Preserving insects for study by mounting them on n insect pins is a practice instituted centuries ago. The pin provides a convenient “handle” to hold an insect. The pin allows information labels to be attached to the insect. The pin allows the insect to be positioned at multiple angles under a microscope so that all the features can be observed. The pin isolates the specimen from contact between its delicate structural parts (legs and wings) and the container. Vibrations and even slight touching can cause brittle parts to detach.
What changes make the dried insect so brittle compared to a live insect? Insect cuticle properties change with the degree of hydration. When the cuticle is swollen with water, its component parts have more freedom of movement and flexibility to respond to shocks. If the cuticle is dehydrated, the long chitin and protein polymers can no longer slide past each other or change shape easily. Force will cause them to break rather than move.
Many non-lviing materials have similar properties. Most are familiar with chewing gum. A fresh, well hydrated (unchewed) stick of gum is flexible; if allowed to dry, it becomes brittle and will shatter. The insect cuticle is similar, maintaining flexibility when hydrated, becoming brittle when dry.
The wings of live butterflies are pliable and not as brittle as preserved specimens. The same is true for the legs. Entomology teachers often simplify the circulatory system of insects to the movement of liquid in its major body cavity (the hemocoel). However, insects have complex structures and means to actively move water from the central cavity into tissues such as wings and legs to keep them hydrated. Once the insect dies, the water ceases to move and the specimen dries. Drying the preserved insect specimen fixes the wings and legs into an unmovable position. Attempts to move them results in breakage rather than the desired movement.
There are some curation alternatives to mounting insects on pins such as storage in ethanol or mounting them on cotton. However, for convenience of study, pins have been the preferred method and over centuries. Some insect specimens, curated centuries ago are still intact. No one has developed an improved method for curation of insects for study since that time. Plastic resins have some use, but limit the manipulation of specimens. Perhaps advances in technology will eventually replace the insect pin, but for now we plod along with pinned insects, carefully handling them to prevent damage.