Insects, like all organisms, must store energy to survive periods of scarcity or fuel high-demand activities. While the colloquial question asks if they can “get fat,” insects do not use the same anatomical structures as mammals. Instead of subcutaneous adipose tissue, insects utilize a unique, multifunctional organ that acts as the central metabolic and storage hub for their entire body.
The Primary Energy Molecules
The scientific answer to what insects store involves two major classes of molecules: lipids (fats) and carbohydrates (sugars). Lipids, primarily stored as triglycerides, are the most energy-dense form of reserve and represent the long-term fuel supply for the insect. Because lipids are stored in an anhydrous, or water-free, form, they offer a higher caloric content per unit of weight compared to carbohydrates, making them the preferred fuel for extended activities like diapause or migration.
Carbohydrates are stored as glycogen, a polymer of glucose, which functions as a readily accessible, short-term energy source. Glycogen is rapidly broken down and converted into trehalose, the main circulating sugar in the insect’s blood, or hemolymph, for quick transport to muscles. This fast-acting fuel is used for immediate, high-power bursts of activity, such as initiating flight or escaping a predator. The ratio between these two reserves changes depending on the insect’s stage of development and its immediate environmental demands.
The Fat Body: Insect’s Central Storage Organ
The organ responsible for storing these energy molecules is called the fat body, a diffuse tissue that plays roles analogous to a combination of the vertebrate liver, adipose tissue, and even immune system. Unlike the clearly defined organs of mammals, the insect fat body exists as sheets or ribbons of cells suspended in the hemocoel, the insect’s body cavity. This dispersed structure facilitates a rapid exchange of metabolites because the cells are in immediate contact with the hemolymph, the insect’s circulating fluid.
The cells within the fat body, known as trophocytes, are specialized for the synthesis, accumulation, and release of energy molecules, including triglycerides and glycogen. It is within these trophocytes that lipids are held in large, visible cytoplasmic lipid droplets. Beyond storage, the fat body also synthesizes most of the proteins that circulate in the hemolymph and plays a central role in detoxification and the production of immune proteins.
Energy Accumulation and Life Stages
Insects accumulate these energy reserves during periods of abundant food to prepare for times when feeding is impossible or limited. The most significant period of energy accumulation occurs during the larval or nymphal stages, which are dedicated feeding phases. During this time, dietary carbohydrates, fatty acids, and proteins are converted into lipids and glycogen for future use.
A major reason for this massive storage is the preparation for metamorphosis in insects that undergo complete transformation, like butterflies and beetles. During pupation, the insect does not feed and must rely entirely on the reserves stored in the fat body to fuel the complete rebuilding of its body into the adult form. Similarly, insects preparing for diapause, a dormant state used to survive harsh winters or dry seasons, accumulate greater lipid reserves than their non-diapausing counterparts. These stored nutrients must be sufficient for the survival of the dormant period and the energy-intensive activities that follow.
Mobilizing Reserves for High-Demand Tasks
The stored energy must be quickly and efficiently mobilized to meet the needs of high-demand tasks like sustained flight or reproduction. The process of releasing these reserves is controlled by hormones, most notably the adipokinetic hormone (AKH) produced by neurosecretory cells. AKH acts on the fat body, signaling the release of stored molecules into the hemolymph for transport to the flight muscles or reproductive organs.
For long-distance flight, insects often switch their fuel source as the journey progresses. They may start by rapidly consuming their glycogen reserves for the initial burst of power, but they quickly transition to utilizing lipids for sustained endurance. The fat body releases the stored triglycerides as diacylglycerides, which are carried by specialized lipoprotein molecules through the hemolymph to the thoracic flight muscles. A trade-off often exists in females between the energy needed for flight and for reproduction, known as the oogenesis-flight syndrome. In migratory females, abdominal lipids are prioritized for fueling flight, often delaying ovarian development until the migration is complete.