Two short articles in this week’s Science link the orb-weaving spiders back to a common ancestor in the Early Cretaceous, with both physical and molecular evidence. What we have is a 110-million-year-old piece of amber that preserves a piece of an orb web and some captured prey, and a new comparative study of spider silk proteins that ties together the two orb-weaving lineages, the Araneoidea and the Deinopoidea, and dates their last common ancestor to 136 million years ago.

Araneoids and Deinopoids build similar looking webs—a radial frame supporting a sticky spiral—but they differ in how they trap prey. Deinopoids spin dry fibers that they fluff into threads that adhere electrostatically to small insects; Araneoids secrete glue onto the the strand, which takes less work (no fluffing), and is much more strongly adhesive. The differences are enough to make one question whether there was a single origin of orb weavers, or whether the two groups independently stumbled on the same efficient form of architecture.

First, the pretty specimen. This is piece of Spanish amber which captured a small piece of a web 110 million years ago. The threads are fine enough that you can’t really see them in the picture, but the authors have marked their positions, and the insets show what the strands look like at high magnification. The geometry of the fragmentary strands suggests that this could be a scrap of an orb web, but isn’t conclusive—other forms could also be compatible with it. There are also suggestions of glue droplets along the strands.

(click for larger image)
Spider web with adhered arthropods from the Early Cretaceous of Spain (strands are not visible in the photomicrograph, so they have been drawn in here). A map of silk strands in the amber portion, with a Microphorites fly and a mite (CPT-963 and CPT-964), also shows details of the rectilinear strand with droplets to which a mite adheres (upper box shows the measured droplets, two big and three small, arrows). Five strands are in the same plane and have a similar orientation and thickness (b1 to b5), and three of them are connected perpendicularly to an incomplete strand (b6), but the other two strands are possibly also connected. Two consecutive main strands are connected by two very thin strands (c1 and c2); one of them was broken inside the resin and appears bent (c1). Another long strand is crossed by a longer rectilinear strand with the same thickness (d). These two strands cross at an acute angle, like the intersection of two silks in this amber portion but which lie in a different plane and are unconnected to the main group (e1 and e2). Other views of the amber piece reveal a more extensive web. Scale bar, 1 mm.

The other paper is a little more abstract, but just as interesting. Araneoid spider silk genes have already been identified and sequenced, and different proteins are used for different structural functions. Web silks aren’t uniform—in some places, spiders need strength, in others elasticity, in others stickiness. Some of the proteins are:

(i) MaSp1 and MaSp2, major ampullate proteins of the frame and radii, (ii) MiSp, minor ampullate scaffolding protein, and (iii) Flag, flagelliform protein of capture spirals.

Garb et al. sequenced silk proteins from deinopoid spiders. If deinopoids evolved their web architecture independently from the araneoids and converged independently on the same style, then we’d expect they’d have adopted or generated different proteins to meet the engineering demands of their webs. If araneoids and deinopoids diverged from a common ancestor that already had these diverse proteins in place, then we’d expect to find homologous proteins in their silks.

The answer is diagramed below: both araneoids and deinopoids use related MaSp1, MaSp2, MiSp, and Flag proteins, suggesting that the last common ancestor of both had all of those proteins present.

Relationships of orb-weaving spiders and spidroins. (A) Deinopoid (purple) and araneoid (green) lineages (2), depicting inferred ancestral web and spidroins. Orb-web frame and radii composed of MaSp1 (blue) and MaSp2 (brown) are shown along with temporary spiral with MiSp (black) and capture spiral with Flag (red). (B) Summarized phylogeny of spidroin family members.

The implication is that the evolution of the orb web preceded the divergence of these major spider groups, probably at some time in the Jurassic. It would have been an important innovation that would lead to an explosion of spider diversity in the Cretaceous as a consequence of another event: the co-evolution of flowering plants and insect pollinators. Flowers may be pretty, but they’re also important pasturage for spider dinners.

Garb JE, DiMauro T, Vo V, Hayashi CY (2006) Silk Genes Support the Single Origin of Orb Webs. Science 312(5781):1762.

Peñalver E, Grimaldi DA, Declòs X (2006) Early Cretaceous Spider Web with Its Prey. Science 312(5781):1761.