Patterns of morphological simplification and innovation in the megadiverse Holometabola (Insecta)

Evolution of insect metamorphosis - an update

Metamorphosis endowed the insects with properties that enabled them to conquer the Earth. It is a hormonally controlled morphogenetic process that transforms the larva into the adult. Metamorphosis appeared with the origin of wings and flight. The sesquiterpenoid juvenile hormone (JH) suppresses wing morphogenesis and ensures that metamorphosis takes place in the right ontogenetic time. This review explores the origin of insect metamorphosis and the ancestral function of JH. Fossil record shows that the first Paleozoic winged insects had (hemimetabolous) metamorphosis and their larvae were likely aquatic. In the primitive wingless silverfish that lacks metamorphosis JH is essential for late embryogenesis and reproduction. JH production after the embryo dorsal closure promotes hatching and terminal tissue maturation.

Revisiting the four Hexapoda classes: Protura as the sister group to all other hexapods

Insects represent the most diverse animal group, yet previous phylogenetic analyses based on morphological and molecular data have failed to agree on the evolutionary relationships of early insects and their six-legged relatives (together constituting the clade Hexapoda). In particular, the phylogenetic positions of the three early-diverging hexapod lineages-the coneheads (Protura), springtails (Collembola), and two-pronged bristletails (Diplura)-have been debated for over a century, with alternative topologies implying drastically different scenarios of the evolution of the insect body plan and hexapod terrestrialization. We addressed this issue by sampling all hexapod orders and experimenting with a broad range of across-site compositional heterogeneous models designed to tackle ancient divergences. Our analyses support Protura as the earliest-diverging hexapod lineage ("Protura-sister") and Collembola as a sister group to Diplura, a clade corresponding to the original composition of Entognatha, and characterized by the shared possession of internal muscles in the antennal flagellum. The previously recognized 'Elliplura' hypothesis is recovered only under the site-homogeneous substitution models with partial supermatrices. Our cross-validation analysis shows that the site-heterogeneous CAT-GTR model, which recovers "Protura-sister," fits significantly better than homogeneous models. Furthermore, the morphologically unusual Protura are also supported as the earliest-diverging hexapod lineage by other lines of evidence, such as mitogenomes, comparative embryology, and sperm morphology, which produced results similar to those in this study. Our backbone phylogeny of hexapods will facilitate the exploration of the underpinnings of hexapod terrestrialization and megadiversity.

Cephalic anatomy highlights morphological adaptation to underground habitats in a minute lacewing larva of Dilar (Dilaridae) and conflicting phylogenetic signal in Neuroptera

Dilaridae are a distinctive and phylogenetically ambiguous neuropteran family. So far, the anatomy of the immature stages remains largely unknown. We examined the 1st instar larvae of Dilar montanus in detail and present results of live observations for the first time. The minute, cryptic larvae display features correlated with their underground lifestyle: for instance, a strongly flattened head, stout antennae, eyelessness, and burrowing forelegs. In contrast to molecular data, several characters suggest a 'dilarid clade' combining Dilaridae with Mantispoidea, for instance a very thin and curved or reduced tentorial bridge, and an elongated postmentum. We found intrinsic antennal muscles and Johnston's organ, the first record of these structures in holometabolous larvae. This proves that the first 2 larval antennomeres are homologous with the scapus and pedicellus. The described characters are discussed and analyzed with an updated matrix of neuropteran larval characters. Alternative scenarios of character evolution are presented. Additionally, we show how the 1st-instar larvae move and feed in the substrate, and also provide a high-resolution video recording of the function of the elongate tubular ovipositor and the egg-laying behavior in an adult female under natural conditions.

Distal leg structures of Zoraptera - did the loss of adhesive devices curb the chance of diversification?

The distal leg structures of Zoraptera are documented and discussed with respect to their functional morphology and evolutionary aspects. We investigated eight species using scanning electron microscopy. We analyzed material compositions of the tarsus in three representative species using confocal laser scanning microscopy. When possible, we included both sexes, wing morphs, and nymphs and compared the structures among them. The distal leg structure is unusually uniform across zorapterans regardless of the sex, morphs, and developmental stages. The observed features combine simplification with innovation. The former is likely partially correlated with cryptic microhabitats and miniaturization. Innovation includes a protibial cleaning organ. This is very likely an autapomorphy of Zoraptera. The tarsi are composed of two tarsomeres covered with setae. The pretarsus distally bears an unguitractor plate and well-sclerotized claws. The tarsomeres appear less-sclerotized than the covering setae. The articulation between the basitarsus and tarsomere 2 is hinge-like, implying that tarsomere 2 moves only mediolaterally. The simplified and specialized tarsal morphology is likely suitable for the typical zorapteran microhabitat, under bark. However, the irreversible complete loss of adhesive devices prevented zorapterans to make use of a broader spectrum of environments and was presumably one reason for the species paucity of the group.