Abstract
The present ultrastructural investigation using scanning and transmission electron microscopy as well as light and fluorescence microscopy describes in detail the attachment devices and tarsal gland of the bug Coreus marginatus (L.) (Hemiptera: Coreidae). In particular, the fine structure of pulvilli reveals a ventral surface rich with pore channels, consistent with fluid emission, and a folded dorsal surface, which could be useful to enhance the pulvillus contact area during attachment to the substrate. The detailed description of the tarsal gland cells, whose structure is coherent with an active secretory function, allows us to consider the tarsal gland as the plausible candidate for the adhesive fluid production. Scolopidia strictly adhering to the gland cells are also described. On the basis of the fine structure of the tarsal gland, we hypothesise a fluid emission mechanism based on changes of the hydraulic pressure inside the gland, due to the unguitractor tendon movements. This mechanism could provide the fluid release based on compression of the pad and capillary suction, as demonstrated in other insects. The data here reported can contribute to understanding of insect adhesive fluid production, emission and control of its transport.
Highlights
Insects successfully survive in almost all ecosystems due in part to their ability to safely walk and attach to many different types of surfaces owing to the presence of claws together with smooth or hairy attachment pads on the distal portion of their legs (Beutel and Gorb 2001)
The attachment devices are represented by an unpaired hairy pad and by paired pretarsal claws and pulvilli
The adhesive setae of the hairy pad are socketed and have a blunt tip when observed with cryo-Scanning electron microscopy (SEM) (Fig. 1b), while they are curled when air-dried and observed with SEM (Fig. 1c)
Summary
Insects successfully survive in almost all ecosystems due in part to their ability to safely walk and attach to many different types of surfaces owing to the presence of claws together with smooth or hairy attachment pads on the distal portion of their legs (Beutel and Gorb 2001). Notwithstanding a great variety of attachment devices, all insects count on a wet adhesion: in all insect groups studied so far, fluid mediates adhesion to the substrate (see review in Dirks and Federle 2011a) In both smooth and hairy attachment devices, a nanometer thin layer of fluid plays an important role in the adhesion based on both capillarity (surface tension) and viscosity principles (Edwards and Tarkanian 1970; Dixon et al 1990; Vötsch et al 2002; Dirks and Federle 2011a, b). In spite of the numerous studies performed so far on insect adhesion, the chemical composition of the adhesive fluid in different insect orders, its exact origin, the secretion mechanism and how the secretion is controlled are still not completely elucidated (Dirks and Federle 2011a, b; Schmitt and Betz 2017)
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