Numerous studies have focussed on the mechanisms of entry of pesticides into insect body parts such as oral intake, penetration through the integument of the body wall, and inhalation through spiracles. However, little is known about how insecticides spread to the points of entry or the paths on the body surface that are used to reach the target sites. In this study, elemental signals of pesticide-mimicking test solutions were tracked and their routes of spreading in experimental insects (Blattella germanica L.) were investigated using NanoSuit (a method of surface modification) and energy dispersive X-ray spectroscopy, combined with high-resolution scanning electron microscopy. When the test solution initially adhered to the dorsal and/or ventral body surface, it tended to spread horizontally to reach lateral plates. Whereas, when the solution directly adhered to the anterior side of the lateral plates, it spread to posterior segments. In this case, however, spreading in the opposite direction (i.e., the solution directly adhered to the posterior side of the lateral plates) was interrupted at a boundary erected by different groups of fine structures; each protrusion was large, and the arrangement was rather dense in the posterior segments. Morphological features of these fine structures and chemical characteristics of the hydrophobic surface substances potentially regulate the strength of the capillary force, which determines pesticide spreading.
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