Abstract
Miniaturization is a major evolutionary trend prominent in insects, which has resulted in the existence of insects comparable in size to some unicellular protists. The adaptation of the complex antennal multisensory systems to extreme miniaturization is a fascinating problem, which remains almost unexplored. We studied the antennal sensilla of Scydosella musawasensis Hall, 1999 (Coleoptera: Ptiliidae), the smallest free-living insect, using scanning electron microscopy. The antenna of S. musawasensis bears 131 sensilla; no intraspecific variation in the number or position of the sensilla has been revealed. Nine different morphological types of sensilla are described according to their external morphological features and distribution: four types of sensilla trichodea, one type of sensilla chaetica, two types of sensilla styloconica, and two types of sensilla basiconica. Morphometric analysis of the sensilla of S. musawasensis, based on measurements of the lengths and diameters of sensilla and their location and number, showed the absence of significant differences between females and males. Comparative allometric analysis of S. musawasensis and larger Coleoptera showed that the number of sensilla and the size of sensilla chaetica decrease with decreasing body size. However, the number of the types of sensilla and the length and diameter of the multiporous sensilla basiconica revealed no correlation with the body size. Comparison of the acquired data with the results of our earlier study of the antennal sensilla of some of the smallest parasitic wasps is used to put forward hypotheses on the common principles of miniaturization of the antennal sensory systems of insects.
Highlights
Antennae are present in a vast majority of Hexapoda (Schneider, 1964)
The antenna of S. musawasensis consists of ten antennomeres: the scape, the pedicel, and eight flagellomeres, which include the two-segmented club (Fig. 1)
A decrease in body size leads to a significant decrease in the number of individual sensory units in the antennal sensory systems of both parasitic and free-living insects
Summary
Antennae are present in a vast majority of Hexapoda (Schneider, 1964). The antennae are complex multimodal organs involved in such processes as the search for a sexual partner, a host, or food, or as intraspecific communication, enemy detection, orientation and navigation during flight, and many others (Altner, Sass & Altner, 1977; Guerenstein et al, 2004; Kamikouchi et al, 2009). Relatively few studies are devoted to the most speciose order of insects, Coleoptera. Most of those few studies use scanning electron microscopy and describe the external morphology of the antennal sensilla (Skilbeck & Anderson, 1996; Merivee et al, 2002). The results of such studies are often used for taxonomic purposes as diagnostic characters (Pérez-González & Zaballos, 2013). Several studies deal with the involvement of the antennal sensory system in interspecies communication and behavioral responses in Coleoptera (Lopes, Marques & Araújo, 2005; Ali et al, 2016)
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