Early web-spinner found in amber

From the BBC.

14 June 2006

Spiral orb webs, which to many people typify spiders, were catching insects in their sticky silk while the dinosaurs still walked the Earth. True orb weaving spiders found trapped in amber from 121-115 million years ago are the oldest of their type yet found. The spiral webs have proven an extremely successful strategy for catching prey - evidenced by the great diversity of orb weavers present today. Two specimens are described in the UK Royal Society journal Biology Letters. The fossil spiders were found embedded in amber from Alava in northern Spain. They date to the Lower Cretaceous.

Amber is a form of protective resin extruded from trees that has hardened over millions of years. It is very useful to scientists studying the history of past life because ancient animals and plants are often preserved in the gem-like material. David Penney of the University of Manchester, UK, and Vicente Ortuno of the University of Alcala, Spain, assign the arachnids to a new species: Mesozygiella dunlopi. Typical orb webs consist of outer frame lines to which radial (spoke-like) lines are attached, providing support for the characteristic spiral sticky line that occupies most of the web's surface. By using two different types of silk - one strong and rigid, the other weaker but stretchy - the orb weaver creates a web with the required strength and flexibility to cope with the impact of fast-flying insects - and the struggling which occurs once the prey is captured in the sticky trap.

The evolutionary success of this design can be seen in the high diversity of true orb weavers, which currently number 2,847 living species. This astonishing diversity also owes much to the way in which the basic design can be easily modified. "One modification to the web is quite fantastic," Dr Penney told the BBC News website. "Picture a normal, spiral orb web and picture running down from it a ladder-type structure which is also made from sticky silk. This has evolved to trap moths, which have scales that rub off. "When a moth flies into a normal orb web, it's the scales that stick and the moth tumbles out of it. But with the ladder structure, the moth tumbles down until all the scales come off and eventually it gets caught."

In Biology Letters, Penney and Ortuno write that spiders may have expanded in number and diversity during the Cretaceous. An explosion in the abundance of flowering plants begot an expansion of the insects which pollinated them. These in turn provided prey for the spiders, the authors suggest, which prospered as a result. There are fossil spiders that date from the Devonian (350-420 million years ago) - long before even the dinosaurs. In some of these mineral fossils, it is possible to see evidence of spinnerets, the organs spiders use to spin their web silk. But it is often unclear how fossil spiders used them; some species spin web silk to line their burrows and to protect egg sacs.