Instituto de Biodiversidad Neotropical (IBN), CONICET- Facultad de Ciencias Naturales, Universidad Nacional de Tucumán (U.N.T.), Miguel Lillo 205, 4000, Tucumán, Argentina.
Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT), 76344, Eggenstein-Leopoldshafen, Germany.
BMC Biol. 2023 Nov 23;21(1):268. doi: 10.1186/s12915-023-01750-8.
Mayflies are basal winged insects of crucial importance for the understanding of the early evolution of Pterygota. Unlike all other insects, they have two successive winged stages, the subimago and the imago. Their forewings feature so-called bullae, which are desclerotized spots in the anterior main veins. Up to now, they have been considered to play a major role in wing bending during flight.
We investigated bullae by multiple methods to reveal their structure and arrangement and to gain new information on the evolution of insect flight. Bullae are mostly present in the anterior negative wing veins, disrupting the otherwise rigid veins. High-speed videography reveals that mayfly wings do not bend during flight. Likewise, different arrangements of bullae in different species do not correlate with different modes of flying. Observations on the moulting of subimagines unravel that they are essential for wing bending during the extraction of the imaginal wing from the subimaginal cuticle. Bullae define predetermined bending lines, which, together with a highly flexible wing membrane enriched with resilin, permit wing bending during subimaginal moulting. Bullae are only absent in those species that remain in the subimaginal stage or that use modified modes of moulting. Bullae are also visible in fossil mayflies and can be traced back to stemgroup mayflies of the Early Permian, the 270 million years old Protereismatidae, which most probably had bullae in both fore- and hind wings.
Bullae in mayfly wings do not play a role in flight as previously thought, but are crucial for wing bending during subimaginal moulting. Thus, the presence of bullae is a reliable morphological marker for a subimaginal life stage, confirming the existence of the subimago already in Permian Protereismatidae. A thorough search for bullae in fossils of other pterygote lineages may reveal wheather they also had subimagines and at what point in evolution this life stage was lost. In mayflies, however, the subimago may have been retained due to selective advantages in connection with the transition from aquatic to terrestrial life or due to morphological requirements for a specialized mating flight.
蜉蝣是有翼昆虫的基干类群，对于理解翼类动物的早期演化具有重要意义。与所有其他昆虫不同，它们具有两个连续的有翅阶段，亚成虫和成虫。它们的前翅具有所谓的泡，这是在前主脉中的去骨化斑点。到目前为止，它们被认为在飞行中的翅膀弯曲中起主要作用。
我们通过多种方法研究了泡，以揭示它们的结构和排列，并获得有关昆虫飞行演化的新信息。泡主要存在于前负翅脉中，破坏了原本坚硬的脉。高速录像显示，蜉蝣的翅膀在飞行中不会弯曲。同样，不同物种中泡的不同排列方式与不同的飞行模式没有相关性。对亚成虫蜕皮的观察表明，它们对于成虫翅从亚成虫外皮中抽出时的翅膀弯曲是必不可少的。泡定义了预定的弯曲线，这些弯曲线与富含弹性蛋白的高度灵活的翅膜一起，允许在亚成虫蜕皮期间进行翅弯曲。只有那些停留在亚成虫阶段或使用修改后的蜕皮模式的物种才没有泡。化石蜉蝣中也可以看到泡，并且可以追溯到 2.7 亿年前的早二叠世的祖蜉蝣科的基干蜉蝣科，它们最有可能在前翅和后翅都有泡。
以前认为蜉蝣翅膀中的泡在飞行中不起作用，但对于亚成虫蜕皮期间的翅膀弯曲至关重要。因此，泡的存在是亚成虫阶段的可靠形态标记，证实了二叠世祖蜉蝣科中已经存在亚成虫。在其他翼类动物谱系的化石中对泡进行彻底搜索，可能会揭示它们是否也有亚成虫，以及这个生命阶段在进化中何时丢失。然而，在蜉蝣中，由于与从水生到陆生生活的过渡有关的选择性优势或由于专门交配飞行的形态要求，亚成虫可能已经被保留下来。
