For several years I studied spiders, which you may be familiar with if you have delved into the beginnings of My World. When I first began I was studying silk gland physiology and biochemistry, but by the time I moved on to other fields of research, my interest drifted to examining the evolutionary origins of silk glands. When I returned to a spider research group, I struggled to take my developing hypotheses regarding this and turn it into feasible research. I have since changed labs and research focus, but in many ways the passion I felt and continue to feel for the ideas of this evolutionary hypothesis is the source of both my current research and the entire premise of The Deep: Please enjoy the long overdue presentation of this concept as a hypothetical explanation of the origins of the silk apparatus in spiders:.
My old mentor Dr. Tillinghast gave me a copy of Endless Forms Most Beautiful by Sean Carroll in I believe late 2007, and so since reading that and taking a course on developmental biology, I’ve had the basic concept of evo-devo and its astonishing complexity as a sort of “regularity” (I still don’t feel as confident as I’d like that I’m using that term correctly) to build my own personal hypotheses. Spiders produce their silk from a sets of what are typically called “spinning apparatuses”, comprised of the glands within the body which produce the silk, the spigots from which the silk is extruded, and the spinnerets upon which the spigots are mounted, allowing for mobility of deployment of the spigots and precision placement of silk fibers (and glues, as appropriate). A classic (one of my all time favorites) paper, The Origin of the Spinning Apparatus in Spiders (Schultz, 1987), is one of the last broad and deep examination of the possible evolutionary origins of the spinning apparatus, and it is, of course, woefully out of date.
There are a number of different hypotheses and ideas that feed into my own personal model(s) of spider silk evolution, some of them are individual observations from evo-devo literature, such as the fact that just as wings are derived from the biramous appendages in early arthropods, spiders too feature similar genes in play in the development of their silk glands (Damen et al, 2002). Similarly, the spigots of the silk glands are well established to be homologous to some manner of spider setae (Bond, 1994); which kind remains unclear (the presence of lymph containing chemosensory hairs, once mistaken for silk producing setae on the feet of tarantulas, may or may not be an explanatory red herring; Perez-Miles et al, 2009, 2012; Rind et al, 2011; Foelix et al, 2012, 2013). Finally there is the fact that in more “primitive” (I had a whole question on “primitiveness” in Gavin’s exam, so I’ll just concisely use quotation marks and move on) spiders, one pair of spinnerets are segmented, innervated, and muscled just like the spider’s legs. This piece of the puzzle to me is one that is yet to be well clarified, but is made particularly complex, in my mind, since all other arachnids, and in fact all other chelicerates, for that matter, lack any appendage-like structures on their opisthosoma (abdomen), and there are even early ancestors of spiders (Uraraneidae, Selden & Shear, 2008) that feature spigots, but no spinnerets.
So from where did these leg-like spinnerets (assuming that these are in fact representative of an ancestral state for spiders) come from? Well, there seems to be a number of possible explanations, but from what I can figure, they all involve evo-devo mechanisms, namely ones responsible for producing leg-like structures:, either the elimination of an inhibitory genetic element preventing abdominal appendages after hundreds of millions of years of activity, a novel activation of the appendage forming gene toolkit in this one lineage or possibly that, that the sclerotized ventral abdominal plates found in more “primitive” arachnids are not in fact plates, but greatly reduced abdominal limbs, and so in our spiders we are not seeing what amounts to denovo growth of appendages, but rather the reversal of an ancestral reduction.
While the difference between the two theories is mostly metaphysical or semantic (and I suspect, impossible to differentiate even with the full suite of genetic techniques available; evolutionary time I suspect has eroded such fine traces), it would seem to me that with all of these individual traces, partial regularities, and evolutionary developmental theory, that there remains insight to be had in our understanding of such a significant evolutionary adaptation.
Bond, J. (1994). Seta-spigot homology and silk production in first instar Antrodiaetus unicolor spiderlings (Araneae: Antrodiaetidae). Journal of Arachnology.
Damen, W., Saridaki, T., & Averof, M. (2002). Diverse adaptations of an ancestral gill: a common evolutionary origin for wings, breathing organs, and spinnerets. Current Biology.
Foelix, R., Erb, B., & Rast, B. (2013). Alleged silk spigots on tarantula feet: Electron microscopy reveals sensory innervation, no silk. Arthropod Structure & Development.
Foelix, R., Rast, B., & Peattie, A. (2012). Silk secretion from tarantula feet revisited: alleged spigots are probably chemoreceptors. Journal of Experimental Biology.
Pérez-Miles, F., & Ortíz-Villatoro, D. (2012). Tarantulas do not shoot silk from their legs: experimental evidence in four species of New World tarantulas. Journal of Experimental Biology.
Pérez-Miles, F., Panzera, A., & Ortiz-Villatoro, D. (2009). Silk production from tarantula feet questioned.
Rind, F., Birkett, C., & Duncan, B. (2011). Tarantulas cling to smooth vertical surfaces by secreting silk from their feet. Journal of Experimental.
Selden, P., & Shear, W. (2008). Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order. Proceedings of the.