Larval Hooks Hold on Tight

photograph of a glass-winged butterfly hanging off of cocoon

Biological Strategy

Eleanor Banwell

Image: Darkdiamond67 / Shutterstock / Some rights reserved

Modify Size/Shape/Mass/Volume

Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.

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Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Attach Permanently

A living system can conserve energy by attaching permanently to a particular site because it can take advantage of resources that come its way, rather than expending energy to move to resources. A permanent attachment, intended to last the lifetime of the living system, creates special challenges. For example, physical mechanisms, such as the anchor that holds a marine algae to the ocean’s bottom, must be able to withstand forces that can pull it off its substrate. Chemical mechanisms, such as a barnacle’s glue, must avoid both physical and chemical breakdown, such as being dissolved by water.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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Hooks and silk of butterfly larvae form a strong attachment under leaves

Most insects go through complete metamorphosis, where their body pattern alters from one form to a new one. The change occurs inside a hard casing, called a pupa, and is a complete remodeling of their entire bodies. During metamorphosis, the body of the lava breaks down completely into a sort of cellular soup that then reforms into an entirely new shape. During this process, the insect is unable to defend itself and is very vulnerable. Butterflies and moths have a number of ways of hiding their pupae, including mimicking leaves and other plant parts so as to be less obvious to predators. One species, the Greta oto, or glass-winged butterfly adds to the protection from camouflage by suspending itself from the underside of leaves when pupating.

Greta oto caterpillars weave a silken pad that is glued to the underside of the leaf. The silk fibers in the pad are deliberately tangled as they are spun to create large numbers of tiny loops. Once the pad has been spun, the animal inserts a specialized array of hooks, called a cremaster, into the silk pad. The cremaster is made of tough chitin that will not snap and it becomes a permanent part of the case of the pupa as the insect inside metamorphoses into the adult butterfly.

The hooks of the cremaster are highly specialized for hanging securely from the silk pad. The hooks are arrayed over the surface of the cremaster in a ball shape, which makes it easier for the hooks to penetrate the center of the pad without squashing it, whilst also increasing the available hooked surface area.

The hooks themselves are also highly specialized. Each hook ends with a backward facing barb that ensures that, while the threads of the silk can slip easily over the end of the hook, it cannot so easily slide off. Each barb is split into segments, forming mini barbs with a groove in between. Silk fibers can also sit in these grooves, forming a further barrier that prevents the hooked loops from sliding off.

The silk pad and cremaster of the Greta oto creates a sort of super hook-and-loop fastener that is at least 50 times stronger than is necessary to support the weight of the chrysalis. It is likely the extra strength is to protect the animal from the extra stresses caused by swaying caused by the strong winds that are common in its native habitat.