Structure of the Cell Covering

Abstract

Scales from all species consist of a base plate and a posterior upturned rim (Figs. 2.1–2.2). The base plate is most often perforated with minute pores, referred to as base plate pores, although in a few species the pores are inconspicuous and perhaps lacking (e.g. Sectio Papillosae). Base plate pores may be spaced evenly over the scale, lie concentrated in certain regions (e.g. the posterior of the shield) or be lacking altogether from specific areas (e.g. flanges). For most species the dome is devoid of such pores. The posterior margin of the scale is upturned and bent (recurved) back over the scale forming a rim (Figs. 2.1–2.2). Posterior rims may be narrow or quite broad and usually encircle about one-half of the perimeter of the scale. The posterior rim usually extends an equal distance around both sides of the scale, however, for scales of some species (e.g. M. retrorsa) and often for scales positioned close to the ends of the cell, the rim may extend further along one margin (usually the right margin) resulting in an asymmetric design. The degree to which the rim overlaps the scale is herein referred to as the canopy of the rim (Fig. 2.2). The end of the scale with the rim is referred to as the proximal end, whereas the front end is designated as the distal end.