The prothoracic repellent glands of stick and leaf insects: Reconstructing the morphological and chemical evolution of an elaborate arthropod defensive system

• Applicants: Privatdozent Dr. Sven Bradler; Professor Dr. Thomas Schmitt
• Subject Area: Systematics and Morphology (Zoology) ; Animal Physiology and Biochemistry ; Evolution, Anthropology
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 445558934

Animals are permanently under strong selection to avoid predation, therefore predator-prey interactions are major driving forces of evolution and have given rise to a plethora of effective defensive strategies. Colouration is arguably the most obvious among primary defenses and might enhance crypsis to reduce detection or warn predators of the prey’s unpalatability or toxicity. Both strategies can be found in stick and leaf insects (Phasmatodea), a mesodiverse lineage of large terrestrial herbivorous arthropods well known for their remarkable capability to camouflage themselves as plant parts. Most species are nocturnal, remain motionless during the day and exhibit masquerade crypsis by mimicking twigs, leaves and bark. In contrast, a minority of species displays striking forms of aposematism (warning colouration) insinuating the use of their efficient prothoracic repellent glands. Chemical spraying from these glands is just one of numerous secondary defensive mechanisms in phasmatodeans (i.e., those initiated when a predator attacks), comprising startle display, defensive stridulation, thanatosis or escape by flight, limb autotomy or counterattack via heavily armend legs. Although most species largely rely on cryptic appearance to avoid predator detection, their secondary defensive system is quite elaborate, and repugnatorial glands in the prothorax are supposed to be widely present among extant species, in fact constituting a derived ground pattern feature (autapomorphy) of all Phasmatodea.The chemical substances involved in this powerful defensive mechanism are known only for a dozen species out of the more than 3.000 described taxa, so far with no information available for some major and pivotal lineages. The goal of the present project is (i) to generate a profound information base for the presence and anatomical diversity of the prothoracic repellent glands across a broad and representative taxon sampling of 100 stick and leaf insect species, (ii) to identify the chemical substances involved, and (iii) to reconstruct the step-wise anatomical and chemical evolution of this elaborate defensive system in a phylogenetic framework. The obtained information will be combined with existing data on body form and size, armature, colouration and behavioural observations of the examined species in order to reveal correlations and transformations of crucial traits and to detect key innovations that might have had an effect on diversification rates and shaped the evolution of stick and leaf insects. Since the few hitherto reported repellent substances show an unexpectedly huge diversity between taxa, exhibiting highly specific characteristics such as odor or causing skin irritations (as perceived by humans), and include some previously unreported natural products, we also expect our project to demonstrate the value of stick and leaf insects as sources of new bioactive compounds.

• DFG Programme: Research Grants