Revisiting the classification of squamate adhesive setae: historical, morphological and functional perspectives

Abstract

Research on gecko-based adhesion has become a truly interdisciplinary endeavour, encompassing many disciplines within the natural and physical sciences. Gecko adhesion occurs by the induction of van der Waals intermolecular (and possibly other) forces between substrata and integumentary filaments (setae) terminating in at least one spatulate tip. Gecko setae have increasingly been idealized as structures with uniform dimensions and a particular branching pattern. Approaches to developing synthetic simulacra have largely adopted such an idealized form as a foundational template. Observations of entire setal fields of geckos and anoles have, however, revealed extensive, predictable variation in setal form. Some filaments of these fields do not fulfil the morphological criteria that characterize setae and, problematically, recent authors have applied the term 'seta' to structurally simpler and likely non-adhesively competent fibrils. Herein we briefly review the history of the definition of squamate setae and propose a standardized classificatory scheme for epidermal outgrowths based on a combination of whole animal performance and morphology. Our review is by no means comprehensive of the literature regarding the form, function, and development of the adhesive setae of squamates and we do not address significant advances that have been made in many areas (e.g. cell biology of setae) that are largely tangential to their classification and identification. We contend that those who aspire to simulate the form and function of squamate setae will benefit from a fuller appreciation of the diversity of these structures, thereby assisting in the identification of features most relevant to their objectives.

Keywords: Dactyloidae; Gekkota; bio-inspired adhesion; epidermal outgrowths; microstructures; spinules.

Figure 1.

Subdigital epidermal fibrillar outgrowths of the Tokay gecko (Gekko gecko). (a) Expanded scales (scansors). (b) Elaborated, branched outgrowths (setae) (c) Triangular-shaped tips (spatulae) (d) Another form of subdigital epidermal outgrowth—branched prongs.

Figure 2.

(a) ‘Essential setal morphology’ values reported by Peattie [70] plotted against data collected from studies examining setal morphometrics along the proximodistal axis of the digits of both Chondrodactylus bibronii [22] and Gekko gecko [20,32]. The schematic of a gecko digit at the left depicts the pad regions selected for measurements of setal morphometrics. prox, proximal; int, intermediate; dist, distal. (b) Schematic of the variability in the overall form of squamate epidermal outgrowths and corresponding definitions by our classification scheme. Not drawn to scale.