six), no antennae, and their eyes are single lenses rather than compound eyes. They have pedipalps (or just palps), at the base of which are coxae or maxillae next to their mouth that aid in ingesting food; the ends of the palp are modified in adult males into elaborate and often species-specific structures used for mating. Since they don't have any antennae, they are using specialised and sensitive hairs on their legs to pick up scent, sounds, vibrations and air currents.
Because they can't chew their food, they have, like other Arachnids, a tiny mouth they use as a short drinking straw to suck up the liquid parts of their prey. Some (like Pholcus phalangioides) have the ability to completely suck out a mosquito from the tip of one of its legs, although this may take up to twelve hours. They are able to eat their own silk.
[edit] Eyes, vision, and sense organs
Spiders usually have eight eyes in various arrangements, a fact which is used to aid in taxonomically classifying different species. Most species of the Haplogynae have six eyes, although some have eight (Plectreuridae), four (eg., 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 colors.
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). [3]
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. Also changes in the air pressure can be detected in the search for prey.
[edit] Abdomen
[edit] Spinnerets
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 Mesothelae is unique in having only two types of silk glands - thought to be the ancestral condition. All other spiders have the spinnerets further towards the posterior end of the body where they form a small cluster, and the anterior central spinnerets on the tenth segment are lost or reduced (suborder Mygalomorphae), or modified into a specialised and flattened plate called the cribellum (suborder Araneomorphae). The cribellum (usually separated into a left and a right half) produces a thread which is made up of hundreds to thousands of very fine dry silk fibers (about 10 nm thick) around a few thicker core fibers, which then are combed into a woolly structure by using a group of specialized hairs (setae) on their fourth pair of legs. It is suspected their woolly silk is charged with static electricity, causing its fine fibres to attach to trapped prey. Once all araneomorph (modern) spiders had a cribellum, but today it only remains in the cribellate spiders (although it is sometimes missing even here), which are widespread around the world. Often, this plate lacks the ability to produce silk, and is then called the colulus; an organ which zoologists have not identified a function for. The colulus is reduced or absent in most species. The cribellate spiders were the first spiders to build specialised prey catching webs, later evolving into groups which used the spinnerets solely to make webs, instead using silk threads dotted with droplets of a sticky liquid (like pearls on a necklace) to capture small arthropods, and a few large species even small bats and birds. Other spiders don't build webs at all, but have become active hunters, like the highly successful jumping spiders.
[edit] Internal anatomy
[edit] Circulation
Spiders have an open circulatory system; i.e., they do not have true blood, or veins to convey it. Rather, their bodies are filled with haemolymph, which is pumped through arteries by a heart into spaces called sinuses surrounding their internal organs.
[edit] Respiration
Spiders have developed several different respiratory anatomies, based either on book lungs, a tracheal system, or both. Mygalomorph and Mesothelae spiders have two pairs of book lungs filled with haemolymph, where openings on the ventral surface of the abdomen allow air to enter and diffuse oxygen. This is also the case for some basal araneomorph spiders like the family Hypochilidae, but the remaining members of this group have just the anterior pair of book lungs intact while the posterior pair of breathing organs are partly or fully modified into tracheae, through which oxygen is diffused into the haemolymph or directly to the tissue and organs. This system has most likely evolved in small ancestors to help resist 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.
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 dryer habitats.
[edit] Digestion
Digestion is carried out internally and externally. Spiders that do not have powerful chelicerae secrete digestive fluids into their prey from a series of ducts perforating their chelicerae. These 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 glob 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.
[edit] Reproductive system
Almost all spiders reproduce sexually. They are unusual in that they do not transfer sperm directly, for example via a penis, but the males transfer it to specialized pedipalps and then wander about to search for a mate[4]. These palps are then introduced into the female's epigyne. This was first observed 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 in the male palpi. While the widened palpal tarsus of Filistata hibernalis (Filistatidae) only forms a kind of bulb containing the coiled blind duct, members of the genus Argiope have a highly complex structure.
[edit] Glossary
This is an incomplete list of abbreviations that often appear in scientific descriptions of spider specimens.
| acronym | meaning |
|---|---|
| ALE | anterior lateral eyes |
| 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 |
[edit] References
- ^ Reiskind, J. (1965). The Taxonomic Problem of Sexual Dimorphism in Spiders and a Synonymy in Myrmecotypus (Araneae, Clubionidae) (PDF). Psyche 72: 279-281.
- ^ Foelix, R. F. (1992). Biologie der Spinnen. Thieme. ISBN 3-13-575802-8. (German)
- ^ How spiders see the world. Australian Museum (2002).
- ^ Baez, E. C. & J. W. Abalos (1963). On Spermatic Transmission in Spiders (PDF). Psyche 70: 197-207.
[edit] External links
- Atlas of Entelegynae. Californian Academy of Sciences.
- Levi, H. W. (1965). Techniques for the study of spider genitalia (PDF). Psyche 72: 152-158.
