Polarized microscopy study of an antennal sensillum of Triatoma infestans: an ordered distribution of chitin fibrils and associated components.

Aspects of the molecular organization of the antennal sensilla trichodea of Triatoma infestans have been investigated with the use of both polarization and scanning electron microscopy. The sensilla have a smooth surface with minute bulbs on the tapered end. They showed strong positive birefringence, irrespective of the refractive index of the imbibing medium. The plotting of a form birefringence (FB) curve for native material showed slight increases in the retardation values with increases of the refractive index and at least 2 inflection points. On the other hand, a FB curve constructed for alkali-treated structures reveals higher values for the form birefringence and slight decreases in retardation values with raising refractive index. These results demonstrate that chitin fibrils are preferentially aligned with the sensilla long axis. Interestingly, the alkali treatment introduced no alterations in the retardation values measured at n = 1,435, which corresponds to the intrinsic birefringence of chitin. It is suggested that components removed by alkali-treatment have electronic transitions disposed perpendicularly to the chitin filaments and that the ill definition of the form birefringence curve of chitin is associated with the incomplete removal of chitin-associated components, which keep chitin fibrils apart from but strongly bound to each other. In addition, it is apparent from the results that cross-linking in a system with a parallel array of the chitin fibrils occurs predominantly perpendicular to the chitin fibrils.