Evolution of proboscis musculature in Lepidoptera

Abstract

The suctorial proboscis of adult Lepidoptera represents a key morphological innovation that enabled these insects to gain access to new food sources. In the ancestral condition of the lepidopteran proboscis only extrinsic galeal muscles are present in the basal joint region. The presence of additional muscles (i.e., the intrinsic galeal muscles) is regarded as a morphological novelty of the Myoglossata that evolved after the galeae were modified to form suctorial mouthparts. The present comparative investigation of the galeal anatomy in representatives of all major taxa revealed that the intrinsic galeal muscles are derived from the basal galeal musculature. In the examined Neopseustoidea, Exoporia, Nepticuloidea, Incurvarioidea, and Tischerioidea all galeal muscles have their origin in the stipes-galea joint and/or in the proximal region of the galea. Two muscle units form the basal galeal musculature of the joint region and one to three longitudinal muscles extend into the galea lumen. Multiple intrinsic galeal muscles, of which both the origin and attachment sites are markedly distal from the basal joint region are regarded as a groundplan autapomorphy of the Ditrysia. Some slightly oblique muscles may occur along the lateral wall; these were lost in species with extremely slender galeae. In most investigated Obtectomera two series of intrinsic galeal muscles occur; these are the (1) oblique lateral intrinsic galeal muscles, which are arranged one upon the other along the lateral proboscis wall and (2) the median intrinsic galeal muscles, which run more or less longitudinally along the ventral wall. Oblique muscle arrangement probably evolved in concert with the functional demands of a long lepidopteran proboscis. A likely evolutionary pathway to account for the serial arrangement of galeal muscles is proposed.