Characterization and mapping of sensilla on the head appendages of noterid larvae (Coleoptera: Noteridae), and development of a preliminary biometric method for taxa delimitation

Abstract

Larvae of the burrowing water beetle family Noteridae are distributed worldwide and are often abundant in a broad range of aquatic habitats, playing an important role in structuring freshwater communities, yet they have remained among the most poorly studied groups of aquatic beetles. Studies on sensillar equipment of aquatic insect larvae are largely lacking, despite their potential use in phylogeny and biometric identification methods. In this article, the external morphology and distribution of sensilla on the head appendages of first instar larvae of selected genera of Noteridae were examined using scanning electron microscopy. Seven main types were distinguished based on their morphological structure: basiconica (3 subtypes), campaniformia (2 subtypes), chaetica (7 subtypes), coeloconica (6 subtypes), coniform complex (2 subtypes), placodea, and styloconica (3 subtypes). The apex of the labial palpus was found to be the most variable and informative region in regard to the number, relative position, and topology of sensilla. Fingerprint models were, therefore, generated for this region in each of the studied genera, allowing their identification.

Morphology and distribution of sensilla on head appendages in the water beetle Hygrobia hermanni (Coleoptera: Adephaga: Hygrobiidae)

“…The presence of a deep cavity at the apex of antennomere IV, in which the bases of these three elongated pegs are inserted, was observed in Noteridae (Urcola et al, 2020) and Hygrobiidae (this study, Figure 6c), whereas in Carabidae there is no apical cavity (Sitar et al, 2021). Few additional sensilla are observed in the apical region of antennomere IV (in Noteridae, one sensillum styloconicum and one sensillum interpreted as a “coniform complex”, Urcola et al, 2020; in H. hermanni , two sensilla coeloconica of different subtypes, Figure 6h and 6j,k).…”

“…Based on available evidence, there appears to be a common ground plan for antennal sensory structures in Adephaga larvae, to which H. hermanni conforms. In these larvae, the antennomere surfaces bear few sensilla, mainly a low number of elongated sensilla (which can be either trichodea, chaetica or basiconica) and flat sensilla, which have been described as “pores” in the larval morphology literature, but which appear to be sensilla campaniformia or coeloconica based on SEM pictures (Urcola et al, 2020 and this study). Notably, on the surface of antennomere IV, a single sensillum campaniformium is present in H. hermanni (cam1, Figure 6b and 6i), as in Carabidae (Sitar et al, 2021), Noteridae (Urcola et al, 2020) and Dytiscidae (Alarie & Michat, 2014 referring to “one lateral pore”).…”

“…In these larvae, the antennomere surfaces bear few sensilla, mainly a low number of elongated sensilla (which can be either trichodea, chaetica or basiconica) and flat sensilla, which have been described as “pores” in the larval morphology literature, but which appear to be sensilla campaniformia or coeloconica based on SEM pictures (Urcola et al, 2020 and this study). Notably, on the surface of antennomere IV, a single sensillum campaniformium is present in H. hermanni (cam1, Figure 6b and 6i), as in Carabidae (Sitar et al, 2021), Noteridae (Urcola et al, 2020) and Dytiscidae (Alarie & Michat, 2014 referring to “one lateral pore”). The apex of antennomere IV bears three sensilla with elongated pegs in Carabidae (Sitar et al, 2021), Noteridae (Urcola et al, 2020), Dytiscidae (Alarie & Michat, 2014) as well as Hygrobiidae (this study, Figure 6b,c).…”

“…More remarkably, sensory structures on the larval palp apices in H. hermanni differ spectacularly from what has been described for the aquatic larvae of Noteridae (Urcola et al, 2020) and Dytiscidae (Zacharuk, 1985). In Noteridae, the larval palp apex bears a single sensory field of several dozens of circumvallate sensilla, among which three main subtypes can be distinguished.…”

“…There are no data for Aspidytidae (SEM photographs of the head have been published but with no details on the sensilla, Beutel et al, 2016; Ribera et al, 2002a). Concerning larvae, although morphology and chaetotaxis have been described for many aquatic Adephaga species (mainly in publications by Yves Alarie), information is limited for cephalic appendage sensilla at the resolution scale of the present study, except for several genera of family Noteridae thanks to the recent study by Urcola et al (2020). A SEM photograph of the apex of the labial palp of an undetermined Dytiscidae larva was published by Zacharuk (1985).…”