The feeding mechanism of the butterfly Pieris Brassicae L

Abstract

The paper gives a detailed anatomical and functional analysis of the butterfly’s proboscis. The proboscis consists of two maxillary galeae joined ventrally by exocuticular toothed hooks and dorsally by overlapping plates. Between them they form the food tube. The whole is a complex structure in which (i) flexible endocuticle and more rigid exocuticle, (ii) muscles, (iii) an internal skeleton of longitudinal septa, and (iv) haemocoelic turgidity play their part in the process of extension. Extension of the proboscis is restricted to the sagittal plane. When extended there is always a point of ventral bending at about one-third of the proboscis length. The angle of this bend is variable and the butterfly is enabled thereby to explore different food sources without movement of the body. The transverse section of a coiled proboscis shows a flat dorsal surface which becomes convex in extension. Contraction of the intrinsic primary oblique muscles of each galea pulls the dorsal and ventral walls towards each other. This sets up an internal pressure in the haemolymph which is maintained by a stipital valve. Because of the preponderance of flexible endocuticle at the junction of the outer walls of the galeae with their inner walls it also causes a transverse dorsal convexity to be imparted to the proboscis. The inner wall of the galea bounding the food tube consists of transversely placed exocuticular laminae so disposed in a lattice arrangement as to make possible easy movement of the proboscis in the sagittal plane as in coiling and extension, but to prevent lateral movement. Coiling of the proboscis is due to the elasticity of an exocuticular dorsal longitudinal bar which lies in each galea in the upper wall of the food tube. Along the proboscis in the dorsal haemolymph there are about one hundred and fifty pairs of large unicellular glands whose secretions are poured into the region of the dorsal linkage. They serve to lubricate the dorsal plates as they slide over each other in coiling and extension, and possibly to seal up the dorsal part of the food tube when the proboscis is extended. As a result of examining the proboscis structure and its movements, and from numerous operations involving nerve sections and perforations of the haemocoele, it is concluded that all existing theories on proboscis extension must be rejected. Extension is not caused by inflation, nor by direct muscle action, nor by elasticity of the proboscis wall. Primary intrinsic muscles, under conditions of haemocoelic turgidity maintained by a stipital valve, have effect in producing a transverse dorsal convexity of the proboscis. Extension is a consequence of this dorsal convexity in an elastic system and only indirectly of the contraction of intrinsic muscles. Assisting in the production of this changed shape of the proboscis as seen in transverse section, are (i) the longitudinal internal septa which control the degree of movement of the proboscis walls and (ii) the strong wall of the food tube which acts as a fulcrum on which the movements of the outer galea wall are made. At the point of ventral bending in the extended proboscis there is a secondary set of intrinsic oblique muscles in each galea which work antagonistically to the primaries and so prevent full extension here. The pharyngeal sucking pump is described.