Classification

Like the scorpions, the true spiders (Araneae) belong to the class Arachnida (arachnids). They are the largest order in this class, with around 30,000 species, and are divided into the 3 suborders Mesothelae, Mygalomorphae (Orthognatha) and Araneomorphae (Labidognatha), and further into numerous families. The Mesothelae are a small group of Southeast Asian spiders that have barely any clinical relevance. Some dangerous species are found among the Mygalomorphae (Bird spiders in the wider sense), but it is Araneomorphae (Labidognatha), by far the largest suborder, which includes the greatest number of dangerous species. Listed below are those families known to include dangerous species. The most important members in medical terms are marked with an asterisk *:

Mygalomorphae (Orthognatha):

• Actinopodidae: Actinopus sp., Missulena sp.
• Dipluridae: Trechona sp.
• Hexathelidae: Atrax sp*., Hadronyche sp.
• Theraphosidae: Anophonopelma sp., Harpactirella sp., Pterinochilus sp.

Araneomorphae (Labidognatha):

• Araneidae: Aranea sp.
• Ctenidae: Phoneutria sp.* (= Ctenus sp.)
• Lycosidae: Lycosa sp.
• Miturgidae: Chiracanthium sp.(*)
• Salticidae: Phidippus sp., Mormon sp.
• Sicariidae: Loxosceles sp.*, Sicarius sp.
• Theridiidae: Latrodectus sp.*, Steatoda sp.

Characteristics that can be used to distinguish between mygalomorph and araneomorph spiders are the arrangement and direction of movement of the venomous fangs (see below and Fig. 4.38b, 1 and 2). Accurate identification at the family or genus level is not easy for laypersons due to the difficulty in locating and observing the spider's identifying characteristics, but some of the most important species do have simple and characteristic features (see Biomedical database entries).

Fig. 4.38

a Schematic representation of a spider.
b Arrangement and direction of movement of the venomous fangs.
1 Mygalomorph spider
2 Labidognath spider

Morphological characteristics (Fig. 4.38)

The spider's body is surrounded by a chitin-containing exoskeleton and is divided into 2 main parts, the cephalothorax ("head") and abdomen. The cephalothorax is covered by a hard plate (carapace), on the anterior side of which the eyes are located. Most spiders have 8 eyes, but some have only 6 (e.g. Loxosceles sp.), 4 or 2. Some cave-dwelling species have no eyes at all. From the region of the mouth there arise a pair of chelicerae (mouth parts) that end in horn-like fangs. In the mygalomorphs these are arranged parallel to each other and each contain a venom gland in the basal part that opens outwards close to the tips of the fangs. These spiders bite by moving the fangs downwards. The chelicerae of the labidognath spiders are arranged in such a manner that the fangs can be moved towards each other when these spiders bite. Generally the venom glands are located in the anterior part of the cephalothorax, and the chelicerae contain venom ducts that open outwards through an opening close to the tips of the fangs.

Between the chelicerae and the first pair of legs there is a pedipalp on each side. In female spiders the pedipalps are small. In male spiders they function as secondary mating organs (spermatophores), with the help of which sperm is introduced into the female's epigyne. For this purpose they are enlarged at the distal end (Fig. 4.38a shows a male pedipalp).

Underneath the cephalothorax there are 4 pairs of legs. The cephalothorax is attached to the soft-skinned abdomen by a thin pedicel. The openings of the respiratory and sexual organs are located underneath the abdomen. At the posterior end of the abdomen are the spinnerets, arranged in pairs.

Venom apparatus

For the structure and function of the venom apparatus, see above. Spiders are exclusively predators, and the venom apparatus is closely associated with the way they eat. With the exception of the family Uloboridae (hackled orbweavers, around 200 species), all spiders possess venom glands. The venom firstly serves to quickly immobilise prey. In addition, most spiders have to externally predigest and liquefy their prey through proteolytic venom components, so that they can suck in the fluids with the help of a specially developed sucking stomach. The necrotising effects that may occur following a spider bite are associated with these lysing venom components.

The vast majority of species are extremely reluctant to use their venom apparatus as a means of defence. Defensive bites generally only occur when the animals are squashed. Exceptions to this rule are Phoneutria sp. and Atrax sp., which display aggressive defensive behaviour when threatened.

Range of venom effects

See Clinical flowchart: Spiders.

Way of life

There are hardly any terrestrial habitats that are not inhabited by spiders. Among the extreme habitats are desert areas, intertidal zones and the Arctic. The species Argyroneta aquatica has even successfully adapted to living under water.

Spiders are generally solitary creatures that only come together to mate. The female animals generally live longer (up to around 20 years for some mygalomorphs!) and are larger than the males, which often die shortly after mating. It is thus usually the larger females among the medically relevant species that are dangerous for humans; in Atrax robustus it is the males.

The vast majority of spiders build themselves a niche to live in with the help of their silk. These dwellings may be in the form of a lined and closable tube in the ground, or the most diverse constructions, be it a confused-seeming gossamer or geometric web, close to the earth or high above the ground. The silk threads also serve as vibration-sensitive structures for catching prey and as winding or sticky threads to immobilise prey.

On the basis of their way of life, spiders can be divided into sedentary and wandering species. Among the sedentary spiders there are some that commonly establish themselves in human habitations. Examples of these are the harmless cellar spiders (Pholcus sp.) or house spiders (Tegenaria sp.). However, there are also dangerous species, such as Latrodectus sp., Loxosceles sp. or Chiracanthium sp., which establish themselves in cellars, living rooms and bedrooms or in protected niches outside the house or in tool sheds and garages. Wandering spiders, such as the South American wandering spiders (Phoneutria sp.), may also enter human habitations while hunting prey or searching for a mate.

Epidemiology

Although over 99% of the well-known spider species have venomous fangs, the majority of species are harmless for humans for various reasons; their venom fangs may be too short to penetrate human skin, the venom may only be weakly toxic for humans or the amount of venom available may be too small. Only a few species are known to cause severe envenoming. Nonetheless, under certain circumstances bites from several other species, for which to date there has been only poor clinical documentation or none at all, can also lead to more serious envenoming. 

Spider bites are relatively frequently given as the cause of accidents with venomous animals in warmer zones, but in many cases the cause of the bite is not identified. Moreover, transient local symptoms of envenoming that are blamed on spiders may also be caused by a variety of other stinging or biting arthropods.

Accidents often occur inside or close to human habitations. However, spider bites may also occasionally occur during farming work in banana plantations (Phoneutria sp.), in grain or cotton fields or in vineyards (Latrodectus sp.). There is also a risk of encountering spiders in other situations in which humans enter the natural distribution areas of dangerous species, e.g. while camping or during military manoeuvres.

Accidents caused by the widespread species Latrodectus sp. and Loxosceles sp. are known from various regions of the world. Envenoming due to other dangerous species is generally restricted to their limited areas of distribution, such as South America (Phoneutria sp.) or southeastern Australia (Atrax sp. and Hadronyche sp.).

Prevention

Precautionary measures are primarily relevant for living quarters. The following recommendations are essentially based on those of Lucas (1988).

Remove junk, piles of newspapers, boxes etc. which offer narrow, dark hiding places. Regular and thorough cleaning in and around the house makes these places inhospitable for spiders. Particularly important in this regard are corners of rooms, cracks in walls etc. When cleaning, furniture and beds should be moved aside and pictures should be taken down.

Shoes and clothes should not be left lying around, as they offer excellent hiding places for spiders. Shake out shoes before putting them on and inspect clothes. Before going to bed check the bedclothes. In warm and tropical regions do not plant bushes or shrubs directly around the house and mow the grass regularly. Wear sturdy shoes and gloves while working in the garden or cleaning.

To make it more difficult for spiders to enter the house, make sure doors and windows are properly sealed. When moving into a house that has been empty for a while (e.g. a holiday house) it is strongly recommended to clean thoroughly as described above.

Literature

General reviews: Bücherl 1971b, Foelix 1979, Heimer 1988, Hubert 1979, Levi et al. 1968, Lucas and Meier 1995b, Preston-Mafham and Preston-Mafham 1984, Schmidt 1986
Regional reviews: Baehr and Baehr 1987, Bellmann 2001, Kaston 1978, Smith 1990, Mascord 1983, Newlands and Atkinson 1988, Lucas 1988, Maretic and Lebez 1979 

Classification: The World Spider Catalog