Sensation of the tiniest kind: the antennal sensilla of the smallest free-living insectScydosella musawasensis(Coleoptera: Ptiliidae)

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.

The world’s tiniest land snails from Laos and Vietnam (Gastropoda, Pulmonata, Hypselostomatidae)

“…There are several proposed evolutionary mechanisms that favour smaller individuals of a species and select against larger ones (Blanckenhorn, 2000;Grebennikov, 2008). For snails, the most obvious consideration is that smallness enables snails to enter a previously unoccupied niche, either by being able to forage in narrow spaces, and/or by utilizing food particles that would otherwise remain unconsumed by larger animals (Grebennikov, 2008;Diakova & Polilov, 2020). Moreover, by being small, a snail may avoid predation either because it falls below the minimum food item size of predators or by being able to enter spaces devoid of predators.…”

“…Despite these advantages, miniaturization also subjects an organism to harsh physical and physiological limitations that probably function as evolutionary constraints. In ptiliid beetles and egg parasitoids, it is suggested that sensory receptors and the processing of stimuli in the brain cease to function below a certain size, simply because the number of available cells drops below a viable threshold (Van der Woude & Smid, 2017;Diakova & Polilov, 2020). However, all these animals have juvenile stages that are much smaller than the adults, so it is not clear why brains and sensory organs would be too small to function in the adult, while they clearly function in much smaller larvae.…”

“…In egg parasitoid wasps, for example, extremely small body size, down to 0.3 mm, enables specialisation on parasitizing the small eggs of small-bodied hosts (Van der Woude & Smid, 2017). In soil invertebrates, such as mites and 10.1163/18759866-bja10025 | páll-gergely et al feather-winged beetles (Ptiliidae), comparably small body sizes allow food particles to be ingested that would otherwise remain unconsumed, while tininess also enables access into crevices too small for larger-bodied individuals (Grebennikov 2008;Diakova & Polilov, 2020). In land snails, small body size is probably driven mostly by the accessibility of small spaces in the subsoil.…”