Data from: Structural mouthpart interaction evolved already in the earliest lineages of insects

In butterflies, bees, flies and true bugs specific mouthparts are in close contact or even fused to enable piercing, sucking or sponging of particular food sources. The common phenomenon behind these mouthpart types is a complex composed of several consecutive mouthparts which structurally interact during food uptake. The single mouthparts are thus only functional in conjunction with other adjacent mouthparts, which is fundamentally different to biting–chewing. It is, however, unclear when structural mouthpart interaction (SMI) evolved since this principle obviously occurred multiple times independently in several extant and extinct winged insect groups. Here, we report a new type of SMI in two of the earliest wingless hexapod lineages—Diplura and Collembola. We found that the mandible and maxilla interact with each other via an articulatory stud at the dorsal side of the maxillary stipes, and they are furthermore supported by structures of the hypopharynx and head capsule. These interactions are crucial stabilizing elements during food uptake. The presence of SMI in these ancestrally wingless insects, and its absence in those crustacean groups probably ancestral to insects, indicates that SMI is a groundplan apomorphy of insects. Our results thus contradict the currently established view of insect mouthpart evolution that biting–chewing mouthparts without any form of SMI are the ancestral configuration. Furthermore, SMIs occur in the earliest insects in a high anatomical variety. SMIs in stemgroup representatives of insects may have triggered efficient exploitation and fast adaptation to new terrestrial food sources much earlier than previously supposed.在蝴蝶、蜜蜂、双翅目昆虫以及半翅目真蝽类昆虫（true bugs）中，特化的口器彼此紧密贴合甚至发生愈合，以此实现针对特定食物源的穿刺、吸食或舐吸行为。这类口器类型背后的共性机制，是由多个连续排列的口器结构组成的复合体，该复合体在食物摄取过程中会发生结构层面的相互作用。因此，单个口器仅在与其他相邻口器协同配合时才能发挥功能，这与典型的咀嚼式口器存在本质差异。然而，由于该作用机制在多个现存及已灭绝的有翅昆虫类群中曾多次独立演化出现，因此结构口器互作（structural mouthpart interaction, SMI）的起源时间至今仍不明确。本研究首次在两类最原始的无翅六足类群——双尾目（Diplura）和弹尾目（Collembola）中，报道了一种新型的结构口器互作现象。研究发现，上颚（mandible）与下颚（maxilla）通过位于下颚茎节（maxillary stipes）背侧的关节突实现彼此耦合，且该结构还可由下咽舌（hypopharynx）和头壳（head capsule）的相关结构提供支撑。这类互作结构在食物摄取过程中是至关重要的稳定支撑要素。在这些原始无翅昆虫中存在结构口器互作，而在可能作为昆虫祖先的甲壳类类群中却不存在该现象，这表明结构口器互作是昆虫的基底衍征（groundplan apomorphy）。因此，本研究结果与当前学界公认的昆虫口器演化观点相悖——该观点认为，不具备任何形式结构口器互作的咀嚼式口器才是昆虫的祖先型口器构型。此外，结构口器互作在最原始的昆虫类群中展现出极高的解剖学多样性。昆虫干群类群中的结构口器互作，可能早在人们此前预期的时间之前，就推动了昆虫对新型陆生食物源的高效利用与快速适应。