Morphology of the prothoracic discs and associated sensilla of Acanthocnemus nigricans (Coleoptera, Acanthocnemidae)

Abstract

The Australian ‘little ash beetle’ Acanthocnemus nigricans (Coleoptera, Cleroidea, Acanthocnemidae) is attracted by forest fires. A. nigricans has one pair of unique prothoracic sensory organs and it has been speculated that these organs may play a role in fire detection. Each organ consists of a cuticular disc, which is fixed over an air-filled cavity. On the outer surface of the disc, about 90 tiny cuticular sensilla are situated. The poreless outer peg of a sensillum is 3–5 μm long and is surrounded by a cuticular wall. One ciliary sensory cell innervates the peg. As a special feature, the outer dendritic segment is very short already terminating below the cuticle. A massive electron-dense cylindrical rod, which most probably represents the hypertrophied dendritic sheath, extends through the cuticular canal connecting the tip of the outer dendritic segment to the peg. The dendritic inner segment and the soma are fused indistinguishably. Thin, leaflike extensions of glial cells deeply extend into that conjoint and considerably enlarged compartment which also contains large numbers of mitochondria. In summary, the sensilla of the sensory disc of A. nigricans represent a new type of insect sensillum of hitherto unknown function. The possible role of the prothoracic sensory organ in fire detection is discussed.

Introduction

Acanthocnemus nigricans is a small dark beetle having a body length of only 3–6 mm. The species nigricans is the only recent species within the genus Acanthocnemus and is distributed all over Australia. Since the middle of the 19th century A. nigricans has also been found on other continents like Asia, Africa and Europe (Lawrence and Britton, 1994). Most probably, the reason for this recent expansion all over the world is the commercial export of Australian wood or cereals infested with A. nigricans. Therefore, the species can be also regarded as a cosmopolitan to warmer parts of the Old World (Champion, 1922). In contrast to its inconspicuous appearance, A. nigricans shows a remarkable behaviour as beetles of both sexes are attracted by forest fires. The beetles invade a freshly burnt area immediately and approach areas where glowing remnants of trees or hot ashes are still present (Champion, 1922, Schmitz and Schmitz, 2002). The beetles often land very close to spots with still high surface temperatures. After a short period of quickly running around, the beetles hide in small openings in the ash or under the bark of burnt trees. The reason for this so-called pyrophilous behaviour is unknown but it can be speculated that the described ‘hot spots’ serve as meeting places for the sexes. Most probably, the females deposit their eggs into the ash or under the bark of burnt trees. Currently, the larval food is unknown. Because it is reasonable to suppose that larvae were exported out of Australia inside wood (see above), it is very probable that the cambium layer of freshly burnt trees represents the primary source of food for the first instars.

In two pyrophilous genera of buprestid beetles, Melanophila and Merimna, unique extraantennal sensory organs have been found which are used in fire detection. Beetles of the genus Melanophila have two pairs of metathoracic infrared (IR) organs which contain about 70 arch-shaped photomechanic IR sensilla (Evans, 1966a, Evans, 1966b, Vondran et al., 1995, Schmitz et al., 1997). The Australian ‘fire-beetle’ Merimna atrata has one to three pairs of abdominal IR receptors (Schmitz et al., 2001, Mainz et al., 2004) which are innervated by thermoreceptive multipolar neurones (Schmitz and Trenner, 2003). Therefore, the receptors function as microbolometers. However, the prothoracic sensory organs in A. nigricans are located on a different part of the body and, moreover, are fundamentally different from the IR receptors of the pyrophilous buprestid beetles. A. nigricans possesses two prothoracic sensory organs which are located directly anterior to the coxae of the prothoracic legs. Each of the organs consists of a tiny cuticular disc (diameter 110–185 μm), which is held by a lateral cuticular stalk above an air-filled cavity. First morphological investigations have already shown that the disc is filled with the somata of many sensory cells, which contain large numbers of mitochondria. By these findings, a sensory function of the disc was made very probable (Schmitz et al., 2002). Additionally, on the outer surface of the disc, small pegs were discovered which represent the cuticular apparatus of sensilla. In the present paper, we describe the morphology of the cuticular sensilla in detail. The possible role of the prothoracic sensory organ in fire detection is discussed.