Addition to article (for those who have already read version 1)

The first araneomorphs to employ silk as a capture method used cribellate silk, an elaborate structure composed of an axial core of paired pseudoflagelliform silk strands, progenitor to the capture silk of araneids, which act as a scaffold for the actual capture threads, cribellate silk. This silk does not employ glue, in fact the webs are completely dry such that placed in an arid environment the web retains its ability to capture prey almost indefinitely (supposing it is not subject to particulates). Rather, the capture threads entangle their prey. When one invests some thought into this problem, this concept is actually quite non-intuitive and difficult to understand without a detailed examination of the microscopic structure. How does a web built in a flat plane capture and hold its prey without the sticky capillary forces of liquids? Furthermore, an insect’s incredibly hard exoskeleton is composed of chitin (embedded in a proteinaceous matrix), a modified polysaccharide often shaped into smooth, flat or rounded lines, geometries extremely difficult to hold onto, especially when the insect is struggling to free itself. Like the seed of the burdock (burr) which fastens to the loops in objects by means of tiny hooks, cribellum silk takes this concept and reduces it to the nanoscale 10^-9 metres); cribellar silk (loops) bind to the prey surface (hooks) by means of innumerable VDWs bonds and hygroscopic forces. Additionally, the flexibility of the silken strands increases the surface area of contact while dissipating the stress along the longitudinal plane. Cribellar threads are not extruded from the spinnerets like other silk, but rather are produced in an anterior plate called the cribellum. This plate is formed from a dense array of spigots (40-60,000), each one producing a cribellar thread as thin as 10-100nm in diameter. These are then hackled in a laborious process achieved through comb-like appendages on the meta-tarsi of cribellate spiders, the calamistrum. It is this hackling which charges the cribellate fibrils, causing them to repel one another and expand into a disorganized, entangling matrix of characteristic wooly appearance. The concept of organisms working towards greater complexity through evolution is a great misconception, one which is well illustrated by a comparison between viscid silk in the Araneids and cribellate silk of their ancestors. The idea of increased complexity is supported by the modular design of many biological systems, each innovation built on the previous one until it reaches the purported pinnacle of the present day. However, biological systems respond only to environmental pressures. Thus any system which can minimize its energy expenditure while enabling the propagation of its genes has a selective advantage. Thus all other considerations being equal, a different biological system has an equal likelihood of being adopted and propagated within a population and a simpler biological system has a greater likelihood of being adopted over a complex one if it leads to the aforementioned advantages. The latter consideration has a greater probability during early development of a trait when its inherent advantage is smaller or else during environmentally dynamic periods. 95% of araneomorphs fall within the araneids, employing viscid silk. How then was the complex cribellate silk superseded by viscid silk? Capture thread was once thought to offer greater extensibility, enabling the capture of a greater diversity of prey. Recent studies have shown however that despite the greater extensibility of capture silk relative to the pseudoflagelliform silk forming the axial core, the cribellar threads increase the range of extensibility (up to 500%) so that a web may maintain its structural integrity after the rupture of the core strands. Unlike capture silk it also fails gradually, as different groups of nanofibres succeedingly fail (unlike capture silk which fails suddenly). In fact, more than 90% of energy absorption occurs after core failure. The answer to Araneid dominance appears to lie in the rise of a key innovation, glue, which drastically reduces that energetic expenditure. However it is the very complexity of cribellate silk, requiring a large investment of energy and time that makes such a viable alternative.Thus it is how extensibility and stickiness is achieved, rather than the degree of extensibility. Moreover, major ampullate (flagelliform) silk is not only tougher than pseudoflagelliform silk, but also capable of handling greater stress, enabling its use in supplementary applications. The rapidity with which it can be extruded, coincident with glycoprotein globule incorporation, offers an advantage especially in ecological niches where web destruction is prevalent. Other factors like decreased UV reflectance are also thought to have played a role.


About pbertner

Studied cell biology and genetics at UBC in Canada with a focus in microbiology. However, have gravitated more recently towards ecology and biodiversity. Have traveled the rainforests of Brazil, Brunei, Cambodia, Colombia, Costa Rica, DRC, Ecuador, Guyana, Indonesia, Madagascar, Malaysia, Panama, Philippines, Rwanda, Tanzania, Thailand, Vietnam with plans to visit many more.
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