Half of all insects eat plants, and unique sterols have been recorded from different plant species.Additionally, most individual plant species contain multiple sterols; in some cases, more than unique sterols have been identified. It has been suggested that the physicochemical properties of the proteins involved in sterol synthesis may determine the stereospecific Compound Libraries formation of sterols are a type of steroid similar to sterols.They are also found in plants, but usually only in small amounts. Most sterols have the general stereostructure needed for function in cellular membrane, but variation in the type, amount, and ratio of different dietary sterols has speciesspecific effects on the rate and total growth of insects. Insects generally acquire sterols from two main sources: parental loading during oogenesis and food.The sterol content in embryostypically more than half being conjugated to fatty acids such as palmitate, oleate, or stearate derives mostly from maternal loading of sterols.This is particularly true for parthenogenic insects like aphids. Ultimately, insufficient cholesterol ingestion during larval stages can affect oogenesis and lead to reduced fecundity. This suggests that there may be a sterol threshold for oogenesis.As immature insects grow and develop, dietary sterols are mostly allocated toward cellular membranes, although there is tissuespecific distribution of different sterols, and development is severely impaired when sterol supply is interrupted. Most insects can use cholesterol directly; examples of exceptions include one dipteran. However, insect herbivores, unlike carnivorous insects, rarely encounter sufficient amounts of dietary cholesterol.Some insects can dealkylate but are not particularly efficient.The flexibility to use sterols other than cholesterol as membrane inserts can be highly beneficial for insects, especially for those that use ecdysone as their hormone.However, the ability of insects to use a mixture of sterols in their membranes varies from species to species. However, much less is known about the genetic basis of sterol metabolism, especially dealkylation.Interestingly, lepidopteran insects can metabolize ketosteroids into two diastereomers, and cholestanol, potentially by reductase and reductase, respectively.These two enzymes were previously described in the metabolism of ketoecdysone.The multiple functions of sterolmetabolizing enzymes hint at the complexity of the sterol metabolism network in insects. Auchenorrhynchan insects can survive on sterolpoor xylem, and cholesterol is the major sterol in these insects. Additionally, it appears that a shared metabolic pathwaybetween the host and the fungal symbiontdirects metabolism of zymosterol to cholesterol.However, verifying this is challenging, as is often the case for research using omicsbased approaches and technologies. For example, in vertebrates, sterol regulatory element binding proteins and subsequently coordinate cholesterol synthesis and cholesterol uptake machinery.Nonetheless, all eukaryotes have mechanisms in place that regulate the flow of sterols into and out of cell and organelle membranes, as well as facilitating their intracellular transportation once inside a cell.Sterols can also be found in other aqueous milieu in eukaryotes, including the gut lumen and blood.In the insect gut lumen, free sterols combine with other free lipids to form soluble micelles that can be absorbed across the peritrophic membrane and into the lipid bilayer of enterocytes.In the aqueous hemolymph, free sterols are solubilized by specialized transport particles that shuttle hydrophobic sterols between different organs and tissues.