Leaving the Web: Upgrade or Trade-off?

The typical spiders wait in webs, but jumping spiders hunt on foot. One tempting explanation is that webs are costly, so shedding them must be an upgrade. But a plausible story remains only a hypothesis; science consists of constant questioning and testing. Upon a closer look, abandoning web foraging looks less like an upgrade than a trade-off, and the evidence becomes far messier while trying to retrace historical evolution.

If webs are energetically costly, hunters should theoretically thrive ahead. In reality, the picture is a lot more balanced. Web-buildings spiders may raise their metabolic rate as much as 10-20 times above resting levels while spinning, but building is often brief, and in certain species, recycling web protein can substantially reduce the cost of making silk (Wilder 2011). Jumping spiders expends its energy differently: the jumping spider Maripissa muscosa had a cost of transport nearly double that of a wolf spider at top sustainable running speed, despite a lower resting metabolic rate (Schmitz 2004, 2005).

Testing the theory from a historical perspective, If abandoning webs was indeed a major advantage, webless lineages should have produced far more species than their web-spinning counterparts. Fernandez et al. (2018) analyzed about 2500 genes from 159 spider species and found no significant association between losing foraging webs and faster diversification, suggesting that habitat variety or biotic interactions may play a bigger influence. However, Garrison et al. (2016) found that the mostly ground-dwelling, webless RTA clade diversified faster than orb weavers, possibly because a rise in Cretaceous non-flying insects favored spiders feeding on ground-dwelling prey. Together they suggest that new food or habitats, not web loss itself, may have mattered, but historical tests as such can only show associations.

The factor that drove web loss is compensated in jumping spiders with superior senses. Jumping spiders hunt by sight, and their eyes solve a design trade-off by dividing the labour: wide-angle side eyes that are sensitive to motion, and principal eyes with the sharpest spatial acuity known among terrestrial invertebrates (Jakob et al. 2018). If the side eyes guide the principal eyes toward movement, masking them should stop the principal eyes from tracking moving targets. Jakob et al. (2018) tested this with a specialized eye-tracker: with the side eyes masked, the principal eyes generally stayed and relaxed instead of following moving disks. The web’s replacement comes with trade-offs of its own.
The assumption “leaving the web was an upgrade” thus functions better as a starting point than a conclusion. Broken into testable hypotheses, it yields an energy budget with costs on both sides, a contested historical record, and a hunting toolkit built on its own trade-off between acuity and motion detection. The studies present the evolution branching of jumping spiders as not simply progressively better, but instead a specialized lineage that traded one set of costs and benefits for another.

References

Fernández, Rosa, Robert J. Kallal, Dimitar Dimitrov, et al. 2018. “Phylogenomics, Diversification Dynamics, and Comparative Transcriptomics across the Spider Tree of Life.” Current Biology 28 (9): 1489–97.e5. https://doi.org/10.1016/j.cub.2018.03.064.

Garrison, Nicole L., Juanita Rodriguez, Ingi Agnarsson, et al. 2016. “Spider Phylogenomics: Untangling the Spider Tree of Life.” PeerJ 4: e1719. https://doi.org/10.7717/peerj.1719.

Jakob, Elizabeth M., Skye M. Long, Duane P. Harland, et al. 2018. “Lateral Eyes Direct Principal Eyes as Jumping Spiders Track Objects.” Current Biology 28 (18): R1092–93. https://doi.org/10.1016/j.cub.2018.07.065.

Schmitz, Anke. 2004. “Metabolic Rates during Rest and Activity in Differently Tracheated Spiders (Arachnida, Araneae): Pardosa lugubris (Lycosidae) and Marpissa muscosa (Salticidae).” Journal of Comparative Physiology B 174 (7): 519–26. https://doi.org/10.1007/s00360-004-0440-6.

Schmitz, Anke. 2005. “Spiders on a Treadmill: Influence of Running Activity on Metabolic Rates in Pardosa lugubris (Araneae, Lycosidae) and Marpissa muscosa (Araneae, Salticidae).” Journal of Experimental Biology 208 (7): 1401–11. https://doi.org/10.1242/jeb.01497.

Wilder, Shawn M. 2011. “Spider Nutrition: An Integrative Perspective.” Advances in Insect Physiology 40: 87–136. https://doi.org/10.1016/B978-0-12-387668-3.00002-7.