Studies on the cuticle of the human body louse, Pediculus humanus humanus.

The cuticular ultrastructure and surface topography of the human body louse, Pediculus humanus humanus, were examined. Under light microscopy, pore canals were readily observed in unstained, cryostat sections of cuticle from various locations on the insect, particularly those areas composed of smooth, sclerotised cuticle. Transmission electron microscopy (TEM) studies revealed that, after attachment at the epidermis, the canals appeared to pass unbranched through the endocuticle (average thickness 4.5ji) with diameters around IOOOÁ. Branching occurred in the exocuticle (thickness l.Ofi) and finally in the epicuticle (thickness 2000A) to form wax canals with diameters around 100A. Pores or openings with diameters around lOOA were found during scanning electron microscopic (SEM) studies of the epicuticular surface and corresponded with the wax canals noted in the TEM examinations. A chemical analysis of the cuticular lipids of the louse was conducted using thin-layer chromatography (TLC), gas-liquid chromatography (GLC) and gaschromatography/mass spectroscopy (GC/MS) and the hydrocarbons were found to be the most abundant class (almost 75 per cent). Cholesteryl esters, triglycerides, 1,3-diglycerides, cholesterol and fatty acids were also present in that order of abundance. Cuticular hydrocarbons were comprised of over 43 components which contained from 21 to 35 total carbons per molecule. Four homologous series were present and consisted of n-alkanes (51.8 per cent of total cuticular hydrocarbons), n-alkenes (26.4 per cent), internally branched monomethylalkanes (12.1 per cent) and a tentatively identified series comprising dimethylalkanes (4.7 per cent). The effect of insecticide solvents upon the cuticular wax layer was investigated in vitro by comparing the solubility of a simulated cuticular wax mixture in a series of ten potential solvents. Results are discussed along with those from toxicity tests performed with the solvents on adult female lice and indicate a relationship between cuticular wax solubility and louse mortality. Studies with a fluorescent dye, demonstrated that wax-dissolving solvents entered the insect via at least two routes, i.e. cuticle and spiracles. The results are compared with the published work on other insect cuticular systems and the implications involving potential insecticide carriers discussed.