Spider Webs Behavior, Function, and Evolution

You would think that after centuries of studying spider webs we have a pretty good grasp of them. Yet a thorough, book-length review of their construction, function, and evolution has been missing. Emeritus Professor William Eberhard has taken on that colossal task, based on his nearly 50 years of observing spiders and their webs. Some works go on to define their discipline. Spider Webs has all the trappings of becoming the arachnological benchmark for many years to come.

At 21 × 28 cm and 658 pages, Spider Webs is big. A 12-page table of contents reveals the book's highly nested structure, many sections going down 4-6 levels (you may find yourself reading section 3.3.4.2.2.5 on the importance of UV reflectance of silk stabilimenta). Almost 300 black-and-white photos and drawings of webs, often of species not included in previous literature. And if that was not enough, a 91-page online supplement with small print containing additional observations, tables, and musings. What makes these numbers mildly terrifying is that Eberhard almost apologetically mentions he avoids so much detail as to overwhelm the reader. Yes, 750 pages is what Eberhard holding back on his encyclopedic knowledge looks like.

So, what is in the book? Eberhard includes much older literature often skipped over in review papers, and has included numerous of his own unpublished observations. He starts with a chapter on the spider's hardware (silk glands, spinnerets, and legs) and ends with two chapters on evolution. The construction and function of webs in chapters 3-8 forms the core of the book, though the order in which topics are discussed is not necessarily how a spider proceeds. Thus, Eberhard begins with an extended chapter on the function of orb webs, including 45 pages discussing incorrect and unsupported ideas, and the many trade-offs between its different properties and parts. Then follow chapters on web construction and cues used. For non-orb weavers this consists of "scattered bits and pieces of information" (p. 273). Orb weavers have been much better studied, and the descriptions of building behaviour (how a spider moves its legs and body to make an orb web) and the cues used to decide where to lay and attach lines span 120 pages in two chapters. And thus the first step in the process, how a spider decides where and whether to build a web, is discussed last. The properties of spider silk are only discussed briefly.

Spider Webs is not a particularly difficult book. Eberhard wanted it to be accessible to the general reader and has by and large succeeded. Any reader serious about tackling a book this size is probably already familiar with the terminology and conventions used here. If not, the spider novice could start with e.g. Biology of Spiders. Spider Webs is, however, particularly rich in detail and minutiae. This is where the highly nested chapter structure comes into its own. I found it useful to bookmark the contents and refer to them while reading to keep my bearings. Throughout, there are summaries in subsections and summaries at the end of chapters, and summaries of summaries. The take-home messages are spelt out clearly, and I will highlight a few prominent ones.

Eberhard warns against typology: published descriptions of webs and spider behaviour make it appear as if there are only a few ways of doing things, but that is a limitation of our language, not a reflection of reality. "The" web does not exist: there is much variation on the level of families, genera, species, and even individuals in the details of web construction, much of it poorly documented. There are fascinating variations on the theme (the elongated ladder webs to catch moths are but one of many examples included here). And this is before you even consider the material here on non-orb weavers. Tying in with that, the idea that spiders are little web-spinning automatons is mistaken. True, freshly hatched spiderlings succeed at their first attempt, and there is little indication that spiders improve with practice, but they combine information from a wide variety of cues (environmental and otherwise) during web construction and can modify their behaviour as needed.

Despite claims to the contrary, it has never been shown empirically that orbs webs are optimum structures, and doing so would be very hard, argues Eberhard. Nor are orb webs the pinnacle of evolution. The higher-level phylogeny of spiders might not be fully resolved, but there is little doubt that orbs have evolved and secondarily been lost again multiple times. The large-scale evolutionary pattern is one of both high diversity and frequent convergence, resulting from "an especially ancient and thorough exploration of the adaptive space associated with webs" (p. 479).

Next to everything we know, Spider Webs is as much a book about what we do not know. Eberhard reviews the many suggested functions of web traits (e.g. silk stabilimenta), only to point out that we often lack empirical data to decide one way or another. A particular sticking point is ecological realism. Much data that is collected (on e.g. number of prey available in the environment or their nutritional value) is flawed or of questionable relevance to spiders: "the cues that are used by spiders are not necessarily the same as the variables that are commonly measured by researchers" (p. 435). And we must not forget that spiders evolved in habitats undisturbed by humans until recently. Most data is now gathered in environments that spiders did not evolve in.

Eberhard berates fellow arachnologists throughout, but is equally critical of his own failures and owes up to his changes of mind. Table 9.4 lists beautiful ideas slain by ugly facts and "gives a sampler of disappointments (with my own failures perhaps over-emphasized)" p. 480. As a bonus, there is a 15-page table in the supplement with hundreds of unanswered questions for future students and researchers to pursue. One might think Eberhard a curmudgeon, but that would ignore the fact that nature is complex and does not readily yield to the order that we wish to impose upon it. Here is someone who has studied spiders intensely and I must agree with his outlook that "intellectual rigor is useful; little is gained (and much is lost) by ignoring the limitations of data to support or discard particular conclusions" (p. 87).

Spider Webs is one of those rare books that sets a discipline's research agenda for decades to come and cannot be ignored. This book has set the bar and has set it high. For professional arachnologists this is a vital and invaluable reference work. For serious spider enthusiasts it is an absolute treasure trove. Beyond the spider community, however, there is much here of interest to ethologists and evolutionary biologists.