Antennal morphology and sensilla ultrastructure of the web-spinning sawfly Acantholyda posticalis Matsumura (Hymenoptera: Pamphiliidae)

Acantholyda posticalis (Hymenoptera: Pamphiliidae) is an important pine pest with a world-wide distribution. To clarify the olfactory receptive mechanism of A. posticalis, scanning electron microscopy and transmission electron microscopy were used to examine the morphology, ultrastructure, and distribution of antennal sensilla of adults from two sites in China. The antennae were filiform, and the flagella comprised 32–35 flagellomeres. Six sensillum types were found. Sensilla chaetica were straight setae with sharply pointed tips and without dendrites in the lumen. Sensilla trichodea were characterized by a parallel-grooved wall and one terminal pore and were innervated by four dendrites at the base. Sensilla basiconica I possessed longitudinally grooved surfaces and multiple terminal pores, with five dendrites in the lumen. Sensilla basiconica II not only had a distinct terminal pore but also had numerous tiny wall pores and many dendritic branches within the sensillum lymph. Sensilla coeloconica had deep longitudinal grooves, one terminal pore and six dendrites, while sensilla campaniformia were thick-walled with a terminal opening and sensory nerve bundles in the lumen. Sensilla chaetica and s. trichodea were most abundant and distributed over the entire antennae, while s. basiconica I and II, s. coeloconica, and s. campaniformia were restricted to the ventral flagellar surfaces. Although the shape and structure of antennae were similar in males and females, females had significantly longer antennae than males, and males had significantly more s. basiconica I than females. We compared the morphology and structure of these sensilla to other Hymenoptera and discussed their possible functions.