The development of the nervous system and other ectodermal derivatives in tenebrio molitor L. (insecta, coleoptera)

Abstract

Previous work has shown Tenebrio molitor to be a relatively unspecialized beetle, and therefore more suitable for head segmentation studies than those hitherto investigated. Such studies must, however, be preceded by a description of the nervous system and other segmental ectodermal structures, and this forms the subject of the present paper. The development of the nervous system is typical of insects. The ventral nerve cord develops from neuroblasts situated in a pair of lateral cords and in a median cell strand, which develop from the intercalary to the tenth abdominal segment. Teloblasts bud off preganglionic cells which, by division, give rise to the ganglion cells. The median strand contributes to the transverse commissures and to the dorsal parts of the ganglia. The definitive cord comprises the suboesophageal ganglion (fused mandibular, maxillary and labial ganglia), three thoracic and nine abdominal pairs of ganglia. The brain is the product of fusion of the proto-, deuto- and trito-cerebral gangliomeres, which develop in series with the ventral ganglia. The stomodaeal nervous system arises from three invaginations in the roof of the stomodaeum, and consists of the frontal and ventricular ganglia; and the recurrent nerve. The neurilemma forms from flattened peripheral ganglion cells. Pairs of ectodermal invaginations develop in the antennary, gnathal meso- and metathoracic, and the first eight abdominal segments. Those of the antennary and maxillary segments form the tentorium ; the mandibular, the flexor apodeme of that segment and the labial diverticula arise from the posterior gnathal segment. The small apodeme of the m andibular extensor muscle is considered to have been derived from the transient intercalary segment. Post-gnathal invaginations give rise to trachea. Groups of oenocytes arise from the ectoderm above the spiracles. They are situated laterally in the haemocoel of the first eight abdominal segments. The segmental affinities of the brain, stomatogastric system and cephalic invaginations are discussed. The conclusions indicate a six segmented head in Tenebrio .