PATTERNS OF EMBRYONIC NEUROGENESIS IN A PRIMITIVE WINGLESS INSECT, THE SILVERFISH, CTENOLEPISMA-LONGICAUDATA - COMPARISON WITH THOSE SEEN IN FLYING INSECTS

Neurogenesis was examined in the central nervous system of embryos of the primitively wingless insect, the silverfish, Ctenolepisma longicau data, using staining with toluidine blue (TB) and the incorporation of bromodeoxyuridine (BUdR). The silverfish has the same number and posi tioning of neuroblasts as seen in more advanced insects and the relati ve order in which the different neuroblasts segregate from the neuroec toderm is highly conserved between Ctenolepisma and the grasshopper, S chistocerca. Of the 31 different neuroblasts found in a thoracic segme nt, one (NB 6-3) has a much longer proliferative period in silverfish. Of the remainder, 14 have similar proliferative phases, while 16 neur oblasts have extended their proliferative period by 10% of embryogenes is or greater in the grasshopper as compared with the silverfish. Both insects had similar periods of abdominal neurogenesis except that in the silverfish terminal ganglion a prominent set of neuroblasts contin ued dividing until close to hatching, possibly reflecting the importan ce of cereal sensory input in this insect. This comparison between sil verfish and grasshopper shows that the shift from wingless to flying i nsects was not accompanied by the addition of any new neuronal lineage s in the thorax. Instead, selected lineages underwent a proliferative expansion to supply the additional neurons presumably needed for fligh t. The expansion of specific thoracic lineages was accompanied by the reduction of the terminal abdominal lineages as flying insects began t o de-emphasize their cereal sensory system.