The specialised catching silks
Cribellate and sticky catching silks evolved to increase the prey holding efficiency of snare webs. They represent totally different solutions to the problem of web-based prey capture. Both are spun in association with supporting lines to maintain their structural integrity in the web.
Cribellate silk
Sheet web building spiders, like the common black house or window spiders, make their webs with this sort of catching silk (sheet or shawl webs), as do the net casting spiders and their relatives. Cribellate silk is produced from many tiny, silk glands placed beneath a specialised, flattened spinning organ called the cribellum. The cribellum is placed in front of the spinnerets and is derived from spinnerets (the anterior median spinnerets) present in ancestral araneomorphs. Its surface is covered by hundreds or thousands of tiny, elongate spigots, each producing a single fibril of cribellate silk about 0.00001 mm thick. All of these spigots act together to produce a single cribellate thread made up of thousands of the silk fibrils. They are supported on thicker lines produced by spigots on the posterior and median spinnerets. A web made with a meshwork of these composite 'wool-like' threads is particularly effective at tangling the bristles, spines and claws of insect prey. The fine fibrils of cribellate
silk also appear to have some type of 'dry adhesive' properties (possibly electrostatic in nature) and will even cling to smooth beetle cuticle.
Cribellate spiders all possess a row of toothed bristles (the calamistrum) on the metatarsal segment of the last leg. These bristles are used to simultaneously comb out the mass of cribellate fibrils and their supporting silk lines from the cribellum and spinnerets.
This remarkable innovation allowed spiders to produce the first specialised prey catching silk. All araneomorph spiders were once cribellate, and a lot still are, but the cribellum has been lost in many descendent lineages. These include the hunting spider groups (wolf, huntsman and jumping spiders, etc.) and most of the orb weavers and their relatives, the latter having evolved a quite different sort of catching silk.
Tip of abdomen showing spinnerets and cribellum. Six spinnerets with different types of spigots and a specialised spinning organ called the cribellum.
Spigots on cribellum.
Cribellate silk.
Comb-like calamistrum.
Calamistrum.
Cribellate silk line, Miagrammopes sp.
Wool-like cribellate silk.
Net casting spider spinning cribellate net. Photo: M Gray © Australian Museum.
Sticky droplets on orb web spiral lines.
Sticky silk
A more recent evolutionary innovation was the development of a glue-like silk for use in prey capture - that is, a silk that remained liquid rather than being produced as a fibre. This type of catching silk evolved in the ancestors of the highly successful orb web weaving and comb-footed spider families and their relatives. As with cribellate silk, the sticky liquid catching silk had to be carried on fibrous silk support lines - for example, the spiral line of the orb web or the vertical lines of a redback spider's web.
Sticky silk and its supporting line are produced simultaneously from a 'triad' of spigots on each of the posterior lateral spinnerets (PLS) - a central spigot provides the supporting line from the flagelliform silk gland, while two spigots on either side coat the line with liquid silk from the two aggregate silk glands. The two coated lines from each PLS coalesce a little way from the spinnerets, forming a doubled sticky line. Surface tension effects subsequently cause the sticky silk coating to break up into droplets. At the centre of each droplet is a core of very sticky glycoprotein material. In orb weavers the sticky silk is also hygroscopic (absorbs moisture from the air) and this 'wetting' of the spiral line is probably a significant factor in increasing its ability to stretch.
