Araneae

Silk used for various functions: spiders
 

Individual spiders are able to use silk for a variety of tasks by varying the properties of the silks they produce.

 
  "Certainly the most extraordinary material among those tabulated here is spider silk (that of silkworm moths is substantially less extreme)--it has the greatest tensile strength, astonishing extensibility, and by far the greatest strain energy storage…silks vary considerably in their properties, quite clearly tuned by natural selection to their particular tasks…A single araneid spider makes frame silk for the main members of its orb, viscid silk for the spiral threads that catch prey, cocoon silk, prey-wrapping silk, and so forth. Other kinds of spiders make other kinds of silks for other tasks…Spider silks do have an unusual combination of properties. But I know of no evidence that these can be achieved (if one wants them) only by a sequence-specific heteropolymer of amino acids, something unlikely to lend itself to cheap manufacture." (Vogel 2003:344-345)

Watch Video
  Learn more about this functional adaptation.

Silk assembled on demand: spiders
 

Structural components of spider silk are safely stored and assembled on demand with help from a molecular switch.

 
  "Five times the tensile strength of steel and triple that of the  currently best synthetic fibers: Spider silk is a fascinating material…How do spiders form long, highly stable and elastic fibers from the  spider silk proteins stored in the silk gland within split seconds?…
 

"Spider silk consists of protein molecules, long chains comprising  thousands of amino-acid elements. X-ray structure analyses show that the  finished fiber has areas in which several protein chains are  interlinked via stable physical connections. These connections provide  the high stability. Between these connections are unlinked areas that  give the fibers their great elasticity.

"The situation within the silk gland is, however, very different: The  silk proteins are stored in high concentrations in an aqueous  environment, awaiting deployment. The areas responsible for interlinking  may not approach each other too closely; otherwise the proteins would  clump up instantaneously. Hence, these molecules must have some kind of  special storage configuration…

"The protein chains are  stored with the polar areas on the outside and the hydrophobic parts of  the chain on the inside, ensuring good solubility in the aqueous  environment.

"When the protected proteins enter the spinning duct, they encounter  an environment with an entirely different salt concentration and  composition. This renders two salt bridges of the control domain  unstable, and the chain can unfold. Furthermore, the flow in the narrow  spinning duct results in strong shear forces. The long protein chains  are aligned in parallel, thus placing the areas responsible for  interlinking side by side. The stable spider silk fiber is formed.

 "'Our results have shown that the molecular switch we discovered at  the C-terminal end of the protein chain is decisive, both for safe  storage and for the fiber formation process,' says Franz Hagn." (Science Daily 2010)

  Learn more about this functional adaptation.

Legs detect airborne vibrations: spiders
 

The legs of some spiders detect airborne vibrations of approaching insects thanks to specialized vibration-sensitive hairs, called trichobothria, on certain leg segments.

 
  "Spiders can detect vibrations traveling through the air from sources far away. They can do this thanks to specialized vibration-sensitive hairs, called trichobothria, on certain segments of their limbs. These hairs are able to move in any direction, and tell the spider the direction from which an object is approaching and its size. They are so responsive to airborne vibrations that they can deven detect those caused by the wings of insects in flight, alerting the spider to the approach of a potential victim as it heads toward the spider's web." (Shuker 2001:36)
  Learn more about this functional adaptation.

Legs use hydraulics: spiders
 

Legs of spiders extend by hydraulic pressure of built-up fluids.

 
  "A remarkable and effective hydraulic mechanism is found in the legs of spiders, which have muscles to flex the joints but none to extend them. Spiders stretch their legs by pumping fluid into them. When a spider gets ready to jump, it generates, for a fraction of a second, excess pressure of up to 60 percent of an atmosphere. The legs extend in order to accommodate more fluid." (Tributsch 1984:59)

"One particular hydraulic device is worth a little more attention here, partly because its existence comes as yet another surprise and partly because it achieves antagonism for contractile muscle in an unusual way. The eight legs of a spider differ little from the six of an insect, but a curious special feature of spider legs has been known for almost a century. While properly equipped with flexor muscles (ones that decrease the angle between one segment and another), they lack the antagonistic extensor muscles (ones that increase that angle toward 180 degrees). Biologists casually assumed that elasticity of the interarticular membranes provided the antagonistic force, not on the face of it an unreasonable idea. But Ellis (1944) remembered that spiders die with legs severely flexed. If elasticity did the extension, they would more likely die with legs extended or at least not so flexed--as do insects. He found that cutting off the tip of a leg prevented reextenson until the tip was resealed; and he found that mild exsanguination reduced a spider's ability to extend any of its legs. He suggested that extension in spider legs was hydraulic, not muscular or elastic. The idea was confirmed by Parry and Brown (1959), who measured resting pressures of 6.6 kilopascals and transient pressures of up to 60 kilopascals (over half an atmosphere) in spider legs. An isolated leg could lift more weight as the pressure inside it was increased, and the spiders turned out to have a special mechanism to seal off a joint that prevented fatal depressurization when a leg was lost." (Vogel 2003:421)
  Learn more about this functional adaptation.