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Arthropod Architects Amaze Engineers

Photo credit: Forest & Kim Starr, CC BY 3.0 US , via Wikimedia Commons.

Arthropods are often described as the most diverse phylum of animals, and the most numerous, too. There are well over a million species of arthropods known today, and other extinct ones in the fossil record. 

Three traits define arthropods: an exoskeleton, paired jointed appendages, and a body plan segmented into head, thorax, and abdomen (but sometimes the thorax and abdomen are fused). Arthropods (phylum Euarthropoda, or true arthropods) are subdivided into in five subphyla: Trilobites (extinct), Chelicerates (horseshoe crabs, spiders, mites and scorpions), Myriapods (millipedes and centipedes), Crustaceans (lobsters, crabs, crayfish, shrimp), and Hexapods (insects). Children are most familiar with chelicerates and hexapods; they learn early that spiders have eight legs and insects have six legs (generally speaking). An occasional centipede or millipede or “pill bug” arouses their fascination, too. They soon recognize spiders, but may have trouble realizing that flies, beetles, butterflies, praying mantises, honeybees, crickets, gnats, and aphids are all classified as insects. Today and tomorrow we’ll offer some news about insects and spiders whose architectural skills fascinate scientists.

The earliest arthropod body plan “explodes” onto the scene in early Cambrian strata, meaning that by their first appearance, arthropods had brains, nerves, muscles, a gut, locomotion, sensory equipment, and the ability to reproduce. The most famous of the Cambrian arthropods, the trilobites, with their complex eyes and coordinated legs, inhabited every part of the world. Other Cambrian arthropods, known from Canada’s Burgess Shale and from China, include MarrellaAnomalocaris, and some bivalve forms, although classification of extinct species is sometimes contentious. Some of these are “brought to life” in Illustra’s film Darwin’s Dilemma and further scrutinized in Stephen Meyer’s work, Darwin’s Doubt). Except in some details, the Cambrian species look familiar and would appear at home in the world today. If their complex body plans could emerge in a geological instant, why stop there? Did some of these extinct arthropods also possess complex behaviors and engineering know-how? We may never know, but we know what we can observe today.

Chelicerata: Spider Engineers

Bioengineers already envy spider silk for its exceptional strength and flexibility. A lesser-known but enviable quality is web architecture. Orb webs are admirable for their symmetry, but what about the irregular “tangle webs” that look chaotic, with silk strands going every which way? The tangle web, it turns out, is functionally beautiful; “it filters in prey and protects the spider from predators.” It is also well-built to be strong and resilient.

Seven researchers at MIT and one from Berlin investigated “In situ three-dimensional spider web construction and mechanics” and wrote up their findings in PNAS. Calling spiders an “evolutionary success” but also “nature’s engineers,” they say,

Learning how spiders used their silks and webs to adapt to environmental pressures have fascinated many fields of research such as biomedicine, biology, and engineering. Because of silk’s nanoscale size and the complex web architecture, little is known about the architecture and mechanics of three-dimensional (3D) spider websduring construction. This work comprehensively investigates the structure, mechanics, and functionality of a 3D spider web under construction, using consistent imaging and computational simulations methods. This work could inspire efficient spider-inspired fabrication sequences or fiber geometries in engineered materials, as demonstrated here for 3D-printed prototype materials. [Emphasis added.]

Of interest to them was a spider’s ability to build “lightweight and high-performance web architectures often several times their size and with very few supports.” This ability would be helpful for spacecraft, for instance, where light weight is a priority. Human construction often takes advance planning, collection of materials and a large team of workers to put a structure together. A spider does all the work herself.

The MIT research team took careful photographs of a 3D web under construction from different angles, and then 3D-printed the parts and put them together. What they found is that the spider first builds an escape route, and then keeps the growing web architecturally sound throughout construction. The spider continues to reinforce the original plan. A tangle web also must withstand the force of a prey hitting it. Because stress is localized at all times, the web is robust against failure. If a fiber breaks, 

the load is transferred to its connecting fibers. Because of redundancy in the structure and the nonlinear behavior of dragline silk, the spider web does not fail catastrophically at any stage of construction.

The paper waxes eloquent about the web-building spider’s skill set, too much so to quote it all here. Suffice it to say they were excited at what they observed and how the information might be used by engineers.

Expanding our knowledge of spiders’ web construction, silk recycling, web monitoring, and repair methods could inspire novel self-sufficient, self-repairable, and self-monitored smart structures. This knowledge can also inspire artistic, design, and architectural interventions — such as complex and large-scale tensile structures — via creative collaborations that both engage and inform materials and engineering sciences.

Tomorrow, “Insects and Design: Ant Engineers.”