Caught In a Sticky Situation:The Orb-Weaving Spiderweb
Emily Gastelum and Taylor Stinchcomb
Biology 342 Fall 2011
What is Phylogeny?
The phylogeny of a behavior is its evolutionary development in a species over several generations. If a behavior is believed to derive from genetics, phylogenic questions can be used to study the nature if its morphology over time. Phylogeny can also provide answers to questions of whether a behavior is primitive or derived.
The Evolution of Silk
The phylogenic data that is currently available is not enough to provide solid evidence of a specific sequence of events for the evolution of the orb-web. However, recent investigations have highlighted some important facts about the expressions of spider silk. There are two basic orb-weaver lineages: one large group consisting of the Araneidae, Tetragnathidae and Theridiosomatidae and a second group represented by the Uloboridae and Deinopidae families. The two lineages are distinguished by their silk producing glands; the former have two types of abdominal glands, a pair of flagelliforms and a pair of aggregate glands which act to produce a type of viscid gluey silk while the later spiders possess a flat spinning plate called a cribellum, producing a more dry cribellate silk (as shown below). The identification of these two lineages has led to a controversy over their evolutionary history. The most recent studies have emphasized the single origin theory.
The Single Origin Theory: The single origin of the orb web implies a transition in the webs spiral composition from cribellate silk to viscid gluey silk.
THE BIG QUESTION:
Did spider first lose the cribellum and subsequently evolve the viscid glue?
Did viscid glue evolve first, thus rendering the cribellum “obsolete”?
One of these areas of study focuses on the adhesive properties of each silk. The stickiness of cribellate silk varies with the size of its surface area and when wet, the fibrils mat together, inhibiting adhesion (Opell 1994). Opposite of this, viscid silk loses its adhesion when it dries out. (Opell and Schwend 2008). To prevent dehydration, hydrophilic salts exist in the aqueous coating of viscid silk glue droplets. Recent studies have shown that these salts may actually directly increase the adhesion of glycoprotiens present in the silk (Blackledge et al 1998). This discovery suggests that the evolution of dry cribellate silk to viscid gluey silk may have begun with the incorporation of salts that initially increased adhesion, setting up a situation where their hydrophilic properties eventually selected for aqueous glue droplets. The most recent study on this evolution has been done by Opal et al who hypothesize that viscid glue silk evolved first in young cribellate spiders, whose undeveloped cribellum could not produce adhesive threads, as an alternative tool to induce stickiness (Blackledge et al 1998).