Formation and structure of egg envelopes in Russian sturgeon Acipenser gueldenstaedtii (Acipenseriformes: Acipenseridae)

The covering of the eggs in Russian sturgeon Acipenser gueldenstaedtii consists of three envelopes (the vitelline envelope, chorion and extrachorion) and is equipped with multiple micropyles. The most proximal to the oocyte is the vitelline envelope that consists of four layers of filamentous and trabecular material. The structural components of this envelope are synthesized by the oocyte (primary envelope). The chorion encloses the vitelline envelope. The extrachorion covers the external surface of the egg. Examination of the arrangement of layers that comprise the egg envelopes together with the ultrastructure of follicular cells revealed that the chorion and extrachorion are secondary envelopes. They are secreted by follicular cells and are built of homogeneous material. During formation of egg envelopes, the follicular cells gradually diversify into three morphologically different populations: 1) cells covering the animal oocyte region (cuboid), (2) main body cells (cylindrical) and (3) micropylar cells. The apical surfaces of follicular cells from the first two populations form processes that remain connected with the oocyte plasma membrane by means of gap junctions. Micropylar cells are located at the animal region of the oocyte. Their apical parts bear projections that form a barrier to the deposition of materials for egg envelopes, resulting in the formation of the micropylar canal.     

Micro analysis of the egg shell of Adela metallica (Poda) (Lepidoptera : Adelidae) by energy-filtering transmission electron microscopy (EFTEM)

The egg shell of the incurvarioid moth Adela metallica (Lepidoptera : Adelidae) was studied by conventional (cTEM) and energy-filtering transmission electron microscopy (EFTEM). The shell of the laid egg consists of 3 envelopes. The vitelline envelope is 0.1–0.2μm thick and homogeneous, thus exhibiting the non-exoporian character state. The single-layered chorion, which is covered by a fibrogranular mucous layer, is 0.5–0.9μm thick and homogeneous, thus exhibiting the non-ditrysian character state. The chorion is highly electron-lucent. Neither cTEM nor EFTEM revealed any sub-structural details. However, electron spectroscopic imaging (ESI) and electron energy-loss spectroscopy (EELS), revealing the elemental composition of the egg shell, indicate that the chorion and vitelline envelope are proteinaceous and hence, similar to the egg shells of other lepidopteran species. The presence of high sulphur signals associated with the vitelline envelope and the thin basal lamella of the chorion indicates that these components may be stabilized via sulphur-bridges.     

Eggshell fine structure of three lepidopteran pests: Cydia pomonella (L.) (Tortricidae), Heliothis virescens (Fabr.), and Spodoptera littoralis (Boisd.) (Noctuidae)

The eggshells of 3 moths, Cydia pomonella (Tortricidae), Heliothis virescens, and Spodoptera littoralis (Noctuidae) were investigated by scanning (SEM) and transmission (TEM) electron microscopy. The surface of the noctuid eggs shows structural elements (micropylar rosette, ribs, cross-ribs, and aeropyles) and regional differentiation, all typical of Lepidoptera. The egg of C. pomonella shows a different regional morphology due to its watch-glass shape and its position, lying on the flank. The micropylar structures are on the lower egg face in contact with the substrate. For S. littoralis, the surface structure (sculpturing) of the egg is not species-specific, being indistinguishable from that of S. frugiperda (Salkeld, 1984).In all 3 moths, the eggshell fine structure is basically identical, as revealed by TEM. Both the vitelline envelope and the chorion consist of several distinct layers. The vitelline envelope, bi-layered and several μm thick, undergoes a marked structural change when embryogenesis begins. At the same time, Golgi vesicles bearing dense particles, appear in the periplasm of the egg cell in fertilized eggs of H. virescens and S. littoralis. The chorion of all 3 species consists of a basal layer (C-1), a cavity layer (C-2) supported by trabecles and opening to the exterior via aeropylar canals, and a lamellar layer (C-3), which probably consists of helicoidally arranged stacks of fibrils. In H. virescens and S. littoralis, an additional epicuticle-like layer (C-4) is present. Available data from the literature are summarized and a basic scheme of the radial eggshell fine structure of ditrysian Lepidoptera is proposed.     

Morphology of the egg shell and the developing embryo of the Red Palm Weevil,Rhynchophorus ferrugineus (Oliver)

The harvested eggs of Rhynchophorus ferrugineus are ovo-cylindrical shaped, averaged 1.09 mm in length and 0.43 mm in width, with ratio of L\W 4.42. The chorionic layer of electron dense material is seen covering the exochorion structure of the eggs. The egg main body chorion exhibits a polygonal pattern and architecture surface of the egg is supported by a system of irregular interconnecting grooves. The micropylar apparatus of the eggs of the Red Palm Weevil, R. ferrugineus is described in the present study for the first time. Two micropylar openings are found closed to the center of the posterior wide pole of the egg. Each micropylar opening presents a single small orifice and its surrounding chorion is porous and densely set with tiny projections allowing the spermatozoa to penetrate the egg. Respiratory aeropyles are distributed on the borders of reticulations in the area chorionic surface of egg capsule. The hatching region is detected on the anterior part at the opposite side of the egg. Changes in the appearance and shape of R. ferrugineus eggs as well as the incidence of embryonic development are observed.     

Eggshell fine structure of Bradysia aprica (Winnertz) (Diptera : Sciaridae)

The eggshell fine structure of the dark-winged fungus-gnat Bradysia aprica (Winnertz) (Diptera : Sciaridae) was investigated by scanning and transmission electron microscopy. At the anterior pole of the ovoid egg is a single micropyle, centrally located in a well-defined micropylar area. The latter is covered by many long drumstick-like chorionic processes that are longer and more numerous than those of the rest of the egg surface. Cross-sections of the eggshell show 3 concentric envelopes: the vitelline envelope, wax layer and chorion. The chorion consists of 3 components with different morphological features: the inner, intermediate and outer chorion. The latter 2 layers, involved in the organization of the drumstick-like processes, have homogeneous features, whereas the former is crystalline and resembles the innermost chorionic layer of other Diptera.     

Surface morphology of eggs of Euproctis chrysorrhoea (Linnaeus, 1758)

Candan, S., Suludere, Z. and Bayrakdar, F. 2007. Surface morphology of eggs of Euproctis chrysorrhoea (Linnaeus, 1758). —Acta Zoologica (Stockholm) 88 : 000–000. Filaments covering the egg batches and chorion structure were studied both by light and scanning electron microscopy in the brown‐tailed moth Euproctis chrysorrhoea (Linnaeus, 1758). Females lay eggs in masses on the underside of apple leaves. The egg batches are covered with brown hairs derived from the bodies of the female. Each female lays about 200–400 eggs. The spherical eggs are about 0.84 mm long and 0.47 mm wide. Newly deposited eggs are golden‐yellow and darken after the onset of embryonic development. The micropylar area appears somewhat depressed and has a circular outline. The region is surrounded by a rosette of 10–12 petal‐shaped primary cells, which are completely surrounded by a series of secondary and tertiary cells. The remainder of the egg is largely smooth, but shows aeropyles. These are located in the corners of ill‐defined polygons.     

Fine structure of the eggshell of Ommatissus binotatus Fieber (Homoptera,Auchenorrhyncha, Tropiduchidae)

The external morphology and fine structure of the eggshell of Ommatissus binotatus Fieber (Homoptera : Tropiduchidae) was investigated by light, scanning and transmission electron microscopy. The egg surface has 2 main regions: a specialized area and an unspecialized egg capsule. The specialized area is characterized by a large respiratory plate containing the operculum and a short respiratory horn. The latter consists of an external hollow tube and an internal coneshaped projection hosting a micropylar canal. The eggshell has 4 layers: the vitelline envelope, a wax layer, the chorion and an outer mucous layer. The chorion has inner, intermediate and outer parts. The functions of the different parts of the eggshell are discussed. Characters useful to define the eggs and the oviposition habit in the family Tropiduchidae were provided. The size and morphology of the egg, plate, respiratory horn and operculum are suggested as useful characters for ootaxonomic analysis.     

Ultrastructural features and formation of the micropylar apparatus in the cherry fly Rhagoletis cerasi

The micropylar apparatus (MA) in Rhagoletis cerasi (Diptera, Tephritidae) is located at the anterior pole of the egg and consists of two parts: an outer chorion and an inner vitelline membrane. Sperm entry takes place through the micropylar canal, 2.0–2.5 μm in diameter, which penetrates the micropylar endochorion and terminates in the thick vitelline membrane, thus forming the “pocket.” The pore of the micropylar canal, i.e., the micropyle, is covered by the exochorionic tuft. The formation of the MA is accomplished by 40 micropylar cells during oogenesis. These cells secrete the successive eggshell layers: the vitelline membrane, the wax layer, the innermost chorionic layer, the micropylar endochorion, and the exochorion. Two among 40 micropylar cells differentiate and form two tightly connected projections. The latter contain a bundle of parallel microtubules and participate in the formation of the micropylar canal and the pocket. At the tip of the projections there are two thin extensions full of microfilaments. In late developmental stages the two projections and the extensions degenerate and leave the canal and the pocket behind. We also discuss the structural features of the MA in relation to its physiology among Diptera.     

Egg structure of Zorotypus caudelli Karny (Insecta, Zoraptera, Zorotypidae)

The structural features of eggs of Zorotypus caudelli Karny are described in detail. The egg is elliptic with long and short diameters of 0.6 and 0.3 mm respectively, and creamy white. The egg shows a honeycomb pattern on its surface, without any specialized structures for hatching such as an operculum or a hatching line. The fringe formed by a fibrillar substance secreted after the completion of the chorion encircles the lateral surface. The egg layer is composed of an exochorion, an endochorion, and a vitelline envelope. The exochorion and endochorion are electron-dense and homogeneous in structure. The exochorion shows a perforation of numerous branching aeropyles. The exo- and endochorion are connected by numerous small columnar structures derived from the latter. The vitelline envelope is very thin and more electron-dense than the chorion. A pair of micropyles is present at the equator on the dorsal side of the egg. Originating at the micropyle, the micropylar canal runs through the chorion obliquely. The structural features of the eggs of Zoraptera were compared with those of other polyneopteran and paraneopteran orders.     

The eggshell of the almond wasp Eurytoma amygdali (Hymenoptera, Eurytomidae) - 2. The micropylar appendage

Micropylar apparatuses in insects are specialized regions of the eggshell through which sperm enters the oocyte. This work is an ultrastructural study and deals with the structure and morphogenesis of the micropylar appendage in the hymenopteran Eurytoma amygdali. The micropylar appendage is a 130 mum long cylindrical protrusion located at the posterior pole of the egg, unlike other insects i.e. Diptera. in which the micropylar apparatus is located at the anterior pole. In mature eggs there is a 0.4 mum wide pore (micropyle) at the tip of the appendage leading to a 6 mum wide micropylar canal. The canal contains an electron-lucent substance, it travels along the whole appendage and finally reaches the vitelline membrane of the oocyte. The vitelline membrane is covered by a wax layer and an electron-lucent layer, whereas the chorion surrounding the canal consists of a granular layer (fine and rough) and a columnar layer. The morphogenesis of the appendage starts in immature follicles: four central cells located at the posterior tip of the oocyte near the vitelline membrane, differing morphologically from the adjacent follicle cells. These central cells degenerate during early chorionic stages, thus assisting in the formation of the micropylar canal. The adjacent, peripherally located cells secrete the electron-lucent substance which fills the canal and at the same time, the fine granular layer is formed starting from the base towards the tip of the appendage. The secretion persists at late chorionic stages and results in the formation of the chorion around the micropylar canal. The extremely long (compared to other insects) micropylar appendage seems to facilitate the egg passage through the very thin and long ovipositor. The structure and morphogenesis of this appendage differs significantly from the micropylar apparatuses studied so far in other insects i.e. Diptera, and may reflect adaptational and evolutionary relationships.     

Surface ultrastructure of the egg chorion in the dragonfly,Ictinogomphus rapax (Rambur) (Odonata : Gomphidae)

Scanning electron microscopic examination of the egg chorion in Ictinogomphus rapax (Rambur) (Odonata : Gomphidae) shows hexagonal reticulation throughout the surface. The anterior pole of the egg bears a small rounded micropylar stalk with a group of 6 orifices arranged radially around a central boss, while the posterior pole consists of a sessile, truncated cone formed of 50–60 long, coiled filaments. The functional, taxonomic, and phylogenetic significance of various microstructures of the eggshell are discussed.     

Egg chorion architecture in stick insects (Phasmatodea)

Comparative analysis of egg chorion architecture by scanning and transmission electron microscopy is reported in about 50 species of stick insects (Phasmatodea). Particular attention has been paid to: (1) synthesis and structure of egg shell layers; (2) egg shape; (3) morphology of the external chorionic surface; (4) position and structure of the micropylar plate and its cup; (5) morphology and details of the operculum, including capitulum or pseudocapitulum; and (6) posterior pole differentiation (the so-called polar mound), The taxonomic value of the various characters is discussed: some are clearly species-specific, while others (such as general egg shape and micropylar plate) appear to reflect phylogenetic relationships of higher rank. Intraspecific features, such as the fine chorionic and opercular patterns of Bacillus and Clonopsis, have been recognized.In natural hybrids, egg chorion architectures were related to that of the parent species, resembling one of the parents in some cases and being intermediate between the 2. The study of the Phasmatodea egg can provide much taxonomic information that is useful in the definition of natural groups.     

Fine structure of micropylar region during late oogenesis in eggs of the hagfish Eptatretus burgeri (Agnatha)

Using the hagfish, Eptatretus burgeri, the fine structure of formation of the micropylar region in hagfish eggs during the late stages of oogenesis was investigated for the first time, focusing on the bottom region of the micropyle and the egg surface. During these stages, many cells penetrated through the chorion and reached a pit of the egg surface, forming a shovel-like structure in two-dimensional sections. The cells, which we called micropylar cells, were separated from the chorion by a wall of amorphous material. In the pit, another fibrous layer filled the space between the egg surface and the anterior portion of the shovel-like structure. Microvilli coming from the egg surface were embedded in this layer. In later stages, the stack of micropylar cells loosened, and a space appeared between the anterior region of the shovel-like structure and the layer on the egg surface. Microvilli decreased in length and number. The pit region appeared likely to have a role in fertilization. The structures associated with the forming micropyle were markedly different from those observed in the same region of teleost fishes. A hypothesis that hagfish might show transitional structures in gametes from protochordates to teleosts is suggested.     

The role of the follicle cells during oogenesis in the chiton Sypharochiton septentriones (Ashby) (Polyplacaphora,Mollusca)

Summary Histochemical studies and electron microscopic investigations on the role of the follicle cells during oogenesis in the chiton Sypharochiton septentriones showed that the main role of the follicle cells was the deposition of a spiny chorion around each oocyte. The chorion was composed of three layers; an inner, acid mucopolysaccharide layer, which was a primary egg membrane secreted by Golgi bodies in the cortical cytoplasm of the oocyte, an intermediate layer of protein and an outer layer of lipid. The intermediate and outer layers were secreted by the follicle cells and were thus secondary egg membranes.     

The structure and surface properties of the eggshell of Megaselia imitatrix Borgmeier (Diptera, Phoridae) in relation to the respiration of the embryo

The role of the chorion in the mechanism of respiration is examined for the egg of Megaselia imitatrix. An air layer in the chorion is not essential for respiration, and oxygen can be absorbed through a hydrophilic pathway in the chorion by diffusion from the surrounding water. A model based on the diffusion of oxygen through the surface layer of the water down to the depth of the egg is proposed and presented in detail.     

Formation and ultrastructure of the micropylar apparatus in Bombyx mori ovarian follicles

The formation of the micropylar apparatus during oogenesis in the silkworm, Bombyx mori, has been studied using light and transmission electron microscopy. The micropylar apparatus is formed by three types of cells: the micropylar channel-forming cells (MCFCs), the micropylar orifice-forming cells (MOFCs), and the micropylar rosette-forming cells (MRFCs). During the formation of the vitelline membrane and the chorion, each of the MCFCs extends a cytoplasmic projection serving as the mold of a micropylar-channel into the egg envelopes. The detachment and collapse of the projections takes place at the end of choriogenesis. The micropylar channels possess a common external orifice on the chorion and several internal orifices within the vitelline membrane. The MOFCs interact closely with the MCFCs and contribute to the formation of the external micropylar orifice. A petal-like rosette surrounding the orifice is imprinted on the outer chorionic surface by the MRFCs which enclose a group of the MCFCs and MOFCs.     

Scanning electron microscopy of the fetal membranes of an oviparous squamate,the corn snake Pituophis guttatus (Colubridae)

Although the fetal membranes of viviparous squamates have received much study, morphology of their homologues among oviparous reptiles is poorly understood. The scarcity of information about these membranes in egg‐laying reptiles hampers attempts to distinguish specializations for viviparity from ancestral oviparous features. We used scanning electron microscopy to examine fetal membranes of an oviparous snake (Pituophis guttatus) throughout the developmental period from oviposition to hatching. The external surface of the chorion contains broad, flattened cells that lack surface features; these cells form a continuous layer over the allantoic capillaries and offer a minimal barrier to respiratory exchange. In contrast, the surface epithelium of the omphalopleure bears elaborate surface ridges suggestive of absorptive capabilities. These ridges are prominent in the first few weeks after oviposition, but diminish thereafter. During development, the isolated yolk mass (IYM) of the omphalopleure becomes depleted, and the tissue becomes heavily vascularized by allantoic vessels. Surface features of the omphalopleure progressively take on the appearance of the chorioallantois, but the changes are not synchronous with loss of the IYM or membrane vascularization. Previous studies on viviparous snakes suggest that the chorioallantois and omphalopleure are respectively specialized for gas exchange and absorption in the intrauterine environment. Our studies of fetal membranes in P. guttatus offer evidence that cytological specializations for these functions originated under oviparous conditions, reflecting functional capacities that predate viviparity. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Oviposition behaviour in Paysandisia archon (Lepidoptera: Castniidae)

Paysandisia archon (Burmeister, 1880) (Lepidoptera: Castniidae) is a South American insect developing on palms and recently introduced in Europe where it damages most palm species. Understanding the oviposition behaviour would be decisive for risk assessment and pest management but key points on oviposition behaviour are missing. Using wind tunnel and field experiments, we investigated the oviposition timing, the attraction behaviour of mated females to palms and the different steps of oviposition behaviour. Results showed that oviposition behaviour occurred between 3 pm and 6 pm. In the field, gravid females were significantly more attracted by the palm crown than virgin females. The ovipositing females exhibited distinctive steps before ovipositing. Subsequent to alighting on the crown, pre-oviposition behaviour was characterized by two main behavioural steps: walking and probing the surface with antennae and ovipositor. After the choice of oviposition place, the gravid female remains motionless and the extendible ovipositor is deeply introduced into the upper fibrous part of the crown. About 10 eggs can be lays at the same place. Finally, the female starts to walk again and reinitiates the same behavioural sequences. This paper supports the hypothesis that odours from the crown may play a key role in gravid female attraction. The study assessed that P. archon lays on the palm crown, the part of the palm that should be treated for population monitoring.     

The invasive moth Paysandisia archon in Europe: Biology and control options

The palm borer moth (Paysandisia archon, Burmeister) is a member of the Castniidae family originally from South America and is currently included in the A2 list of the OEPP/EPPO. This moth was introduced to Europe in 2000 through ornamental palms. Since its accidental introduction, it has become a major threat for natural stands of native palms, as well as for nurseries and gardens in the Mediterranean basin. To date, neither preventive nor control methods have been implemented for managing this pest under field conditions. In this review, we highlight the most relevant information on the biology of P. archon and summarize the available control strategies with a special focus on biocontrol-based treatments.