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Flash pyrolysis-gas chromatography/mass spectrometry was used to assess the chemical composition of the head louse's nit sheath. The pyrolyzate of the female insect's secretions, which form a cement-like cylinder holding the egg onto the hair, is dominated by amino acid derivatives and fatty acids. No chitin-specific compounds were detected in the sheath. These results, contrary to previous reports, show that the polymeric complex of the sheath is composed of proteinaceous moieties, possibly cross-linked to aliphatic components. This study constitutes the first chemical characterization of the pyrolysis products of insect (louse) glue and unequivocally confirms that louse sheaths are not chitinous, as suggested by earlier histochemical studies. Development of agents that might loosen nits from the hair shaft is dependent on research that addresses the chemical composition of the nit sheath.