Phylogenetic implications based on an ultrastructural study with emphasis on the tracheal system of the compound eyes of three species of nymphalid butterflies

Compound eyes are the prominent visual organs of insects and can provide valuable information for the reconstruction of insect phylogeny. Although the largest butterfly family (Nymphalidae) has been well defined, the infrafamilial phylogenetic relationships remain controversial hitherto. In the present study the ultrastructure of the compound eyes of three nymphalids Neptis beroe, Childrena zenobia, and Palaeonympha opalina was investigated using light and transmission electron microscopy in an attempt to seek potentially important phylogenetic characters. The compound eyes of the nymphalids share a tracheal system in a "1-4-8" branching pattern. The eight tracheal subbranches exhibit distinct distribution patterns along the basal retinula cell as follows: the tracheal subbranches of Palaeonympha opaline are close to the rhabdom in the distance from the distalmost part of the basal retinula cell to the rhabdom end, while those of N. beroe and C. zenobia are on the periphery of the retinula along almost the whole basal retinula cell and become close to the rhabdom just at the proximal end of the basal retinula cell. The tracheal structure of the three nymphalids is discussed for their potential phylogenetic 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.

Ultrastructural comparison of the compound eyes of the Asian corn borer Ostrinia furnacalis (Lepidoptera: Crambidae) under light/dark adaptation

The Asian corn borer Ostrinia furnacalis is one of the most destructive pests of maize throughout eastern Asia and the South Pacific. In the present study the fine structure of the compound eyes of adult O. furnacalis was investigated under light/dark adaptation using light and electron microscopy. The compound eyes of male and female O. furnacalis are superposition eyes with electron-lucent clear zones. The sexual differences of the compound eyes of O. furnacalis are mainly reflected in eye size rather than ommatidial ultrastructure. Each ommatidium of both sexes contains 12 retinula cells, one of which is the basal retinula cell. All the retinula cells form a centrally-fused, two-tiered rhabdom, whose distal layer passes through the clear zone and distally connects with the crystalline cone. The ultrastructural changes under light/dark conditions mainly involve the rhabdom occupation ratio to retinula cell volume in the proximal layer of the rhabdom as well as the dimensions of the subcorneal zone and the crystalline tract. Pigment movements occur within the retinula cells and primary pigment cells, but are undetectable within the secondary pigment cells. Regardless of light or dark adaptation, in other words, the pigments never migrate into the clear zone.

Trans-generational transmission of altered phenotype resulting from flubendiamide-induced changes in apoptosis in larval imaginal discs of Drosophila melanogaster

The eye and wing morphology of Drosophila melanogaster maintain unique, stable pattern of genesis from larval eye and wing imaginal discs. Increased apoptosis in cells of eye and wing discs was found to be associated with flubendiamide (fluoride containing insecticide) exposure (at the range 0.25-10μg/mL) in D. melanogaster larvae. The chemical fed larvae on attaining adulthood revealed alterations in morphology and symmetry of their compound eyes and wings through scanning electron microscopy. Nearly 40% and 30% of flies (P generation) demonstrated alterations in eyes and wings respectively. Transmission electron microscopic study (at the range 1-20μg/mL) also established variation in the rhabdomere and pigment cell orientation as well as in the shape of the ommatidium. Subsequent SEM study with F₁ and F₂ generation flies also revealed structural variation in eye and wing. Decrease in percentage of altered eye and wing phenotype was noted in subsequent generations (P> F₁>F₂). Thus, the diamide insecticide, flubendiamide, expected to be environmentally safe at sub-lethal concentrations was found to increase apoptosis in larvae and thereby cause morphological alteration in the adult D. melanogaster. This study further demonstrated trans-generational transmission of altered phenotype in three subsequent generations of a non-target insect model, D. melanogaster.