The dietary requirement for essential fatty acid(s) is small in some species, suggesting a specific function such as hormone precursor. In this case the situation could be comparable to the vertebrate requirement of essential fatty acids as prostaglandin precursors. Dietary linoleic acid has a sparing effect on the amount of linolenic acid required, and the standard artificial diet used for rearing this species contains about equal amounts of these unsaturated fatty acids. Larvae reared on this diet, in fact, accumulate more linoleic than linolenic acid into their tissue lipids. When P. brassicae larvae are reared on the leaves of Brassica species, the essential fatty acid content of larval tissues parallels that of the diets, exceeding 40% of fatty acids in the case of linolenic acid. The utilization of dietary neutral lipids is illustrated by an examination of triglyceride hydrolysis and subsequent tissue uptake of the liberated fatty acids.