Simple Anomalies Disturb the Pattern of Segmentation
Anomalies in the seriality of segments are also of interest in this context. This concerns the occurrence of half-segments and in particular the spiral or helical arrangement of segmental structures (helicomery or helicomerism) (Figure l. 11) (e.g., Morgan 1895; Lesniewska et al. 2009). Half-segments and helicomery occur through regeneration after injuries or disturbances in the embryonic development during the segmentation process. Half-segments lead to a difference in the number of serial structures on the left and right sides, showing once again the differential independence of the series formation on the two sides of the body. Helicomery, however, substantially changes the seriality. Merely a break in a segment or its embryonic anlage in combination with a slight spatial shift the length of a segment can lead to helicomery. In the normal case, most structures referred to as parts of segments, for example the legs, are arranged in bilaterally symmetrical series along the longitudinal body axis. In the case of helicomery, the segment rings that normally appear one behind the other form a continuous spiral around the body (at least in part of it) (Figure 1.11). Segmental structures on this spiral, such as the appendages, no longer appear as pairs on the respective rings, but as a continuous series. The integrity of the segments is thus eliminated. Strictly speaking, the body in the helicomerous regions is not segmented (see also Fusco et al. 2008). However, the normal seriality along the
FIGURE 1.11 Anomaly: “Helicomery.” A. Schematic representation. Left: The normal arrangement of segmental structures as a series along the longitudinal body axis. The colored dots represent the paired, serially arranged segmental structures, such as extremities. Right: The segmentation is eliminated due to helicomery. Segments form a continuous spiral band. The serial segmental and the paired arrangements, as well as the bilateral symmetry, are thus eliminated and replaced by a series on a spiral or screw (spiral or helical symmetry). B. The larva of a mealworm beetle with helicomery on some segments of the abdomen (modified after Balazuc 1948, ©Publications Scientifiques du Museum national d’Histoire naturelle).
body axis is also fundamentally changed by a spiral series arrangement of the parts (Figure 1.11).