Chemical Ecology of Cave-Dwelling Millipedes: Defensive Secretions of the Typhloiulini (Diplopoda, Julida, Julidae)

Simple protocol for combined extraction of exocrine secretions and RNA in small arthropods

The integration of data from multiple sources and analytical techniques to obtain novel insights and answer challenging questions is a hallmark of modern science. In arthropods, exocrine secretions may act as pheromones, defensive substances, antibiotics, as well as surface protectants, and as such they play a crucial role in ecology and evolution. Exocrine chemical compounds are frequently characterized by gas chromatography-mass spectrometry. Technological advances of recent years now allow us to routinely characterize the total gene complement transcribed in a particular biological tissue, often in the context of experimental treatment, via RNAseq. We here introduce a novel methodological approach to successfully characterize exocrine secretions and full transcriptomes of one and the same individual of oribatid mites. We found that chemical extraction prior to RNA extraction had only minor effects on the total RNA integrity. De novo transcriptomes obtained from such combined extractions were of comparable quality to those assembled for samples that were subject to RNA extraction only, indicating that combined chemical/RNA extraction is perfectly suitable for phylotranscriptomic studies. However, in-depth analysis of RNA expression analysis indicates that chemical extraction prior to RNAseq may affect transcript degradation rates, similar to the effects reported in previous studies comparing RNA extraction protocols. With this pilot study, we demonstrate that profiling chemical secretions and RNA expression levels from the same individual is methodologically feasible, paving the way for future research to understand the genes and pathways underlying the syntheses of biogenic chemical compounds. Our approach should be applicable broadly to most arachnids, insects, and other arthropods.

Chemical defence of a centipede (Clinopodes flavidus)

Chemical substances are of utmost importance for the biotic interactions between animals and their predators/parasites; many of these semiochemicals are emitted for defence purposes. One of the most deterrent and toxic biogenic substances we know of is hydrogen cyanide, which can be stored by certain insects, millipedes, centipedes and arachnids in the form of stable and less volatile molecules. The aim of this study was to analyse the biology and chemistry of such a defence mechanism in a geophilomorph centipede (Chilopoda). The cyanogenic secretion of Clinopodes flavidus is discharged from the ventral glands, whose glandular units are located in the space between the cuticle and the trunk muscles and do not extend deep into the segment. In addition to hydrogen cyanide, the ventral secretion contains 2-methylpentanoic acid, benzaldehyde, benzoyl cyanide, 2-methyl branched C-9 carboxylic acid (tentatively identified as 2-methyloctanoic acid), methyl 2-phenylacetate, benzoic acid and mandelonitrile as well as four major proteins with a molecular weight of 150, 66.2, 59 and 55 kDa. The correlation between the presence of ventral glands and guarding with the female's ventral side facing away from the eggs and young indicates a functional link between these two traits. We hope that the specificity of the chemical composition of the ventral secretion could serve as a criterion for chemotaxonomy and that the analysis of more species will help to clarify the phylogenetic relationships within the Geophilomorpha.

The genus Inversotyphlus Strasser, 1962, stat. nov. and Inversotyphlusammirandus sp. nov., a new bizarre, highly modified troglobiotic millipede (Diplopoda, Julida, Julidae) from Albania, Balkan Peninsula

Inversotyphlus Strasser, 1962, stat. nov. is raised to the genus level and a new hydrophilous species, Inversotyphlusammirandussp. nov., is described from the second deepest pit in Albania. This species is characterized by a highly modified head and body for a presumably semiaquatic or hygropetric life and filtering diet. It is by far the most bizarrely modified cave-dwelling julid known. The new species is diagnosed, described in detail, and richly illustrated. Besides I.ammirandussp. nov., the genus Inversotyphlusstat. nov. includes six species: I.clavatus (Antić, 2018), comb. nov., I.edentulus (Attems, 1951), comb. nov., I.gellianae (Makarov & Rađa, 2006), comb. nov., I.gracilis (Antić, 2018), comb. nov., I.lobifer (Attems, 1951), comb. nov., and I.opisthonodus (Antić, 2018) comb. nov. The subgenus Attemsotyphlussyn. nov. is here considered as a junior subjective synonym of the genus Inversotyphlusstat. nov. Notes are given on the habitat of I.ammirandussp. nov., the taxonomy of the tribe Typhloiulini and the genus Inversotyphlusstat. nov., and adaptive modifications of the mouthparts.

Inferring on Speleomantes Foraging Behavior from Gut Contents Examination

We here provide the first comprehensive analysis and discussion on prey consumed by the European cave salamanders of the genus Speleomantes. Our study stems from the need to shed light on the still unknown foraging behavior adopted by Speleomantes cave salamanders. Starting from the published datasets on gut contents from all Speleomantes species (including hybrids), we here discuss additional information (i.e., species ecology, lower taxonomic level), which were systematically omitted from those data sets. We analyzed a data set consisting of 17,630 records from 49 categories of consumed prey recognized from gut contents of 2060 adults and juveniles Speleomantes. Flying prey accounted for more than 58% of the prey items, while elongated crawling prey accounted for no more than 16% of the diet within a single population. Among the total recognized prey items, only three can be surely ascribed to the group of strictly-cave species (i.e., troglobites), meaning that European cave salamanders mostly forage in surface environment, and therefore represent one of the major drivers of allochthonous organic matter in subterranean environments. Some of the consumed prey seemed to be aquatic, allowing us to hypothesize whether Speleomantes are able to catch prey from a shallow body water. Furthermore, European cave salamanders possess the ability to prey upon taxa characterized by particular anti-predator defenses, while morphological constraints seem to be the most important limit to prey consumption. For each specific case, we provide insights and propose hypotheses concerning the foraging behavior that need to be tested to properly understand the foraging behavior of this cryptic salamanders.

Wild Blonde Capuchins (Sapajus flavius) Perform Anointing Behaviour Using Toxic Secretions of a Millipede (Spirobolida: Rhinocricidae)

Defensive secretions of millipedes are remarkable for containing toxic quinones known to efficiently repell hematophagous arthropods. Here we show that Endangered blonde capuchin monkeys make use of such secretions. We (i) describe the anointing behavior performed by the monkeys (ii) identify the millipede species used in the process (iii) describe the volatile chemical composition of its secretion. The blonde capuchin monkeys selectively searched for millipedes hidden under the ground. We observed three bouts of anointing behavior, performed by 13 individuals of all age classes (from adults to independent infants), both solitarily (1 event) and socially (10 events). The only millipede species used by the monkeys is an undescribed species of the genus Poecilocricus (Spirobolida, Rhinocricidae). The volatile chemical composition of the secretions was predominantly comprised of a mixture of benzoquinones and hydroquinones. The social nature of the behavior and time of the observations (mosquito season), suggest that social bonding and mosquito avoidance is linked to the anointing behavior of the monkeys.

Step-wise evolution of complex chemical defenses in millipedes: a phylogenomic approach

With fossil representatives from the Silurian capable of respiring atmospheric oxygen, millipedes are among the oldest terrestrial animals, and likely the first to acquire diverse and complex chemical defenses against predators. Exploring the origin of complex adaptive traits is critical for understanding the evolution of Earth's biological complexity, and chemical defense evolution serves as an ideal study system. The classic explanation for the evolution of complexity is by gradual increase from simple to complex, passing through intermediate "stepping stone" states. Here we present the first phylogenetic-based study of the evolution of complex chemical defenses in millipedes by generating the largest genomic-based phylogenetic dataset ever assembled for the group. Our phylogenomic results demonstrate that chemical complexity shows a clear pattern of escalation through time. New pathways are added in a stepwise pattern, leading to greater chemical complexity, independently in a number of derived lineages. This complexity gradually increased through time, leading to the advent of three distantly related chemically complex evolutionary lineages, each uniquely characteristic of each of the respective millipede groups.

Methyl N-methylanthranilate: major compound in the defensive secretion of Typhloiulus orpheus (Diplopoda, Julida)

The defensive secretion of the julid diplopod Typhloiulus orpheus contains methyl N-methylanthranilate (MNMA), an ester of N-methylanthranilic acid that comprises more than 99% of secretion of this species. MNMA is accompanied by small amounts of methyl anthranilate and two benzoquinones (2-methyl-1,4-benzoquinone and 2-ethyl-1,4-benzoquinone, respectively). MNMA is a known intermediate in the biosynthesis of both benzoquinones (as present in defensive secretions of juliformians) and glomerin-like quinazolines (chemical defense in Glomerida). The compound may have evolved independently in the pathway to glomeridan chemistry, or may even represent a pivotal branching point in the pathway to different chemical classes of diplopod defensive chemistry.