Larval morphology of Panorpodes kuandianensis (Insecta, Mecoptera, Panorpodidae) and its evolutionary implications

Ultrastructure of the larval eyes of the hangingfly Terrobittacus implicatus (Mecoptera: Bittacidae)

The fine structure of the larval eyes of the hangingfly Terrobittacus implicatus (Huang & Hua) was investigated using scanning and transmission electron microscopy. The results show that the larval eyes of T. implicatus each consist of seven spaced ommatidia. Each ommatidium is composed of a corneal lens with about 45 lamellae, a tetrapartite eucone type of crystalline cone, eight retinula cells, two primary pigment cells, and an undetermined number of secondary pigment cells. The rhabdomeres of eight retinula cells effectively fuse into a centrally-fused, tiered funnel-shaped rhabdom extending from the base of the crystalline cone deeply into the ommatidium. In light of different positions in the ommatidium, the retinula cells can be divided into four distal and four proximal retinula cells. Pigment cells envelop the entire ommatidium. Electron-lucent vesicles are abundant throughout the cytoplasm of the eight retinula cells. The larval ommatidia of T. implicatus are similar to those of the Panorpidae, except for the distal retinula cells that also participate in the formation of the proximal rhabdom. In this case, the larval eyes of T. implicatus may lie in the transitional stage during the larval eye evolution of insects from ommatidia to stemmata.

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.

The sperm pump and genital coupling of Panorpodes kuandianensis (Mecoptera: Panorpodidae)

Males of Panorpodidae possess a special sperm pump, through which they directly transfer seminal fluid to the female spermatheca. However, the sperm pump has not been studied in Panorpodes to date. Here, the structure of the sperm pump and the internal coupling of genitalia were investigated in the short-faced scorpionfly Panorpodes kuandianensis Zhong, Zhang, and Hua, 2011 using light and scanning electron microscopy. The sperm pump mainly consists of a piston, a pumping chamber, the anterior region of the aedeagal complex, the posterior region of the ejaculatory sac, and associated muscles. The piston as a propulsion apparatus is controlled by levator and depressor muscles. Its posterior region connects dorsally to the aedeagus via a joint. The pumping chamber is located between the piston and the aedeagus. The dorsal and ventral parameres were attached by retractor muscles. During copulation, the male phallotreme connects to the female copulatory pore to transfer sperm. Male gonostyli and parameres grasp the female to restrict the genitalia movement and impede her medigynium from retreating. The sperm ejaculatory mechanism of Panorpodes and the evolution of sperm transfer mode in insects are briefly discussed based on the structure of the sperm pump and the internal coupling of genitalia.

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

Bittacidae and Panorpidae are the two largest families in Mecoptera. The larvae of Bittacidae are different from those of Panorpidae in external morphology and habits, and have an interesting habit of spraying the body surface with soil through the anus. However, it remains unknown to date whether the larval midguts are different in structure between the two families. Here the ultrastructure of the larval midguts of the hangingfly Bittacus planus Cheng and the scorpionfly Neopanorpa longiprocessa Hua & Chou were compared using light, scanning, and transmission electron microscopy. The midguts of both species are simple tubes of single layered epithelia with digestive and regenerative cells but without diverticula. The basal plasma membrane of epithelial cells exhibits infolding in B. planus, but is closely apposed to its basal lamina in N. longiprocessa. Lymph spaces are present between adjacent epithelial cells in B. planus, but are absent in N. longiprocessa. The regenerative cells are scattered among the digestive cells in B. planus, but are aggregated in N. longiprocessa. The longitudinal muscle bands are compact in B. planus, but are sparse in N. longiprocessa. The compact longitudinal muscle bands are likely associated with their soil-spraying habit in Bittacidae.

Ultrastructure of male accessory glands in the scorpionfly Sinopanorpa tincta (Navás, 1931) (Mecoptera: Panorpidae)

The ultrastructure of male reproductive accessory glands was investigated in the scorpionfly Sinopanorpa tincta (Navás, 1931) (Mecoptera: Panorpidae) using light and transmission electron microscopy. The male accessory glands comprise one pair of mesodermal glands (mesadenia) and six pairs of ectodermal glands (ectadenia). The former opens into the vasa deferentia and the latter into the ejaculatory sac. The mesadenia consist of a mono-layered elongated columnar epithelium, the cells of which are highly microvillated and extrude secretory granules by means of merocrine mechanisms. The epithelium of ectadenia consists of two types of cells: the large secretory cells and the thin duct-forming cells. These two types of cells that join with a cuticular duct constitute a functional glandular unit, corresponding to the class III glandular cell type of Noirot and Quennedey. The cuticular duct consists of a receiving canal and a conducting canal. The secretory granules were taken up by the receiving canal and then plunged into the lumen through the conducting canal.

Comparative embryogenesis of Mecoptera and Lepidoptera with special reference to the abdominal prolegs

The eruciform larvae of holometabolous insects are primarily characterized by bearing a varying number of abdominal prolegs in addition to three pairs of thoracic legs. However, whether the prolegs are evolutionarily homologous among different insect orders is still a disputable issue. We examined the embryonic features and histological structure of the prolegs of the scorpionfly Panorpa byersi Hua and Huang (Mecoptera: Panorpidae) and the Oriental armyworm Mythimna separata (Walker) (Lepidoptera: Noctuidae) to investigate whether the prolegs are homologous between these two holometabolous insect orders. In the scorpionfly, paired lateral process primordia arise on abdominal segments I-VIII (A1-A8) in line with the thoracic legs in early embryonic stages, but degenerate into triangular protuberances in later stages, and paired medial processes appear along the midventral line before dorsal closure and eventually develop into unjointed, cone-shaped prolegs. Histological observation showed that the lumina of the prolegs are not continuous with the hemocoel, differing distinctly from that of the basic appendicular plan of thoracic legs. These results suggest that the prolegs are likely secondary outgrowths in Mecoptera. In the armyworm, lateral process primordia appear on A1-A10 in alignment with the thoracic legs in the early embryonic stages, although only the rudiments on A3-A6 and A10 develop into segmented prolegs with the lumina continuous with the hemocoel and others degenerate eventually, suggesting that the prolegs are true segmental appendages serially homologous with the thoracic legs in Lepidoptera. Therefore, we conclude that the larval prolegs are likely not evolutionarily homologous between Mecoptera and Lepidoptera.