Lost lovers linked at long last: elusive female Nanophyllium mystery solved after a century of being placed in a different genus (Phasmatodea, Phylliidae)

Taxonomic Revision, Morphology and Natural History of the Stick Insect Genus Xerosoma Serville, 1831 (Insecta: Phasmatodea)

Stick insects (Phasmatodea) are quite diverse in the Neotropical region. Among them, Xerosoma Serville belongs to Pseudophasmatidae and comprises winged, roughly brownish phasmids that resemble bark or dry branches and inhabit the Atlantic Forest in Brazil. In this study, we present a redescription and revision of the genus that include three valid species, Xerosoma canaliculatum, Xerosoma michaelis, and Xerosoma nannospinus sp. nov. Xerosoma senticosum syn. nov. was found to be a junior synonym of X. canaliculatum. We also provide an identification key and geographic records for these three species. Additionally, we present a detailed study on the morphology and natural history of X. canaliculatum with the description of its nymphal stages, egg, male genitalia, ontogeny, oviposition method, life habits, defense mechanisms, mating behavior, and other aspects regarding its biology. The study also highlights the shortcomings related to the classification of Xerosomatinae, since its tribes find themselves without proper characterization and contain heterogeneous genera. We expect to provide a basis for a proper diagnosis of Xerosomatinae and encourage future studies on this group, as there is still much to be discovered about this lineage of Neotropical stick insects.

On seven undescribed leaf insect species revealed within the recent "Tree of Leaves" (Phasmatodea, Phylliidae)

With the recent advance in molecular phylogenetics focused on the leaf insects (Phasmatodea, Phylliidae), gaps in knowledge are beginning to be filled. Yet, shortcomings are also being highlighted, for instance, the unveiling of numerous undescribed phylliid species. Here, some of these taxa are described, including Phylliumiyadaonsp. nov. from Mindoro Island, Philippines; Phylliumsamarensesp. nov. from Samar Island, Philippines; Phylliumortizisp. nov. from Mindanao Island, Philippines; Pulchriphylliumheraclessp. nov. from Vietnam; Pulchriphylliumdelisleisp. nov. from South Kalimantan, Indonesia; and Pulchriphylliumbhaskaraisp. nov. from Java, Indonesia. Several additional specimens of these species together with a seventh species described herein, Pulchriphylliumanangusp. nov. from southwestern India, were incorporated into a newly constructed phylogenetic tree. Additionally, two taxa that were originally described as species, but in recent decades have been treated as subspecies, are elevated back to species status to reflect their unique morphology and geographic isolation, creating the following new combinations: Pulchriphylliumscythe (Gray, 1843) stat. rev., comb. nov. from Bangladesh and northeastern India, and Pulchriphylliumcrurifolium (Audinet-Serville, 1838) stat. rev., comb. nov. from the Seychelles islands. Lectotype specimens are also designated for Pulchriphylliumscythe (Gray, 1843) stat. rev., comb. nov. and Pulchriphylliumcrurifolium (Audinet-Serville, 1838) stat. rev., comb. nov. from original type material.

Leaves that walk and eggs that stick: comparative functional morphology and evolution of the adhesive system of leaf insect eggs (Phasmatodea: Phylliidae)

Phylliidae are herbivorous insects exhibiting impressive cryptic masquerade and are colloquially called "walking leaves". They imitate angiosperm leaves and their eggs often resemble plant seeds structurally and in some cases functionally. Despite overall morphological similarity of adult Phylliidae, their eggs reveal a significant diversity in overall shape and exochorionic surface features. Previous studies have shown that the eggs of most Phylliidae possess a specialised attachment mechanism with hierarchical exochorionic fan-like structures (pinnae), which are mantled by a film of an adhesive secretion (glue). The folded pinnae and glue respond to water contact, with the fibrous pinnae expanding and the glue being capable of reversible liquefaction. In general, the eggs of phylliids appear to exhibit varying structures that were suggested to represent specific adaptations to the different environments the eggs are deposited in. Here, we investigated the diversity of phylliid eggs and the functional morphology of their exochorionic structure. Based on the examination of all phylliid taxa for which the eggs are known, we were able to characterise eleven different morphological types. We explored the adhesiveness of these different egg morphotypes and experimentally compared the attachment performance on a broad range of substrates with different surface roughness, surface chemistry and tested whether the adhesion is replicable after detachment in multiple cycles. Furthermore, we used molecular phylogenetic methods to reconstruct the evolutionary history of different egg types and their adhesive systems within this lineage, based on 53 phylliid taxa. Our results suggest that the egg morphology is congruent with the phylogenetic relationships within Phylliidae. The morphological differences are likely caused by adaptations to the specific environmental requirements for the particular clades, as the egg morphology has an influence on the performance regarding the surface roughness. Furthermore, we show that different pinnae and the adhesive glue evolved convergently in different species. While the evolution of the Phylliidae in general appears to be non-adaptive judging on the strong similarity of the adults and nymphs of most species, the eggs represent a stage with complex and rather diverse functional adaptations including mechanisms for both fixation and dispersal of the eggs.

Convergent Evolution of Adhesive Properties in Leaf Insect Eggs and Plant Seeds: Cross-Kingdom Bioinspiration

Plants and animals are often used as a source for inspiration in biomimetic engineering. However, stronger engagement of biologists is often required in the field of biomimetics. The actual strength of using biological systems as a source of inspiration for human problem solving does not lie in a perfect copy of a single system but in the extraction of core principles from similarly functioning systems that have convergently solved the same problem in their evolution. Adhesive systems are an example of such convergent traits that independently evolved in different organisms. We herein compare two analogous adhesive systems, one from plants seeds and one from insect eggs, to test their properties and functional principles for differences and similarities in order to evaluate the input that can be potentially used for biomimetics. Although strikingly similar, the eggs of the leaf insect Phyllium philippinicum and the seeds of the ivy gourd Coccinia grandis make use of different surface structures for the generation of adhesion. Both employ a water-soluble glue that is spread on the surface via reinforcing fibrous surface structures, but the morphology of these structures is different. In addition to microscopic analysis of the two adhesive systems, we mechanically measured the actual adhesion generated by both systems to quantitatively compare their functional differences on various standardized substrates. We found that seeds can generate much stronger adhesion in some cases but overall provided less reliable adherence in comparison to eggs. Furthermore, eggs performed better regarding repetitive attachment. The similarities of these systems, and their differences resulting from their different purposes and different structural/chemical features, can be informative for engineers working on technical adhesive systems.

Three new genera and one new species of leaf insect from Melanesia (Phasmatodea, Phylliidae)

With the first large-scale Phylliidae molecular phylogeny recently published adding a great deal of clarity to phylliid diversity, several of the rarer species which could not be included were methodically and morphologically reviewed. This review resulted in identification of numerous substantial morphological features that suggest there are Melanesian clades that create polyphyletic groups within the phylliids which should instead be taxonomically recognized as unique. These rarer Melanesia species have historically been considered to be southern representatives of the Pulchriphyllium Griffini, 1898 sensu lato. However, there are notable morphological differences between the Pulchriphyllium sensu stricto and the "schultzei" group. Therefore, two new genera are erected, Vaabonbonphyllium gen. nov. from the Solomon Islands and Papua New Guinea and Rakaphyllium gen. nov. from New Guinea and the Aru Islands. Erection of these two new genera warrants the following new combinations: Rakaphylliumschultzei (Giglio-Tos, 1912), comb. nov., Rakaphylliumexsectum (Zompro, 2001b), comb. nov., and Vaabonbonphylliumgroesseri (Zompro, 1998), comb. nov. Additionally, while reviewing material an undescribed Vaabonbonphyllium gen. nov. specimen was located and is herein described as Vaabonbonphylliumrafidahae gen. et sp. nov. from Mt. Hagen, Papua New Guinea. Additionally, a morphologically unique clade of several species recovered as sister to the Nanophyllium sensu stricto was recognized and their numerous unique morphological features and monophyly leads the authors to erect the new genus Acentetaphyllium gen. nov. which warrants the following new combinations: Acentetaphylliumbrevipenne (Größer, 1992), comb. nov., Acentetaphylliumlarssoni (Cumming, 2017), comb. nov., Acentetaphylliummiyashitai (Cumming et al. 2020), comb. nov., and Acentetaphylliumstellae (Cumming, 2016), comb. nov. With the addition of several new genera, a key to phylliid genera is included for adult males and females.

Multi-modal locomotor costs favor smaller males in a sexually dimorphic leaf-mimicking insect

Background

In most arthropods, adult females are larger than males, and male competition is a race to quickly locate and mate with scattered females (scramble competition polygyny). Variation in body size among males may confer advantages that depend on context. Smaller males may be favored due to more efficient locomotion leading to higher mobility during mate searching. Alternatively, larger males may benefit from increased speed and higher survivorship. While the relationship between male body size and mobility has been investigated in several systems, how different aspects of male body morphology specifically affect their locomotor performance in different contexts is often unclear.

Results

Using a combination of empirical measures of flight performance and modelling of body aerodynamics, we show that large body size impairs flight performance in male leaf insects (Phyllium philippinicum), a species where relatively small and skinny males fly through the canopy in search of large sedentary females. Smaller males were more agile in the air and ascended more rapidly during flight. Our models further predicted that variation in body shape would affect body lift and drag but suggested that flight costs may not explain the evolution of strong sexual dimorphism in body shape in this species. Finally, empirical measurements of substrate adhesion and subsequent modelling of landing impact forces suggested that smaller males had a lower risk of detaching from the substrates on which they walk and land.

Conclusions

By showing that male body size impairs their flight and substrate adhesion performance, we provide support to the hypothesis that smaller scrambling males benefit from an increased locomotor performance and shed light on the evolution of sexual dimorphism in scramble competition mating systems.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-01993-z.

Resolving a century-old case of generic mistaken identity: polyphyly of Chitoniscus sensu lato resolved with the description of the endemic New Caledonia Trolicaphyllium gen. nov. (Phasmatodea, Phylliidae)

With every molecular review involving Chitoniscus Stål, 1875 sensu lato samples from Fiji and New Caledonia revealing polyphyly, the morphology from these two distinct clades was extensively reviewed. Morphological results agree with all previously published molecular studies and therefore Trolicaphylliumgen. nov. is erected to accommodate the former Chitoniscus sensu lato species restricted to New Caledonia, leaving the type species Chitoniscuslobiventris (Blanchard, 1853) and all other Fijian species within Chitoniscus sensu stricto. Erection of this new genus for the New Caledonian species warrants the following new combinations: Trolicaphylliumbrachysoma (Sharp, 1898), comb. nov., Trolicaphylliumerosus (Redtenbachher, 1906), comb. nov., and Trolicaphylliumsarrameaense (Größer, 2008a), comb. nov. Morphological details of the female, male, freshly hatched nymph, and egg are illustrated and discussed alongside the Chitoniscus sensu stricto in order to differentiate these two clades which have been mistaken as one for decades.

A tree of leaves: Phylogeny and historical biogeography of the leaf insects (Phasmatodea: Phylliidae)

The insect order Phasmatodea is known for large slender insects masquerading as twigs or bark. In contrast to these so-called stick insects, the subordinated clade of leaf insects (Phylliidae) are dorso-ventrally flattened and therefore resemble leaves in a unique way. Here we show that the origin of extant leaf insects lies in the Australasian/Pacific region with subsequent dispersal westwards to mainland Asia and colonisation of most Southeast Asian landmasses. We further hypothesise that the clade originated in the Early Eocene after the emergence of angiosperm-dominated rainforests. The genus Phyllium to which most of the ~100 described species pertain is recovered as paraphyletic and its three non-nominate subgenera are recovered as distinct, monophyletic groups and are consequently elevated to genus rank. This first phylogeny covering all major phylliid groups provides the basis for future studies on their taxonomy and a framework to unveil more of their cryptic and underestimated diversity.

Physical constraints lead to parallel evolution of micro- and nanostructures of animal adhesive pads: a review

Adhesive pads are functional systems with specific micro- and nanostructures which evolved as a response to specific environmental conditions and therefore exhibit convergent traits. The functional constraints that shape systems for the attachment to a surface are general requirements. Different strategies to solve similar problems often follow similar physical principles, hence, the morphology of attachment devices is affected by physical constraints. This resulted in two main types of attachment devices in animals: hairy and smooth. They differ in morphology and ultrastructure but achieve mechanical adaptation to substrates with different roughness and maximise the actual contact area with them. Species-specific environmental surface conditions resulted in different solutions for the specific ecological surroundings of different animals. As the conditions are similar in discrete environments unrelated to the group of animals, the micro- and nanostructural adaptations of the attachment systems of different animal groups reveal similar mechanisms. Consequently, similar attachment organs evolved in a convergent manner and different attachment solutions can occur within closely related lineages. In this review, we present a summary of the literature on structural and functional principles of attachment pads with a special focus on insects, describe micro- and nanostructures, surface patterns, origin of different pads and their evolution, discuss the material properties (elasticity, viscoelasticity, adhesion, friction) and basic physical forces contributing to adhesion, show the influence of different factors, such as substrate roughness and pad stiffness, on contact forces, and review the chemical composition of pad fluids, which is an important component of an adhesive function. Attachment systems are omnipresent in animals. We show parallel evolution of attachment structures on micro- and nanoscales at different phylogenetic levels, focus on insects as the largest animal group on earth, and subsequently zoom into the attachment pads of the stick and leaf insects (Phasmatodea) to explore convergent evolution of attachment pads at even smaller scales. Since convergent events might be potentially interesting for engineers as a kind of optimal solution by nature, the biomimetic implications of the discussed results are briefly presented.

Cryptophyllium, the hidden leaf insects - descriptions of a new leaf insect genus and thirteen species from the former celebicum species group (Phasmatodea, Phylliidae)

While the leaf insects (Phylliidae) are a well-supported group within Phasmatodea, the genus Phyllium Illiger, 1798 has repeatedly been recovered as paraphyletic. Here, the Phyllium (Phyllium) celebicum species group is reviewed and its distinctiveness from the remaining Phylliini genera and subgenera in a phylogenetic context based on morphological review and a phylogenetic analysis of three genes (nuclear gene 28S and mitochondrial genes COI and 16S) from most known and multiple undescribed species is shown. A new genus, Cryptophylliumgen. nov., is erected to partially accommodate the former members of the celebicum species group. Two species, PhylliumericoriaiHennemann et al., 2009 and Phylliumbonifacioi Lit & Eusebio, 2014 morphologically and molecularly do not fall within this clade and are therefore left within Phyllium (Phyllium). The transfer of the remaining celebicum group members from Phyllium Illiger, 1798 to this new genus creates the following new combinations; Cryptophylliumathanysus (Westwood, 1859), comb. nov.; Cryptophylliumcelebicum (de Haan, 1842), comb. nov.; Cryptophylliumchrisangi (Seow-Choen, 2017), comb. nov.; Cryptophylliumdrunganum (Yang, 1995), comb. nov.; Cryptophylliumoyae (Cumming & Le Tirant, 2020), comb. nov.; Cryptophylliumparum (Liu, 1993), comb. nov.; Cryptophylliumrarum (Liu, 1993), comb. nov.; Cryptophylliumtibetense (Liu, 1993), comb. nov.; Cryptophylliumwestwoodii (Wood-Mason, 1875), comb. nov.; Cryptophylliumyapicum (Cumming & Teemsma, 2018), comb. nov.; and Cryptophylliumyunnanense (Liu, 1993), comb. nov.

The review of specimens belonging to this clade also revealed 13 undescribed species, which are described within as: Cryptophylliumanimatumgen. et sp. nov. from Vietnam: Quang Nam Province; Cryptophylliumbankoigen. et sp. nov. from Vietnam: Quang Ngai, Thua Thien Hue, Da Nang, Gia Lai, Quang Nam, and Dak Nong Provinces; Cryptophylliumbollensigen. et sp. nov. from Vietnam: Ninh Thuan Province; Cryptophylliumdaparogen. et sp. nov. from China: Yunnan Province; Cryptophylliumechidnagen. et sp. nov. from Indonesia: Wangi-wangi Island; Cryptophylliumfaulknerigen. et sp. nov. from Vietnam: Quang Ngai and Lam Dong Provinces; Cryptophylliumicarusgen. et sp. nov. from Vietnam: Lam Dong and Dak Lak Provinces; Cryptophylliumkhmergen. et sp. nov. from Cambodia: Koh Kong and Siem Reap Provinces; Cryptophylliumlimogesigen. et sp. nov. from Vietnam: Lam Dong, Dak Lak, and Dak Nong Provinces; Cryptophylliumliyananaegen. et sp. nov. from China: Guangxi Province; Cryptophylliumnuichuaensegen. et sp. nov. from Vietnam: Ninh Thuan Province; Cryptophylliumphamigen. et sp. nov. from Vietnam: Dong Nai and Ninh Thuan Provinces; and Cryptophylliumwennaegen. et sp. nov. from China: Yunnan Province. All newly described species are morphologically described, illustrated, and molecularly compared to congenerics.

With the molecular results revealing cryptic taxa, it was found necessary for Cryptophylliumwestwoodii (Wood-Mason, 1875), comb. nov. to have a neotype specimen designated to allow accurate differentiation from congenerics. To conclude, male and female dichotomous keys to species for the Cryptophylliumgen. nov. are presented.

Multifunctional Adhesives on the Eggs of the Leaf Insect Phyllium philippinicum (Phasmatodea: Phylliidae): Solvent Influence and Biomimetic Implications

Leaf insects (Phylliidae) are well-camouflaged terrestrial herbivores. They imitate leaves of plants almost perfectly and even their eggs resemble seeds—visually and regarding to dispersal mechanisms. The eggs of the leaf insect Phyllium philippinicum utilize an adhesive system with a combination of glue, which can be reversibly activated through water contact and a water-responding framework of reinforcing fibers that facilitates their adjustment to substrate asperities and real contact area enhancement. So far, the chemical composition of this glue remains unknown. To evaluate functional aspects of the glue–solvent interaction, we tested the effects of a broad array of chemical solvents on the glue activation and measured corresponding adhesive forces. Based on these experiments, our results let us assume a proteinaceous nature of the glue with different functional chemical subunits, which enable bonding of the glue to both the surface of the egg and the unpredictable substrate. Some chemicals inhibited adhesion, but the deactivation was always reversible by water-contact and in some cases yielded even higher adhesive forces. The combination of glue and fibers also enables retaining the adhesive on the egg, even if detached from the egg’s surface. The gained insights into this versatile bioadhesive system could hereafter inspire further biomimetic adhesives.