Interaction of liquid epicuticular hydrocarbons and tarsal adhesive secretion in Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae)

Abstract

Species of various insect orders possess specialised tarsal adhesive structures covered by a thin liquid film, which is deposited in the form of footprints. This adhesive liquid has been suggested to be chemically and physiologically related to the epicuticular lipid layer, which naturally covers the body of insects and acts as the prime barrier to environmental stresses, such as desiccation. The functional efficiency of the layer, however, is jeopardised by partial melting that may occur at physiological temperatures. In this study, light microscopic images of elytral prints show that the epicuticular lipid layer of the Colorado potato beetle Leptinotarsa decemlineata actually is partially liquid and chemical investigations reveal the high similarity of the epicuticular hydrocarbon pattern and the tarsal liquid. By means of chemical manipulation of the surface hydrocarbon composition of live beetles, the substance exchange between their tarsal adhesive hairs and the body surface is monitored. Histological sections of L. decemlineata tarsi, furthermore, reveal glandular cells connected to individual adhesive setae and departing from these results, an idea of a general mechanism of tarsal secretion is developed and discussed in a functional-ecological context.

How do cuticular hydrocarbons evolve? Physiological constraints and climatic and biotic selection pressures act on a complex functional trait

“…On a physiological level, the CHC layer experiences two basic selection pressures: firstly, its melting point must be high enough to waterproof the insect cuticle at ambient temperatures. Secondly, it must be fluid enough so that substances relevant for nest-mate recognition and/or waterproofing can spread over the cuticle [3,35] and be exchanged among nest-mates.…”

The influence of surface energy on the self-cleaning of insect adhesive devices

“…To enable adhesion, insect feet are inherently 'tacky', employing an adhesive fluid secretion to maximise contact with a substrate (Gorb, 2005;Pohl and Beutel, 2004). The exact composition of the secretion is still unknown, although there is a general acknowledgement that it is an emulsion of lipophilic and hydrophilic components (Votsch et al, 2002), but no hydrophilic liquid has been identified (Geiselhardt et al, 2010). Indeed, Dirks and colleagues state that hydrophobic fluids stick well to both hydrophobic and hydrophilic surfaces (Dirks et al, 2009), and the inclusion of a hydrophilic component to the adhesive secretion used by insects has unclear benefits.…”

Congruence of epicuticular hydrocarbons and tarsal secretions as a principle in beetles

“…A chemical congruence of tarsal and cuticular lipids, therefore, would be expected in these beetle species. In contrast, the species included in the present study are equipped with specialised attachment structures and at least some possess glands, which seem to be involved in the secretion of the adhesive liquid (SF Geiselhardt et al 2010). Hence, the locally restricted production of a specialised formula with optimised adhesive properties would be physiologically possible.…”

“…In the present study, we have chosen a representative selection of mostly plant-inhabiting beetles, in which all possess widened tarsal segments and adhesive hairs (Stork 1980;own observations), presumably connected to specialised glandular cells (Betz 2003;SF Geiselhardt et al 2010). Our focus is on the Chrysomeloidea and Curculionoidea (29 species), where broadened tarsi and adhesive devices seem to be a ubiquitous character that may have evolved in adaptation to life on smooth plant surfaces.…”

“…Yet, in no case has the hydrophilic liquid been successfully characterised. For most beetles, the liquid recovered from footprints had been postulated to comprise the same set of lipophilic substances as the cuticular lipids (Attygalle et al 2000;Ishii 1987;Kosaki and Yamaoka 1996), but only recently has the chemical congruence been evidenced by means of direct sampling [solid phase microextraction (SPME)] of tarsi and elytra of the Colorado potato beetle, Leptinotarsa decemlineata, and the green dock leaf beetle, Gastrophysa viridula (SF Geiselhardt et al , 2010. In L. decemlineata it also has been evidenced that not only the tarsal lipids which are secreted by specialised tarsal glands, but also the cuticular lipids are at least partially liquid, and that both pools are involved in constant substance exchange (SF Geiselhardt et al 2010).…”