To protect themselves, insects cover their bodies with what is called cuticular lipid. The cuticular lipid of an American cockroach has a unique lipid content; the most abundant is a cis-alkadiene, cis,cis-6,9-heptacosadiene, amounting to about 70%, which is followed by a branched alkane 3-methylpentacosane. In order to clarify the structural features o f the unique lipid composition below the critical temperature, the cuticular lipid was studied by Fourier transform infrared (FTIR ) spectroscopy in combination with an attenuated total reflection (ATR ) sampling technique. The infrared spectra measured on an extracted lipid sample at 20 degrees C suggested that the lipid keeps an appreciable level o f conformational and lateral packing regularity, in spite o f a high cis-unsaturated lipid content, and also a highly disordered condition around the methyl terminals and cis-olefin groups. The CH2 scissoring and the CH2 rocking regions showed the characteristics of the O perpendicular to subcell. The same characteristics were observed also by in situ measurements on a forewing of the American cockroach. Combining the spectral features o f these bands and the physicochemical properties of each component, it can be inferred that saturated lipids form highly ordered domains within the liquid containing the cis,cis-diene as the main component. For comparison, the cuticular lipid of a male cricket, which consisted of many different hydrocarbons, including 15% of unsaturated hydrocarbons, showed a lower regularity both in the conformation and in the lateral packing of hydrocarbon chains. These results imply that not only the degree of ds-unsaturation but also the chemical structure diversity of hydrocarbons are the important factors to determine the physicochemical properties of cuticular lipid.