The pregenital abdomen of Enicocephalomorpha and morphological evidence for different modes of communication at the dawn of heteropteran evolution

Moss bugs shed light on the evolution of complex bioacoustic systems

Vibroacoustic signalling is one of the dominant strategies of animal communication, especially in small invertebrates. Among insects, the order Hemiptera displays a staggering diversity of vibroacoustic organs and is renowned for possessing biomechanically complex elastic recoil devices such as tymbals and snapping organs that enable robust vibrational communication. However, our understanding of the evolution of hemipteran elastic recoil devices is hindered by the absence of relevant data in the phylogenetically important group known as moss bugs (Coleorrhyncha), which produce substrate-borne vibrations through an unknown mechanism. In the present work, we reveal the functional morphology of the moss bug vibrational mechanism and study its presence across Coleorrhyncha and in extinct fossilised relatives. We incorporate the anatomical features of the moss bug vibrational mechanism in a phylogeny of Hemiptera, which supports either a sister-group relationship to Heteroptera, or a sister-group relationship with the Auchenorrhyncha. Regardless of topology, we propose that simple abdominal vibration was present at the root of Euhemiptera, and arose 350 million years ago, suggesting that this mode of signalling is among the most ancient in the animal kingdom. Therefore, the most parsimonious explanation for the origins of complex elastic recoil devices is that they represent secondary developments that arose exclusively in the Auchenorrhyncha.

Sexual selection and predation drive the repeated evolution of stridulation in Heteroptera and other arthropods

Acoustic and substrate-borne vibrations are among the most widely used signalling modalities in animals. Arthropods display a staggering diversity of vibroacoustic organs generating acoustic sound and/or substrate-borne vibrations, and are fundamental to our broader understanding of the evolution of animal signalling. The primary mechanism that arthropods use to generate vibroacoustic signals is stridulation, which involves the rubbing together of opposing body parts. Although stridulation is common, its behavioural context and evolutionary drivers are often hard to pinpoint, owing to limited synthesis of empirical observations on stridulatory species. This is exacerbated by the diversity of mechanisms involved and the sparsity of their description in the literature, which renders their documentation a challenging task. Here, we present the most comprehensive review to date on the systematic distribution and behavioural context of stridulation. We use the megadiverse heteropteran insects as a model, together with multiple arthropod outgroups (arachnids, myriapods, and selected pancrustaceans). We find that stridulatory vibroacoustic signalling has evolved independently at least 84 times and is present in roughly 20% of Heteroptera, representing a remarkable case of convergent evolution. By studying the behavioural context of stridulation across Heteroptera and 189 outgroup lineages, we find that predation pressure and sexual selection are the main behaviours associated with stridulation across arthropods, adding further evidence for their role as drivers of large-scale signalling and morphological innovation in animals. Remarkably, the absence of tympanal ears in most Heteroptera suggests that they typically cannot detect the acoustic component of their stridulatory signals. This demonstrates that the adoption of new signalling modalities is not always correlated with the ability to perceive those signals, especially when these signals are directed towards interspecific receivers in defensive contexts. Furthermore, by mapping their morphology and systematic distribution, we show that stridulatory organs tend to evolve in specific body parts, likely originating from cleaning motions and pre-copulatory displays that are common to most arthropods. By synthesising our understanding of stridulation and stridulatory organs across major arthropod groups, we create the necessary framework for future studies to explore their systematic and behavioural significance, their potential role in sensory evolution and innovation, and the biomechanics of this mode of signalling.

External structures of the metathoracic scent gland efferent system in the true bug superfamily Pyrrhocoroidea (Hemiptera: Heteroptera: Pentatomomorpha)

Pyrrhocoroidea represents an important group of true bugs (Insecta: Hemiptera: Heteroptera) which includes fire bugs, cotton stainers and other taxa widely used in experimental studies or known as pests. However, the morphology and phylogeny of Pyrrhocoroidea have been only poorly studied so far. Here, structures of the external scent efferent system of the metathoracic scent glands are examined in 64 out of 71 currently valid genera of Pyrrhocoroidea and scanning electron micrographs are provided for most taxa. Several characters are revealed which define each of the three higher taxa within Pyrrhocoroidea: Larginae (small auriculate peritreme lacking manubrium and median furrow; metathoracic spiracle never surrounded by evaporatorium), Physopeltinae (large, widely open ostiole; large peritremal disc with manubrium [new term], lacking median furrow; mace-like mycoid filter processes of equal shape and size on both anterior and posterior margins of metathoracic spiracle), and Pyrrhocoridae (elongate auriculate peritreme with deep median furrow). Within Pyrrhocoridae, three main types (A, B and C) of the external scent efferent system are distinguished, differring in the amount of reductions. The findings are interpreted in the context of phylogenetic hypotheses available for Pyrrhocoroidea and their close relatives, Coreoidea and Lygaeoidea. An updated identification key to the families and subfamilies of Pyrrhocoroidea applicable for both sexes is provided.

On the morphology and evolution of cicadomorphan tymbal organs

Cicadas and many of their relatives (Hemiptera: Cicadomorpha) generate vibroacoustic signals using tymbal organs located on their first two abdominal segments. Although tymbals are well-studied in Cicadidae, their systematic distribution in other Cicadomorpha and their possible homologies to the vibroacoustic mechanisms of other Hemiptera have been debated for more than a century. In the present study, we re-examine the morphology of the musculoskeletal system of cicadomorphan vibroacoustic organs, and we document their systematic distribution in 78 species drawn from across the phylogeny of Cicadomorpha. We also compare their morphology to the recently-described snapping organ of planthoppers (Fulgoromorpha). Based on the structure and innervation of the metathoracic and abdominal musculoskeletal system, we find that several key elements of cicadomorphan vibroacoustic organs that have previously been assigned to the first abdominal segment in fact belong to the second. We find that tymbal organs are nearly ubiquitous in Cicadomorpha, and conclude based on their phylogenetic distribution, that they are likely to be synapomorphic. The unusual tymbal-like organs of the Deltocephalinae and Typhlocybinae, represent derived modifications. Finally, we propose a standardised terminology for sternal components of the cicadomorphan vibrational organs, which can be used in future taxonomic descriptions.

On the morphology and possible function of two putative vibroacoustic mechanisms in derbid planthoppers (Hemiptera: Fulgoromorpha: Derbidae)

A mechanism involving interaction of the metathoracic wing and third abdominal segment of derbid planthoppers was first discovered over a century ago, and interpreted as a stridulatory organ for sound production. Although referred to occasionally in later taxonomic works, the detailed morphology, systematic distribution, and behavioural significance of this structure have remained unknown, and its proposed use in sound production has never been corroborated. Here we examine the distribution and morphology of the supposed stridulatory organ of Derbidae and the recently-described vibratory mechanism of planthoppers - the snapping organ, across 168 species covering the entire taxonomic spectrum of the family. We find that many derbids possess snapping organs morphologically similar to those of other planthoppers, and find no evidence for the presence of tymbal organs, which were previously thought to generate vibrational signals in derbids. We find the supposed stridulatory mechanism to be widespread in Derbidae, and conclude that it provides several systematically and taxonomically important characters. Nevertheless, its morphology appears unsuitable for the production of sound, and we instead speculate that the mechanism plays a role in spreading chemical secretions or wax. Finally, we observe wax production by tergal glands in derbid larvae, and illustrate their external morphology in adults.

Synopsis of Schizopteridae (Hemiptera, Heteroptera, Dipsocoromorpha) from the United States, with description of seven new species from the US and Mexico

Because species diversity of the small true bug family Schizopteridae is greatest in tropical and subtropical areas, it is not surprising that only four species have been described from the United States. As part of a larger project on the taxonomy and phylogenetics of Schizopteridae, 178 specimens from the United States were examined. This material contained representatives of the previously described species Glyptocombussaltator Heidemann, 1906, Corixideamajor McAtee & Malloch, 1925, Nannocorisarenarius Blatchley, 1926, and Schizopterabispina McAtee & Malloch, 1925, but also six undescribed species. These new taxa are described as Glyptocombushalbertaesp. n., Glyptocombussuterisp. n., Nannocorisanophorussp. n., Nannocorisbrevipilussp. n., Schizoptera (Cantharocoris) rileyisp. n., and Schizoptera (Schizoptera) henryisp. n. Habitus images and genitalic illustrations of the previously described and the new species are provided as well as a map showing distribution ranges of these species in the United States and Mexico. To provide a comprehensive treatment of the small genus Glyptocombus Heidemann, 1906, Glyptocombusmexicanussp. n. is also described that, to our knowledge, occurs only in Mexico, and the female of one additional undescribed Glyptocombus species is documented from Mexico.