Ultrastructure of the larval midgut of Bittacus planus (Mecoptera: Bittacidae) and Neopanorpa longiprocessa (Mecoptera: Panorpidae)

Ultrastructure of the rectum of the soil-spraying larva in Bittacus cirratus (Mecoptera: Bittacidae)

The larvae of Bittacidae have an interesting behavior of spraying soil particles on their body surface through the anus. However, the hindgut specialization associated with this behavior has rarely been studied hitherto. Here, we investigated the fine structure of the larval rectum in the hangingfly Bittacus cirratus Tjeder using light and transmission electron microscopy. The results show that the larvae of B. cirratus have a tubular rectum without rectal pads or papillae. The rectum consists of well-developed visceral muscle layers, a non-cellular basal lamina, a single-layer epithelium with a cuticular intima, and a central lumen. The folded rectal epithelium consists of two types of flattened epithelial cells: electron-dense type I cells and electron-lucent type II cells. The apical and basal plasma membranes are infolded and are associated with mitochondria in the epithelial cells. The epithelial cells are held by septate and scalariform junctions. The lateral cell membranes are combined with mitochondria among type I cells and generate mitochondria-scalariform junction complexes. These features suggest that the epithelial cells are active in water and ion reabsorption. We conclude that the absence of rectal pads or papillae and the presence of developed circular muscles are likely morphological adaptations of these larvae to the soil-spraying behavior.

Habitat divergence shapes the morphological diversity of larval insects: insights from scorpionflies

Insects are the most diverse group of organisms in the world, but how this diversity was achieved is still a disputable and unsatisfactorily resolved issue. In this paper, we investigated the correlations of habitat preferences and morphological traits in larval Panorpidae in the phylogenetic context to unravel the driving forces underlying the evolution of morphological traits. The results show that most anatomical features are shared by monophyletic groups and are synapomorphies. However, the phenotypes of body colorations are shared by paraphyletic assemblages, implying that they are adaptive characters. The larvae of Dicerapanorpa and Cerapanorpa are epedaphic and are darkish dorsally as camouflage, and possess well-developed locomotory appendages as adaptations likely to avoid potential predators. On the contrary, the larvae of Neopanorpa are euedaphic and are pale on their trunks, with shallow furrows, reduced antennae, shortened setae, flattened compound eyes on the head capsules, and short dorsal processes on the trunk. All these characters appear to be adaptations for the larvae to inhabit the soil. We suggest that habitat divergence has driven the morphological diversity between the epedaphic and euedaphic larvae, and may be partly responsible for the divergence of major clades within the Panorpidae.

Ultrastructure of the larval Malpighian tubules in Terrobittacus implicatus (Mecoptera: Bittacidae)

The larvae of Bittacidae, a cosmopolitan family in Mecoptera, have an interesting habit of spraying the body surface with soil through the anus after hatching, and each molts. The fine structure of Malpighian tubules, however, remains largely unknown in the larvae of Bittacidae to date. Here, we studied the ultrastructure of the larval Malpighian tubules in the hangingfly Terrobittacus implicatus (Huang & Hua) using scanning and transmission electron microscopy. The larvae of T. implicatus have six elongate Malpighian tubules at the junction of the midgut and hindgut. The tubule comprises a basal lamina, a single-layered epithelium, and a central lumen. The basal plasma membranes of the epithelial cells are conspicuously infolded and generate a labyrinth. The epithelium consists of two types of cells: large principal cells and scattered stellate cells. Mitochondria and cisterns of rough endoplasmic reticulum are numerous in the principal cells but are sparsely distributed in the stellate cells, indicating that the principal cells are active in transport. On the other hand, spherites are only abundant in the principal cells and are likely associated with the soil-spraying habit of the larvae.