Ontogeny of the sensilla placodea on the antennae of Aulacus striatus jurine (Hymenoptera : Aulacidae)

Abstract

The pore plate sensilla of pupae and imagines of Aulacus striatus (Hymenoptera : Aulacidae) were studied by scanning and transmission electron microscopy. Our results confirm the assumption that these sensilla are derivatives of hair sensilla, probably basiconic sensilla of the Cephus (Hymenoptera : Symphyta) type. Each sensillum is innervated by about 10–20 sense cells and ensheathed by 5 enveloping cells. The innermost envelope cell is the dendritic sheath cell. In mature pupae and imagines, the dendritic sheath is reduced to a collar-like ring encircling the outer dendritic segments. In the young pupa, 2 long processes of the envelope cells 2a and 2b are in contact immediately with the epicuticle of the pore plate “Anlage”. They form the pores and the pore tubules and therefore are interpreted as trichogen cells. So far, this doubling of trichogen cells in insect sensilla has been observed only in pore plates and basiconic sensilla of Hymenoptera, and it is reported here for the first time. The rod-like process of envelope cell 2a is embedded in the half-tube like envelope cell 2b. Around this inner half cylinder envelope cell 3 forms a middle, and envelope cell 4 forms an outer half-tube. In the mature pupa, the processes of envelope cells 2a, 2b, and 3 withdraw almost completely from the pore plate canal, and thus build up the outer receptor lymph cavity that fuses with the inner receptor lymph cavity. Only envelope cell 4 remains in the pore plate canal. In the imago, the outer dendritic segments branch and extend through a proximal opening into the pore plate canal.