Are tarantula’s spiders? Short answer… yes, they are indeed spiders. Any of you who follow me on social media will know that I own a few tarantulas, and this is where this post comes from. The last few times that I have visited an exotic pet shop and asked to see their spiders, I have been met with the response “we don’t really stock any spiders, but we do have a few tarantulas”. Furthermore, I have seen a couple of social media posts and YouTube videos from tarantula keepers in which they state that tarantulas are not actually spiders. This misunderstanding appears to be most prominent within the tarantula keeping hobby, rather than the science community. This article was originally going to be a quick correction of this misunderstanding, but it has decided to turn it into an excuse to write about spider evolution and phylogeny. So, if you are just here to find out that tarantulas are spiders, then you may want to stop here, but if you’re interested in why this is true and the differences between tarantulas and other spiders, then grab a warm drink, sit back, and read on.
Evolutionary History and Phylogeny
Spiders are the most special 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 that date back 380 million years (Garrison et al., 2016). Attercopus fimbrianguis 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). While not a spider, it does show how far back the spider ancestry goes, now onto the extant spiders. 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 some 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 more derived, Araneomorph spiders have a Labidognath cheliceral position where their fangs move side to side, like a pair of scissors (figure 1.0). However, back to the main point for a second, regardless of whether they are a Mygalomorph or Araneomorph, they still belong to the Arachnid class Araneae and therefore are spiders. It can sometimes get confused with the Araneomorphs often being referred to as “True Spiders” but I believe this is simply down to the fact that they make up more of the species and are more commonly encountered. It doesn’t however mean that the Mygalomorphs are “False or Fake Spiders”.
Figure 1.0 The cheliceral positions of the Mygalomorphs and Araneomorphs. 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 distinct 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 (eg 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 2.0.
Figure 2.0 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.
Behavior and Natural History
Along with the differing morphological characteristics, the Mygalomorphs and Araneomorph spiders also differ in their lifestyles. While both groups are incredibly diverse in behavior, there are a few trends that work as a 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). While 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… they’re that distinct.
Some whims, such as the net-casting spiders (Deinopids) use the primitive, non-sticky (cribellate) silk that catches prey through Van der Waal force interactions and physical entanglement (Piorkowski and Blackledge, 2017). The Araneoidea spiders utilize 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 to the success and diversity of the Araneoidea spiders. I could easily go into loads more detail, waffling on about why this is the case, but we are already starting to approach 1,000 words, so I’ll save that for another time. Silk production and use is just one of the multitude of factors that have contributed to the success of spiders and there are many more differences between the groups. I imagine I will address more of these in individual posts in the future.
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 just two examples of Araneomorphs that do not use silk at all for prey capture, but instead actively hunt, more towards the Mygalomorphs. Demonstrating that there is a lot of cross over between the two groups, with the Mygalomorphs being the ancestral condition, they are the basis 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 led 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.
I feel as though I may have gone off track a little bit, but it’s easy to do when spiders possess a cornucopia of fascinating traits and behaviors. Anyway, the point is that, regardless of the huge diversity among all spiders, and the differences between the Mygalomorphs and Araneomorphs, all Mygalomorph spiders (including tarantulas) still belong to the Arachnid class Araneae and therefore are indeed spiders. I hope you’ve enjoyed this somewhat all over the place post about spider groups and their differences, with the underlying aim of displacing a common misunderstanding. It is highly probable that I will do future posts going into more detail on the different silks used by spiders as well as spider evolution, simply because (in case you hadn’t guessed), I found it fascinating. Hopefully you do as well.
Thanks for reading, until next time.
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Blackledge, TA, Scharff, N., Coddington, JA, Szüts, Wenzel, JW, Hayashi, CY and Agnarsson, I. (2009). Reconstructing web evolution and spider diversification in the molecular era. PNAS, 106(13), 5229-34.
Coddington, JA (1991). Systematics and evolution of spiders (Araneae). Annual Review of Ecology and Systematics, 22565-92.
King, GF and Hardy, MC (2013). Spider-venom peptides: Structure, pharmacology and potential for control of insect pests. Annual Review of Entomology, 58, 475-96.
Piorkowski, D. and Blackledge, TA (2017). Punctuated evolution of viscid silk in spider orb webs supported by the mechanical behavior of wet cribellate silk. Naturwissenschaften, 104(67).
Ubick, D., P. Paquin, PE Cushing, and V. Roth (eds). 2005. Spiders of North America: an identification manual. American Arachnological Society. 377 pages.