Flies possess a unique structural system that differs significantly from the internal skeletons found in many animals. Unlike humans and other vertebrates, flies do not have an internal framework of bones. Their body structure relies on an external support system.
Understanding Skeletons
A skeleton is a framework that provides support, shape, and protection to an organism. In the animal kingdom, there are two primary types of skeletons. An endoskeleton is an internal support structure made of bone and cartilage, which grows with the organism. This is the type of skeletal system found in mammals, birds, and fish, providing an anchor for muscles and protecting organs.
In contrast, an exoskeleton is a rigid, external covering that encases the body. Organisms with exoskeletons, such as insects and crustaceans, have their soft tissues contained within this hard outer layer. The key difference lies in their location and how they accommodate growth.
The Exoskeleton: A Fly’s Framework
Flies, like all insects, have an exoskeleton, which serves as their structural support and protective layer. This external framework is composed mainly of chitin, a tough, flexible polysaccharide, interwoven with proteins. This composition provides a strong, resilient casing that maintains the fly’s body shape.
The exoskeleton performs dual roles for the fly. It acts as a protective barrier against physical damage, environmental hazards, and water loss, important for terrestrial insects. Beyond protection, the exoskeleton also provides surfaces for muscle attachment. Muscles attach directly to the inner surface of this external shell, enabling movement of legs, wings, and other body parts.
Growth and the Exoskeleton
The rigid nature of an exoskeleton presents a challenge for growth, as it cannot expand with the organism’s size. To overcome this, flies and other insects undergo a process called molting, or ecdysis. This involves periodically shedding their old, restrictive exoskeleton and forming a new, larger one.
The molting process begins with the separation of the old exoskeleton from underlying epidermal cells, known as apolysis. A new, soft exoskeleton forms beneath the old one. The fly sheds its old outer casing, often by muscular contractions or by taking in air to expand its body. Once free, the new, larger exoskeleton expands and hardens through a process called sclerotization, involving cross-linking of proteins. Immediately after molting, the fly is soft and vulnerable until its new shell hardens.