Patterning of a compound eye on an extinct dipteran wing

We have discovered unexpected similarities between a novel and characteristic wing organ in an extinct biting midge from Baltic amber, Eohelea petrunkevitchi, and the surface of a dipteran's compound eye. Scanning electron microscope images now reveal vestigial mechanoreceptors between the facets of the organ. We interpret Eohelea's wing organ as the blending of these two developmental systems: the formation and patterning of the cuticle in the eye and of the wing. Typically, only females in the genus carry this distinctive, highly organized structure. Two species were studied (E. petrunkevitchi and E. sinuosa), and the structure differs in form between them. We examine Eohelea's wing structures for modes of fabrication, material properties and biological functions, and the effective ecological environment in which these midges lived. We argue that the current view of the wing organ's function in stridulation has been misconstrued since it was described half a century ago.     

The Drosophila wing hearts consist of syncytial muscle cells that resemble adult somatic muscles

In Drosophila, hemolymph circulation in the wings is accomplished by a pair of wing hearts located in the thorax. The embryonic progenitors of these organs were only recently discovered and found to belong to the cardiac mesoderm. In this study, the functional morphology and the structure of mature organs were studied by light and electron microscopy to characterize the tissues arising from this new set of progenitors. Each wing heart consists of 7-8 muscle cells providing the pumping force, a thin layer of non-contractile mononucleated cells separating the muscle cells from the body cavity, and acellular suspending strands opposing the muscle contractions. The muscle cells are multinucleated syncytia attached to the cuticle via epidermal tendon cells. They have central nuclei and sarcomeres with discontinuous Z-discs, A-bands, and I-bands, whereas H-bands and M-bands are indiscernible. From 9 to 11 actin filaments surround each myosin filament. Mitochondria are abundantly interspersed between myofibrils and accumulated in characteristic outpockets of the plasma membrane. The analysis revealed that the wing heart muscles resemble in their ultrastructure and their mode of attachment adult somatic muscles. This suggests that, despite their origin in the cardiac mesoderm, wing heart progenitors are functionally related to somatic adult muscle precursors.     

Synthesis of the major adult cuticle proteins of Drosophila melanogaster during hypoderm differentiation

Differentiating imaginal hypodermal cells of Drosophila melanogaster form adult cuticle during the second half of the pupal stage (about 40 to 93 hr postpupariation). A group of proteins with molecular weights of 23,000, 20,000, and 14,000 is identified as putative major wing cuticle proteins with the following biological properties: These proteins are abundant components of cuticle and are major synthetic products of cuticle-secreting hypodermal cells. They are leucine-rich and methionine-free and are the most prominent proteins of this type synthesized by wing hypoderm at 65 hr, during the period of procuticle formation. Electron microscopic autoradiography shows that leucine-rich, methionine-free proteins specifically localize to the apical cell surface and newly secreted cuticle of 65-hr wing cells. This strongly suggests the export of these proteins to the cuticle. Lastly, these proteins undergo a reduction in extractability just after eclosion, during the period of cuticle protein crosslinking (sclerotization). The synthesis of these major hypoderm proteins is temporally regulated in development. In wing cells, the 14-kDa proteins are synthesized first, from 53 to 78 hr, and the 20- and 23-kDa proteins are synthesized from 63 to 93 hr. The pattern of synthesis for these proteins is similar in abdominal cells but delayed by 6 to 10 hr. Two-dimensional gel electrophoresis shows that each of the 23-, 20-, and 14-kDa size classes contains at least two component polypeptides. Patterns of protein synthesis in cells of the imaginal hypodermis are regulated in a precise temporal sequence during the production of adult cuticle. Their study yields a useful system for the analysis of molecular events in gene control and cell differentiation.     

Beyond aerodynamics: The critical roles of the circulatory and tracheal systems in maintaining insect wing functionality

Insect wings consist almost entirely of lifeless cuticle; yet their veins host a complex multimodal sensory apparatus and other tissues that require a continuous supply of water, nutrients and oxygen. This review provides a survey of the various living components in insect wings, as well as the specific contribution of the circulatory and tracheal systems to provide all essential substances. In most insects, hemolymph circulates through the veinal network in a loop flow caused by the contraction of accessory pulsatile organs in the thorax. In other insects, hemolymph oscillates into and out of the wings due to the complex interaction of several factors, such as heartbeat reversal, intermittent pumping of the accessory pulsatile organs in the thorax, and the elasticity of the wall of a special type of tracheae. A practically unexplored subject is the need for continuous hydration of the wing cuticle to retain its flexibility and toughness, including the associated problem of water loss due to evaporation. Also, widely neglected is the influence of the hemolymph mass and the circulating flow in the veins on the aerodynamic properties of insect wings during flight. Ventilation of the extraordinarily long wing tracheae is probably accomplished by intricate interactions with the circulatory system, and by the exchange of oxygen via cutaneous respiration.     

Tissue remodeling during maturation of the Drosophila wing

The final step in morphogenesis of the adult fly is wing maturation, a process not well understood at the cellular level due to the impermeable and refractive nature of cuticle synthesized some 30 h prior to eclosion from the pupal case. Advances in GFP technology now make it possible to visualize cells using fluorescence after cuticle synthesis is complete. We find that, between eclosion and wing expansion, the epithelia within the folded wing begin to delaminate from the cuticle and that delamination is complete when the wing has fully expanded. After expansion, epithelial cells lose contact with each other, adherens junctions are disrupted, and nuclei become pycnotic. The cells then change shape, elongate, and migrate from the wing into the thorax. During wing maturation, the Timp gene product, tissue inhibitor of metalloproteinases, and probably other components of an extracellular matrix are expressed that bond the dorsal and ventral cuticular surfaces of the wing following migration of the cells. These steps are dissected using the batone and Timp genes and ectopic expression of alphaPS integrin, inhibitors of Armadillo/beta-catenin nuclear activity and baculovirus caspase inhibitor p35. We conclude that an epithelial-mesenchymal transition is responsible for epithelial delamination and dissolution.     

Postembryonic development of the wing imaginal discs in the female wingless bagworm moth Eumeta variegata (Lepidoptera,Psychidae)

The process of wing disc development and degeneration in the bagworm moth Eumeta variegata was investigated histologically. Morphological differences between two sexes first appear in the penultimate (eighth) larval instar. In the male, wing discs proliferate rapidly in the penultimate larval instar and continue proliferating; a conspicuous peripodial epithelium forms in the last (ninth) larval instar. The hemopoietic organs break down in this stage and disappear completely by the prepupal stage. In the female, in contrast, the wing discs remain as in the previous (seventh) instar, without proliferation of cells inside. No peripodial epithelium forms in the penultimate instar or later. Hemopoietic organs are still attached to the wing discs in the last larval instar and the entire wing discs transform into a plain, thick epidermis in the prepupal period. It is suggested that the hemopoietic organs may prevent the wing discs from developing in E. variegata.     

Bursicon signaling mutations separate the epithelial-mesenchymal transition from programmed cell death during Drosophila melanogaster wing maturation

Following eclosion from the pupal case, wings of the immature adult fly unfold and expand to present a flat wing blade. During expansion the epithelia, which earlier produced the wing cuticle, delaminate from the cuticle, and the epithelial cells undergo an epithelial–mesenchymal transition (EMT). The resulting fibroblast-like cells then initiate a programmed cell death, produce an extracellular matrix that bonds dorsal and ventral wing cuticles, and exit the wing. Mutants that block wing expansion cause persistence of intact epithelia within the unexpanded wing. However, the normal progression of chromatin condensation and fragmentation accompanying programmed cell death in these cells proceeds with an approximately normal time course. These observations establish that the Bursicon/Rickets signaling pathway is necessary for both wing expansion and initiation of the EMT that leads to removal of the epithelial cells from the wing. They demonstrate that a different signal can be used to activate programmed cell death and show that two distinct genetic programs are in progress in these cells during wing maturation.     

Wing colors based on arrangement of the multilayer structure of wing scales in lycaenid butterflies (Insecta: Lepidoptera)

Male wing colors and wing scale morphology were examined for three species of lycaenid butterflies: Chrysozephyrus ataxus, Favonius cognatus and F. jezoensis. Measurement of spectral reflectance on the wing surface with a spectrophotometer revealed species‐specific reflection spectra, with one or two peaks in the ultraviolet and/or green ranges. Observations of wing scales using an optical microscope revealed that light was reflected from the inter‐ridge regions, where transmission electron microscopy revealed a multilayer structure. Based on the multilayer dimensions obtained, three models were devised and compared to explain the measured reflectance spectrum. The results showed that the best fit is a model in which thicknesses of thin films of the multilayer system are not constant and air spaces between cuticle layers are more or less packed with cuticle spacers. This suggests that the specific wing colors of the species examined are produced by the species‐specific arrangement of the multilayer structure of wing scales.     

Organisation of wing cuticle in Locusta migratoria linnaeus,Tropidacris cristata linnaeus and Romalea microptera beauvais (Orthoptera : Acrididae)

The composite fibrous architectures of the wing cuticles of Locusta migratoria, Tropidacris (= Eutropidacris) cristata and Romalea microptera (Orthoptera : Acrididae) have been established. The wing cuticle in all the 3 species consists of: (i) an exocuticle, which is either pigmented or birefringent, and which under an electron microscope shows constantly helicoidal architecture of chitin microfibrils; (ii) endocuticle, which shows alternately birefringent and isotropic layers when sectioned transversely across the wing veins; these layers show helicoidal and unidirectional architecture, respectively of chitin microfibrils under the electron microscope. In transverse section, the chitin microfibrils appear as clear rods (2.8 nm in diameter) in a darkly stained matrix. However, in the hinge called the “claval furrow”, these microfibrils are considerably larger, being 25 nm in diameter. This presumably gives sufficient hardness to the claval hinge, which is the most vulnerable area for wear and tear during flight. The pore canals follow the parabolic pattern of microfibrils in the helicoidal layer, but remain straight in the unidirectional layers. The thickness of wing cuticle increases up to about 10–12 days, the time at which the acridids most probably attain the optimum flight ability. It is suggested that changes in the wing cuticle are related to increased wing beat frequency and speed of flight with age, and may help in resisting the simultaneous increase in the bending and twisting forces on the wing.     

The precocious commitment of wing Anlagen in Tenebrio molitor revealed by the addition of 20-hydroxyecdysone

An injection of 20-hydroxyecdysone (10 mug per animal) 6-13 days after the moult of the last larval instar of Tenebrio molitor induces the development of prothetelic larvae and larval-pupal intermediates. The state of larval-pupal switchover, or commitment, is only disclosed at the time of injection of the moulting hormone. Prothetelic A and B larvae, with small and medium sized wing Anlagen, undergo another larval or pupal instar. Prothetelic C larvae with bigger Anlagen are unable to moult, but the adult programme is expressed. Ecdysed larval-pupal intermediates give more or less perfect adults, while unecdysed mealworms, imprisoned in their larval cuticle, also expressed the adult programme. The commitment of Tenebrio is not a global switchover because a significant asynchronisation is noted between the development of organs considered. Animal crowding induces a delay in the appearance of wing Anlagen.     

The expansion of Schistocerca gregaria at the imaginal ecdysis: The mechanical properties of the cuticle and the internal pressure

(1) At the imaginal ecdysis of Schistocerca, the cuticle is viscoelastic rather than elastic. (2) The cuticle becomes softer just before emergence. It is suggested that this is due to an ‘eclosion hormone’. The softening permits the extrication of the appendages and an increase in the size of the locust. (3) The stiffness of the cuticle increases transiently at the end of emergence. (4) In the later part of wing expansion, the cuticle becomes elastic and its stiffness again increases. (5) Maximum pressures are recorded about 10 min into emergence, and pressures in the gut are greater than those in the tracheal system. (6) From these results it is concluded that the expansion of the wings initially depends on the high haemolymph pressure and low stiffness. Only in the last 15 min of wing expansion do the processes involved in autonomous expansion become important.     

Structure of the green lacewing tympanal organ (Chrysopa carnea,neuroptera)

Small swellings near the base of the radial vein in each fore wing of the green lacewing, Chrysopa carnea, resemble typical insect tympanal organs, but some important differences are apparent. The swellings are bounded dorsally and laterally by thick cuticle and ventrally by thin, membranous cuticle. The ventral membrane is formed by a single, thin sheet of exocuticle with flattened hypodermis internally, but lacks the tracheal component that forms part of the tympanum in the typical insect tympanal organ. The portion of the membrane beneath each swelling is rippled while proximally it is smooth. In contrast to typical insect tympanal organs, the swellings in C. carnea are largely fluid-filled since an unexpanded trachea runs through each organ. A distal and a proximal chordotonal organ composed of typical chordotonal sensory units are associated with each swelling. The distal organ contains from five to seven units while the proximal organ is composed of from 18 to 20 units. Each sensory unit is composed of three readily identifiable cells. Distally, an attachment cell unites with the membrane and is contiguous with the scolopale cell, which surrounds the dendrite of the bipolar neuron. On the basis of the morphological evidence, one would not expect these swellings to function as sound receptors. However, the results of physiological and behavioral experiments, presented elsewhere, show that these organs are receptors for ultrasound.     

Navigation and thermophotovoltaic activity by hornets at different light conditions: the influence of UVB blockers

The aim of the present investigation was two-fold: a) to observe the homing of the Oriental hornet, Vespa orientalis (Hymenoptera, Vespinae) from different distances; and b) to study the photothermoelectric activity of hornet cuticle obtained from the subjects of goal (a) and kept frozen for a number of days prior to its testing. In both the above mentioned phases of the investigation, an attempt was made to assess how the covering of the hornets' cuticle with Ultra Violet B (UVB) blockers affects their activity as compared to the control. Flying hornets were observed to return to the nest from distances of up to 7 km, once they had learned the way back. However, covering of the cuticle with UVB blockers increases the percentage of 'non-returners' to nearly 100%. Covering the cuticle completely or partly with a number of UVB blockers (except for Sisley) proves lethal for the hornets within 24 hours. A statistical model on homing is proposed of the effect of range, of covering with UVB blockers and covering ocelli with Tippex. In the wing of the hornet there is increase in the electric current with rise in the temperature and decrease in the current upon drop of the temperature, but light has no effect on this alar (wing) current. Contrariwise, the body cuticle of the hornet responds to both temperature and illumination in terms of its electric current. Coating of the cuticle with UVB blockers causes in the wing (under all conditions of illumination) and in the cuticle (only in the dark) a moderation in the amplitude of the photothermoelectric current.     

Gene expression during Drosophila wing morphogenesis and differentiation

The simple cellular composition and array of distally pointing hairs has made the Drosophila wing a favored system for studying planar polarity and the coordination of cellular and tissue level morphogenesis. We carried out a gene expression screen to identify candidate genes that functioned in wing and wing hair morphogenesis. Pupal wing RNA was isolated from tissue prior to, during, and after hair growth and used to probe Affymetrix Drosophila gene chips. We identified 435 genes whose expression changed at least fivefold during this period and 1335 whose expression changed at least twofold. As a functional validation we chose 10 genes where genetic reagents existed but where there was little or no evidence for a wing phenotype. New phenotypes were found for 9 of these genes, providing functional validation for the collection of identified genes. Among the phenotypes seen were a delay in hair initiation, defects in hair maturation, defects in cuticle formation and pigmentation, and abnormal wing hair polarity. The collection of identified genes should be a valuable data set for future studies on hair and bristle morphogenesis, cuticle synthesis, and planar polarity.     

Mass isolation of cuticle protein cDNAs from wing discs of Bombyx mori and their characterizations

Multiple cloning of cuticle protein genes was performed by sequencing of cDNAs randomly selected from a cDNA library of wing discs just before pupation, and nine different cuticular protein genes were identified. Thirty-one clones of a cuticle protein gene were identified from the 1050 randomly sequenced clones; about 3% were cuticle protein genes in the W3-stage wing disc cDNA library. The sequence diversity of the deduced amino acid sequences of isolated Bombyx cuticle genes was examined along with the expression profiles. The deduced amino acid sequences of the nine cuticle protein genes contained a putative signal peptide at the N-terminal region and a very conserved hydrophilic region known as the R and R motif. The developmental expression of cuticle genes was classified into two types: pupation (five clones were expressed only around pupation) and pupation and mid-pupal (four clones were expressed around this stage). All the isolated genes were expressed in the head, thoracic, and abdominal regions of the epidermis at different levels around pupation, but no expression was observed in the epidermis at the fourth molting stage.