Chelicerata is a highly diverse phylum and you only need to look at the superficial differences between ticks and scorpions to see there is a great range in body morphologies and sizes. The class Arachnida, to which spiders belong, contains the vast majority of recent chelicerates with around 70,000 described species and possibly a million more awaiting discovery. Within its ranks are every living terrestrial chelicerate and also a few which have returned to aquatic environments (Ruppert, et al. 2004).
Specifically spiders are found in the order Araneae. They are distinguishable from other arachnids by the presence of a pedicel between the cephalothorax (head end) and abdomen (Ruppert, et al. 2004). Araneae is an exceptional order given that every member is predatory (Coddington & Levi, 1991). You just won’t find an herbivorous spider. This may seem obvious but then consider that around 80% of the class Arachnida is either a spider or a mite and that each spider is a carnivore (Ruppert, et al. 2004). To have become so successfully diverse whilst remaining purely predatory, spiders must have evolved many adaptive mechanisms to enable them to avoid competition, predation and other selective pressures (I will be aiming to cover these mechanisms in future blogs).
The garden spider: A British example of the infra-order Araneomorphae. 
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Conventionally Araneae is split into 2 sub-orders: Mesothelae and Opisthothelae. The first represents the trap-door spiders, the most primitive spiders which have a segmented abdomen and chelicerae that point downwards (move orthognathously) unlike the majority of spiders which utilise a pincer movement (labidognathously). The second sub-order is split into 2 infra-orders one of which is Mygalomorphae containing such horrors as the bird-eating spiders. These also have primitive downward pointing chelicerae but their abdomens are not segmented. The other infra-order is Araneomorphae, what most people would view as a typical spider (Tudge, 2000). These spiders are very diverse, possibly because they have pincer-like chelicerae and thus do not require a surface to pin prey against. Their hunting methods can be more multifarious because of this, utilising upstanding webs to ambush prey from different angles as well as feeding relatively safely above the ground.
The systematic classification of Araneae has changed greatly over the past two decades and there continues to be research into the relationships between the spider families. One of the more ambitious ventures for creating a taxonomical tree for the Araneae is part of the project “Assembling the Tree of Life” (AToL). The aim is to produce up to date phylogenies on a range of organisms to provide a baseline for future research. Phylogenetic research on spiders specifically (which commenced in 2002) is utilising morphological, fossil, molecular and behavioural data (Penney & Selden, 2005). This phylogeny would include all known spider families and a link to the project work funded by the US National Science Foundation can be found here.
References
- Coddington, J. A. & Levi, H. W. (1991). Systematics and evolution of spiders (Araneae). Annual Review of Ecology and Systematics, 22, 565-592.
- Penney, D., & Selden, P. A. (2005). Assembling the Tree of Life—Phylogeny of Spiders: a review of the strictly fossil spider families. Acta zoologica bulgarica, 1, 25-39.
- Ruppert, E. E., Fox, R. S., & Barnes, R. D. (2004). Invertebrate Zoology: A Functional Evolutionary Approach. 7th ed. South-Western, a part of Cengage Learning.
- Tudge, C. (2000). The variety of life: A survey and a celebration of all the creatures that have ever lived. New York: OxfordUniversity Press.
