Morphology and Distribution of Cutaneous Sensory Organs on the Digits of Anolis carolinensis and A. sagrei (Squamata: Dactyloidae) in Relation to the Adhesive Toepads and Their Deployment

Abstract

Cutaneous sensory organs are characteristic of many squamate lineages. Such organs may occur on the surface of scales as button-like, circular protuberances set off from their surroundings by a noticeable boundary, often taking the form of a moat or furrow. They may be relatively unadorned, clad with the surface micro-ornamentation of the scales on which they are carried, or they may carry one or more bristles of varying length and surface ornamentation. Such bristles may extend away from the body of the organ to interface with the surrounding environment or to contact adjacent scales. Cutaneous sensory organs have been physiologically demonstrated to have a mechanoreceptive function but have also been posited to potentially be involved with additional sensory modalities. Their distribution and structure across the body surface has been shown to be unequal, with some regions being much more extensively endowed than others, indicative of regional differential sensitivity. The digits of Anolis (Iguania: Dactyloidae) carry adhesive toepads that are convergent with those of geckos (Gekkota). Geckos exhibit a high density of cutaneous sensory organs on their toepads and their form and distribution has been associated with the operation and control of the toepads during locomotion. Investigation of the form and topographical distribution of cutaneous sensory organs on the toepads of Anolis shows them to be convergent in these attributes with those of geckos and quite distinct from those of the ancestrally padless Iguana (Iguania: Iguanidae). Their location at scale margins and the direction of their bristles towards adjacent scales indicates that the cutaneous sensory organs play an important role in proprioception during toepad deployment in Anolis.