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

动物永远处于强烈的选择下，以避免被捕食，因此捕食者-猎物的相互作用是进化的主要驱动力，并产生了大量有效的防御策略。着色可以说是初级防御中最明显的，可能会增强隐蔽性，以减少检测或警告捕食者猎物的不适口性或毒性。这两种策略都可以在棒和叶昆虫（Phasmatodea）中找到，这是一种大型陆生草食性节肢动物的中分化谱系，以其将自己伪装成植物部分的卓越能力而闻名。大多数物种是夜行性的，在白天保持不动，并通过模仿树枝，树叶和树皮来展示伪装的秘密。相反，少数物种显示出引人注目的警戒色（警告色），暗示着它们有效的前胸驱避腺的使用。从这些腺体中喷出的化学物质只是phasmatodeans中众多次级防御机制之一（即，当捕食者攻击时引发的那些），包括惊吓表现、防御性的鸣叫、死亡或逃跑、肢体自断或通过重臂腿进行反击。虽然大多数物种在很大程度上依赖于神秘的外观，以避免捕食者的检测，他们的二级防御系统是相当复杂的，和regnatorial腺体在前胸应该是广泛存在于现存的物种，实际上构成了导出的接地图案特征（autapomorphy）这种强大的防御机制所涉及的化学物质在3000多个物种中只有十几种是已知的。描述分类群，到目前为止，没有信息提供的一些主要和关键的血统。本项目的目标是：（一）产生一个深刻的信息基础的存在和解剖多样性的prothoracic驱虫腺在广泛的和有代表性的分类样本的100棒和叶昆虫物种，（二）确定所涉及的化学物质，和（三）重建逐步解剖和化学进化的这个精心设计的防御系统的系统发育框架。所获得的信息将与关于所研究物种的体形和大小、甲胄、颜色和行为观察的现有数据相结合，以揭示关键特征的相互关系和变化，并发现可能对多样化率产生影响并影响竹节虫和叶虫进化的关键创新。由于迄今为止报道的几种驱避物质在类群之间显示出出乎意料的巨大多样性，表现出高度特异性的特征，如气味或引起皮肤刺激（如人类所感知的），并包括一些以前未报道的天然产物，我们还希望我们的项目能够证明竹节虫和叶虫作为新的生物活性化合物来源的价值。

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.