Relationship between tissue-specific hydrocarbon profiles and lipid melting temperatures in the cockroach Blattella germanica

Hydrocarbons (HC) are the most important waterproofing barrier on the cuticle of most terrestrial insects. Yet, the relationships among the type, amount. biophysical properties, and water retardation capacity of constituent HC are poorly understood. Melting temperatures and gas chromatographic profiles of HC of German cockroach tissues of various ages and stages were compared. The melting temperature (T-m) of oothecal HC was highest, T-m of epicuticular HC was substantially lower, and that of hemolymph HC was lowest. The epicuticular HC of older nymphs and adults had higher T-m than HC of the same sex and stage soon after the molt. The HC of females had higher T-m than did male HC. Principal components analysis suggested that normal and 3- and 5-methylalkanes, which were more prevalent on the epicuticle, were associated with higher T-m, implicating these components of the HC blend in waterproofing roles. The cockroach ootheca is particularly well protected by an abundance of n-alkanes and its external HC exhibit the highest T-m of any HC blend. The methyl ketone sex pheromone components, which are derived from HC, appear to only slightly reduce the T-m of the epicuticular HC, probably because the methyl ketones represent only 1.12% of the mass of epicuticular HC. We suggest that the evolution of polar epicuticular chemical signals may be constrained by their tendency to increase water transpiration.