Flies are common insects belonging to the order Diptera, a diverse group within the phylum Arthropoda. Like all arthropods, their physical structure is defined by an external skeleton that provides support and protection. Understanding what flies are made of requires looking at the specialized materials that form this outer shell and the unique biological systems operating within it. The composition extends down to the fundamental molecules that drive all life processes.
The Protective Outer Layer
The fly’s exterior is defined by a tough, segmented exoskeleton, which is a complex, non-living cuticle. This cuticle is primarily constructed from chitin, a polysaccharide and one of the most abundant biopolymers on Earth. Chitin provides the fly’s body with tensile strength and flexibility, acting as the main structural component.
Chitin is organized into a layered structure, secreted by the underlying epidermal cells. The outermost layer, the epicuticle, is waxy and lacks chitin, serving as a water-resistant barrier against desiccation. The deeper procuticle, rich in chitin and proteins, gives the fly its hard form.
The rigidity of the exoskeleton is achieved through a process called sclerotization, often called “tanning.” This process involves the cross-linking of structural proteins within the cuticle using phenolic compounds. Sclerotization stiffens the shell, providing the necessary mechanical support for the fly’s body and attachment points for muscles, particularly in the head and thorax. The degree of sclerotization varies, allowing for movement in the softer parts connecting the head, thorax, and abdomen.
Internal Biological Systems
Inside the hard shell is a complex set of organs and tissues that manage all of the fly’s biological functions. The fly’s circulatory system is open, meaning its fluid does not remain confined to vessels but instead bathes the internal organs directly. This fluid is called hemolymph, the insect analog of blood, which circulates nutrients, hormones, and waste products.
Unlike the blood in vertebrates, hemolymph does not play a significant role in oxygen transport. Respiration is handled by a highly efficient system of air-filled tubes called tracheae. These tubes open to the outside through small pores called spiracles, delivering oxygen directly to metabolically active tissues, such as the flight muscles.
The nervous system consists of a brain and a ventral nerve cord, with clusters of nerve cells called ganglia located in each major body segment. The digestive system is a tube extending through the body, divided into the foregut, midgut, and hindgut. This tract processes food and absorbs nutrients that are then distributed by the hemolymph.
The Basic Molecular Ingredients
At the fundamental level, the fly is composed of the same building blocks that make up all living organisms. A significant percentage of the fly’s mass is water, which is necessary for all cellular functions. The remaining dry mass consists of large biological macromolecules.
Proteins are highly abundant, forming the enzymes that drive chemical reactions, the structural components of cells, and muscle tissue fibers. These proteins are polymers made from chains of amino acid monomers. Lipids include fats, which serve as energy storage, and phospholipids, which form the essential bilayer structure of cell membranes.
The final group of large molecules is the nucleic acids, primarily Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA). These molecules are polymers of nucleotides and carry the genetic code required for the fly’s development, function, and reproduction.