Spiders are the most specious group of venomous animals on the planet, and arguably the most successful terrestrial predators, except for perhaps predatory beetles (King and Hardy, 2013). Currently, there are over 50,000 described species with estimates that the total number may be closer to 120,000 species, and records of spider-like creatures date back 380 million years (Garrison et al., 2016). Attercopus fimbriunguis is a species of prehistoric spider-like creature and is known from a Devonian-aged fossil. It had many spider features, such as the ability to produce silk, but possessed no true spinnerets and possessed a tail-like appendage, not dissimilar to the modern-day whip spiders (Coddington, 1991). Whilst not a spider, it does show how far back spider ancestry goes. Spiders (Araneae) are one of the eleven orders of the invertebrate class Arachnida, which also includes the scorpions (Scorpiones), harvestmen (Opiliones), ticks and mites (Acari) and more (Ubick et al., 2005). Arachnids are Arthropods and sit within the subphylum Chelicerata.
Living spiders are divided into two main groups, the Mygalomorphae and the Araneomorphae both of which possess a number key features that make them spiders, including:
- spinnerets at the end of the abdomen
- eight legs
- fangs and two body segments (Ubick et al., 2005)
Tarantulas belong to the group Mygalomorphae, which contains most of the large, heavy-bodied spiders that are typically more primitive than the Araneomorph spiders and have several defining features. Mygalomorphs have the primitive Orthognath cheliceral position, meaning that their fangs point downwards, like those of snakes. Whereas, the Araneomorph spiders have a Labidognath cheliceral position where their fangs move side to side, like a pair of scissors (Figure 1).
Figure 1 The cheliceral positions of the Mygalomorphs and Araneomorph. The former possessing the primitive downwards facing fangs (Orthognath) and the latter having the more advanced, sideways moving fangs (Labidognath).The cheliceral position is certainly one of the most distinguishably morphological differences, as with many of the others, such as the large sizes that are typical of Mygalomorph spiders are not exclusive. For example, some Araneomorphs such as Lycosidae (wolf) spiders can reach large sizes, whilst the Mygalomorph purse-web spiders (Atypus affinis) and dwarf tarantulas (e.g. Cyriocosmus sp.) are comparatively small.
There is actually a third group of spiders, the Mesothelae, which are the oldest and most primitive group, with only one living family (Liphistiidae) consisting of approximately 100 species, which share the primitive Orthognath cheliceral position. Mesothelae are limited to China, Japan, southeast Asia, and Sumatra (Coddington, 1991). They are the most primitive of the groups and possess greater abdominal segmentation and less developed spinnerets (Coddington, 1991). The phylogeny of these groups and their characteristics can be seen in Figure 2.
Figure 2 The phylogeny of the three spider groups with the Taxa in regular font and characters in italics. Not all the Araneomorph groups are represented, the three shown represent the group and show some of the characters that are shared with the Mygalomorphae (such as the size of Lycosids). Taken from Ubick et al., 2005.
Behaviour and Natural History
Along with the differing morphological characteristics, the Mygalomorphs and Araneomorph spiders also differ in their lifestyles. Whilst both groups are incredibly diverse in behaviours, there are a few trends that work as general rule of thumb when it comes to separating them, for example, how they use silk. The evolution of silk production is undoubtedly one of the main advancements that has led to the success of the spiders (Blackledge et al., 2009). Whilst the exact timeline is still not clear, it is estimated that the Araneomorphs diverged from the Mygalomorphs approximately 240 million years ago (Garb et al., 2007). This divergence and the sudden radiation of Araneomorphs can be, to at least some extent, attributed to their advancements in silk production and use (Garb et al., 2007). The Mygalomorphs are more primitive in their silk use than the Araneomorphs, using it primarily to construct burrows, hides and trapdoors. These spiders use silk in its more basic form, some using it mainly for housing construction and just a few for prey capture (such as the purse-web spider – Atypus affinis). Whereas, the Araneomorph spiders have evolved the ability to capture their prey using webs, spiders are so diverse that there is a huge variety in just the webs of the Araneomorphs, which in some cases can be used as an identifying feature. Whilst some, such as the net-casting spiders (Deinopids), use non-sticky (cribellate) silk that catches prey through Van der Waal force interactions and physical entanglement (Piorkowski and Blackledge, 2017).
The Araneoidea spiders utilise a more advanced form of silk, known as viscid silk, which uses an aqueous glue-like substance that coats the silk core (Piorkowski and Blackledge, 2017). The evolution of the viscid silk is thought to be one of the biggest contributing factors of the success and diversity of the Araneoidea spiders.
Back to the comparison of Araneomorphs and Mygalomorphs. Whilst the spinning of orb webs is exclusive to the Araneomorphs, as is the viscid silk, many of the characters are not exclusive. For example, the Lycosids and Salticids are two examples of Araneomorphs that do not use silk at all for prey capture, but instead hunt actively, more akin to the Mygalomorphs. Demonstrating that there is a lot of cross over between the two groups, with the Mygalomorphs being the ancestral condition, they are base of spider evolution. The Araneomorphs have just managed to innovate these base features, such as the production of silk and go down a myriad of different roots. Using silk in a plethora of unimaginable ways has lead to spiders being remarkably successful and whilst many Araneomorphs use more complex techniques, the building of silken houses and trip lines, combined with sheer brutality and size, has enabled the Mygalomorphs to survive millennia.
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Blackledge, T.A., Scharff, N., Coddington, J.A., Szüts, Wenzel, J.W., Hayashi, C.Y. and Agnarsson, I. (2009). Reconstructing web evolution and spider diversification in the molecular era. PNAS, 106(13), 5229-34.
Coddington, J.A. (1991). Systematics and evolution of spiders (Araneae). Annual Review of Ecology and Systematics, 22, 565-92.
King, G.F. and Hardy, M.C (2013). Spider-venom peptides: Structure, pharmacology and potential for control of insect pests. Annual Review of Entomology, 58, 475-96.
Piorkowski, D. and Blackledge, T.A. (2017). Punctuated evolution of viscid silk in spider orb webs supported by mechanical behaviour of wet cribellate silk. Naturwissenschaften, 104(67).
Ubick, D., P. Paquin, P.E. Cushing, and V. Roth (eds) (2005). Spiders of North America: an identification manual. American Arachnological Society. 377 pages.