The specification of metameric order in the insect Callosobruchus maculatus Fabr. (Coleoptera). I. Incomplete segment patterns can result from constriction-induced cytological damage to the egg.

Eggs of the pea-beetle Callosobruchus were divided into two at different stages of development. Both fragments were allowed to develop into partial larvae. The segment patterns of normal and partial larvae are described using cuticular markers of cell differentiation. To study the contribution of cytological damage to the segment gap phenomenon three different types of constriction were performed: complete and incomplete permanent constriction and complete temporary constriction. Changes in the structure of the egg can produce absence of segments resulting from two different effects. First, partial absence of segments results from a decreased egg circumference in the constriction region and involves the disturbance of a morphogenetic process (dorsal closure). Secondly, cytological damage can result in a gap between two arrays of segments. The loss of segments in the gap occurred in two different ways. In a spatial segment gap the two arrays of segments were physically discontinuous, whereas in a non-spatial gap the segments bordering the gap were juxtaposed in a physically continuous cuticle. The extent to which the gap phenomenon can be attributed to cytological damage is discussed. We also discuss, on the basis of certain dorsal defects, a possible stepwise specification of the dorsal transverse cuticular pattern.