Flagellar Annuli of Blattella germanica (Dictyoptera: Blattellidae).–Changes in Their Numbers and Dimensions during Postembryonic Development

TLDR: In the flagellum of the antennae of the German cockroach, Blattella germanica (L.), new annuli are formed lie fore each molt within the meriston and by division within the next several annuli distal to the Meriston (meristal annuli).

Abstract: In the flagellum of the antennae of the German cockroach, Blattella germanica (L.), new annuli are formed lie fore each molt within the meriston (3rd segment) and by division within the next several annuli distal to the meriston (meristal annuli). Doublets produced by division of meristal annuli do not themselves divide nor do the 17 or 18 terminal annuli (singletons) of the 1st instar. The latter are gradually lost during nymphal development. The overall result is that a 1st-instar flagellum having 24 or 25 annuli may become an adult flagellum having as many as 95 annuli. Meristal annuli range from 4 to 12. The mode changes during development, being least in the 2nd instar (6) and greatest (9) in 4th, 5th, and 6th instars. Only in the 1st instar do the annuli distal to the meriston increase gradually in length without discontinuities. In all subsequent instars there are more or less obvious points in the flagellum at which the progression in length of annulus changes. These saltation points occur between groups of annuli; i.e., between the meristal annuli and the adjacent doublet group, between adjacent doublet groups, and between the distal doublet group and the group of singletons. These discontinuities make it possible to determine the instar of flagella that have singletons. The 2 annuli comprising a doublet are conspicuously unequal in length when first formed. In subsequent instars the difference in length diminishes and may undetectable when the doublet group of the 2nd instar reaches the 6th instar. The growth in length of the flagellum as a whole like that of other hard parts of insects, increasing about 1.3 times at each molt. But within the flagellum the growth ratio ranges from about 4.0 in the meriston 1.0 at the apex, declining continuously from base apex, not only between groups of annuli but within groups. The observations summarized above apply to both sexes.