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Electron microscopic studies of antennal sensilla in the ambrosia beetle Trypodendron lineatum (Olivier) (Scolytidae) Moeck, Henry A.

Abstract

The antennae of the ambrosia beetle Trypodendron lineatum (Olivier) were examined with the light and electron microscopes to determine the types, distribution, and structure of sense organs found thereon. At least six types of sense organs were found, with an additional seventh cuticular structure, the hypodermal gland pore, which is thought to be non-sensory. The sensilla are sensilla chaetica, three types of sensillum tri-choideum, sensilla basiconica, and sensilla campaniformia. Distribution maps of the various sensillum types and the gland pores are presented, for one each of female and male left antenna. Sensilla chaetica, evenly distributed over all parts of the antennae, as well as the rest of the body, consist of a long thick-walled hair 20 to 140 micra long which articulates in a socket composed of a hair root, socket lining, and spongy cylinder. A single bipolar neuron terminates in a scolo-pale attached at one side of the hair base. Sensilla trichoidea, Type I, situated at the base of the scape and the base of the first funicular segment, are short thin hairs articulating in a socket. Their fine structure and innervation are not known. Sensilla trichoidea, Type II, found on the distal periphery of the club only, consist of sharply pointed smooth hairs 18 to 25 micra long, the hair wall being thin and perforated. The hair is solidly joined to the body cuticle. The sensillum has two bipolar neurons, the dendrites of which extend, with slight branching, to the distal limits of the hair lumen. No dendritic endings could be demonstrated at the hair perforations. Sensilla trichoidea, Type III, are evenly distributed over the distal half of the anterior club surface. The hair is 26 to 36 micra long, blunt-tipped, and curved in reverse, with the result that the hairs protrude at right angles to the club surface and beyond all other vestiture. The hair articulates in a socket, and has a double lumen. The dendrites of four to seven bipolar neurons extend through the eccentric small lumen to the hair tip, where, presumably, they are open to the air. Sensilla basiconica cover both club surfaces. At least two types exist, one group being short pegs 6 to 8 micra long, and another group being longer pegs or hairs llj to 18 micra long. The long sensilla basiconica have a thin perforated hair wall, the openings being slit-shaped (700Å by 100 to 200Å). The two nerve cells of this sensillura send two distal processes into the hair where subsequent repeated branching occurs. The relationship of the dendrite branches to the hair perforations is not clear. Sensilla campaniformia are found in small numbers on all parts of the antennae, as well as other parts of the body. They consist of a short thin canal leading from the outside, to a sub-surface dome 3 micra in diameter, in the centre of which lies the nerve ending similar in appearance to the scolo-pale and nerve of the sensillum chaeticum. Also, a cross section of the antennal nerve in the proximal portion of the scape revealed about 2100 axons. A count of the sensilla, corrected for the number of sense cells present per sensillum, gave expected axon numbers of 1845 and 1921 for female and male antennae, respectively, with Johnston's organ not accounted for. Since more axons than expected are present, axon fusion is considered unlikely. This sudy may serve as the basis for further electrophysiological work to determine the functions of the various sense organs.

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