Ultrastructure of the single-chamber stemmata of Arge pagana (Panzer, 1798) (Hymenoptera: Argidae)

Stemmata are peculiar visual organs of most larvae in holometabolous insects. In Hymenoptera, Symphyta larvae exclusively possess a pair of stemmata, whose cellular organizations have not been thoroughly elucidated to date. In this paper, the morphology and fine structure of stemmata were investigated in the large rose sawfly Arge pagana (Panzer, 1798) using light and electron microscopy. The larvae possess a pair of stemmata, which belong to the “unicorneal composite eye” or single-chamber stemmata. Each stemma is composed of a biconvex cornea lens, a layer of corneagenous cells, numerous pigment cells, and hundreds of retinula cells. According to the number of retinula cells forming a rhabdom, the stemma can be divided into two regions, the larger Region I and the smaller Region II. The former occupies the largest area of the stemma and contains the majority of rhabdoms, each of which is formed by the rhabdomeres of eight retinula cells. The latter occupies a narrow posterior margin, where each rhabdom consists of nine retinula cells. Based on the different cellular organizations of rhabdoms, the stemma of Argidae is likely developed by the fusion of two types of ommatidial units.     

Phylogeny of the Myriapoda – Crustacea – Insecta: a new attempt using photoreceptor structure*

According to molecular sequence data Crustacea and not Myriapoda seem to be the sister‐group to Insecta. This makes it necessary to reconsider how the morphology of their eyes fit with these new cladograms. Homology of facetted eye structures in Insecta (Hexapoda in the sense of Ento‐ and Ectognatha) and Crustacea is clearly supported by identical numbers of cells in an ommatidium (two corneageneous or primary pigment cells, four Semper cells which build the crystalline cone and primarily eight retinula cells). These cell numbers are retained even when great functional modification occurs, especially in the region of the dioptric apparatus. There are two different possibilities to explain differences in eye structure in Myriapoda depending on their phylogenetic position in the cladogram of Mandibulata. In the traditional Tracheata cladogram, eyes of Myriapoda must be secondarily modified. This modification can be explained using the different evolutionary pathways of insect facetted eyes to insect larval eyes (stemmata) as an analogous model system. Comparative morphology of larval insect eyes from all holometabolan orders shows that there are several evolutionary pathways which have led to different types of stemmata and that the process always involved the breaking up the compound eye into individual larval ommatidia. Further evolution led on many occasions to so‐called fusion‐stemmata that occur convergently in each holometabolic order and reveals, in part, great structural similarities to the lateral ocelli of myriapods. As myriapodan eyes cannot be regarded as typical mandibulate ommatidia, their structure can be explained as a modified complex eye evolved in a comparable way to the development to the fusion‐stemmata of insect larvae. The facetted eyes of Scutigera (Myriapoda, Chilopoda) must be considered as secondarily reorganized lateral myriapodan stemmata, the so‐called ‘pseudo‐compound eyes’. New is a crystalline cone‐like vitreous body within the dioptric apparatus. In the new cladogram with Crustacea and Insecta as sister‐groups however, the facetted eyes of Scutigera can be interpreted as an old precursor of the Crustacea – Insecta facetted eye with modified ommatidia having a four‐part crystalline cone, etc. as a synapomorphy. Lateral ocelli of all the other Myriapoda are then modified like insect stemmata. The precursor is then the Scutigera‐Ommatidium. In addition further interpretations of evolutionary pathways of myriapodan morphological characters are discussed.     

The Larval Eye of Nannochoristid Scorpionflies (Insecta,Mecoptera)

Abstract The stemmata of last–instar Nannochoristalarvae are compound eyes composed of 10 or more ommatidia. Each ommatidium has four Semper cells, four distal and four proximal retinula cells which form a cruciform and layered rhabdom. The ommatidia are separated by epidermal cells (possibly rudimentary pigment cells). Corneal lenses are lacking. At the posterior edge, aberrant stemma units may be present which lack a dioptric apparatus and have a star–shaped rhabdom composed of at least six retinula cells. The stemmata of Nannochoristaappear to be derived from stemmata of the Panorpa-type (Mecoptera-Panorpidae). Differences between the stemmata of Nannochoristaand Panorpacan be explained as adaptations to aquatic life (flat cornea) or as regression. A compound larval eye is ascribed to the ground plan of the Mecoptera sensu latoand is considered a genuine plesiomorphy. The identical basic number (seven) of stemmata in the Neuropteroid/Coleoptera assemblage, Amphiesmenoptera and some Mecoptera (Bittacidae, Boreidae) is attributed to parallel evolution.     

Post-larval development of compound eyes and stemmata of Chaoborus crystallinus (De Geer, 1776) (Diptera : Chaoboridae): Stage-specific reconstructions within individual organs of vision

During metamorphosis, the dioptric apparatus of the larval compound eye of Chaoborus crystallinus (Diptera : Nematocera) is radically reconstructed. The thin larval cornea of the ommatidia is replaced by strongly curved corneal lenses, and the eucone larval cone is replaced by an imaginal cone of the acone type. Curvature of the future lens is already apparent in very young pupae, in which the cornea consists only of a thin epicuticle with corneal nipples. Fibrillary cuticle is secreted by cone and primary pigment cells throughout pupal development. Lens formation is accompanied by movement of the nuclei of the accessory pigment cells. The larval cone disintegrates unexpectedly late in young, images. During late pupal development, 7 cone cell projections emerge. In contrast to the dioptric apparatus, the retinula cells and rhabdom remain almost unchanged during metamorphosis. The main refractive element of the larval ommatidium appears to be the cone, while that of the imaginal ommatidium is the corneal lens. In addition to the compound eyes, the pairs of stemmata are retained during the whole post-larval development. Pupal stemmata show no structural differences from the larval stemmata. The stemmata are still present in 2-day-old images (“retained stemmata”), but the primary stemma loses its dioptric apparatus and is proximally relocated to the basal region of the compound eye. The reconstructions in the visual system of Chaoborus, which occur during ontogeny, are probably connected with the change from aquatic living larvae to aerial adults, and appear to fulfill stage-specific needs of vision.     

Fine structural organization of the lateral ocelli in two species of Scolopendra (Chilopoda: Pleurostigmophora): an evolutionary evaluation

The lateral ocelli of Scolopendra cingulata and Scolopendra oraniensis were examined by electron microscopy. A pigmented ocellar field with four eyes arranged in a rhomboid configuration is present frontolaterally on both sides of the head. Each lateral ocellus is cup-shaped and consists of a deeply set biconvex corneal lens, which is formed by 230–2,240 cornea-secreting epithelial cells. A crystalline cone is not developed. Two kinds of photoreceptive cells are present in the retinula. 561–1,026 cylindrical retinula cells with circumapically developed microvilli form a large distal rhabdom. Arranged in 13–18 horizontal rings, the distal retinula cells display a multilayered appearance. Each cell layer forms an axial ring of maximally 75 rhabdomeres. In addition, 71–127 club-shaped proximal retinula cells make up uni- or bidirectional rhabdomeres, whose microvilli interdigitate. 150–250 sheath cells are located at the periphery of the eye. Radial sheath cell processes encompass the soma of all retinula cells. Outside the eye cup there are several thin layers of external pigment cells, which not only ensheath the ocelli but also underlie the entire ocellar field, causing its darkly pigmented. The cornea-secreting epithelial cells, sheath cells and external pigment cells form a part of the basal matrix extending around the entire eye cup. Scolopendromorph lateral ocelli differ remarkably with respect to the eyes of other chilopods. The dual type retinula in scolopendromorph eyes supports the hypothesis of its homology with scutigeromorph ommatidia. Other features (e.g. cup-shaped profile of the eye, horizontally multilayered distal retinula cells, interdigitating proximal rhabdomeres, lack of a crystalline cone, presence of external pigment and sheath cells enveloping the entire retinula) do not have any equivalents in scutigeromorph ommatidia and would, therefore, not directly support homology. In fact, most of them (except the external pigment cells) might be interpreted as autapomorphies defining the Pleurostigmophora. Certain structures (e.g. sheath cells, interdigitating proximal rhabdomeres, discontinuous layer of cornea-secreting epithelial cells) are similar to those found in some lithobiid ocelli (e.g. Lithobius). The external pigment cells in Scolopendra species, however, must presently be regarded as an autapomorphy of the Scolopendromorpha.     

Visual behaviour and the structure of dark and light-adapted larval and adult eyes of the New Zealand glowworm Arachnocampa luminosa (Mycetophilidae: Diptera)

Both larval and adult New Zealand cave glowworms exhibit reactions to light; their photoreceptors must, therefore, be regarded as functional. The two principal stemmata of the larva possess large biconvex lenses and voluminous rhabdoms. Approximately 12 retinula cells are present. In light-adapted larvae the diameter of the rhabdom is 8 μm and that of an individual microvillus is 49.5 nm. Dark-adapted eyes have rhabdoms that measure 14 μm in cross section and microvilli with an average diameter of 54 nm. The compound eye of the adult comprises approximately 750 ommatidia, each with a facet diameter of 27–28 μm. A facet is surrounded by 1–6 interommatidial hairs which are up to 30 μm long. The interommatidial angle is 5.5°. Cones, consisting of 4 crystalline cone cells, are of the ‘acone’ type. Pigment granules in the primary pigment cells are twice as large as those of the retinula cells which measure 0.6–0.75 μm in diameter. The rhabdom is basically of the dipteran type, i.e. six open peripheral rhabdomeres surround 2 central rhabdomers arranged in a tandem position. The microvilli of cells 1–6 and cell 8 have diameters ranging from 68 to 73 nm, but those of the distally-located central rhabdomere 7 are 20% larger. This is irrespective of whether the eye is dark or light-adapted. In the latter the cones are long and narrow, the screening pigment granules closely surround the rhabdomeres, and the rhabdom is less voluminous than that of the dark-adapted eye.     

The compound eye of<Emphasis Type="Italic"> Scutigera coleoptrata</Emphasis> (Linnaeus, 1758) (Chilopoda: Notostigmophora): an ultrastructural reinvestigation that adds support to the Mandibulata concept

The lateral compound eye of Scutigera coleoptrata was examined by electron microscopy. Each ommatidium consists of a dioptric apparatus, formed by a cornea and a multipartite eucone crystalline cone, a bilayered retinula and a surrounding sheath of primary pigment and interommatidial pigment cells. With reference to the median eye region, each cone is made up of eight cone segments belonging to four cone cells. The nuclei of the cone cells are located proximally outside the cone near the transition area between distal and proximal retinula cells. The connection between nuclear region and cone segment is via a narrow cytoplasmic strand, which splits into two distal cytoplasmic processes. Additionally, from the nuclear region of each cone cell a single cytoplasmic process runs in a proximal direction to the basement membrane. The bilayered rhabdom is usually made up of the rhabdomeres of 9–12 distal retinula cells and four proximal retinula cell. The pigment shield is composed of primary pigment cells (which most likely secrete the corneal lens) and interommatidial pigment cells. The primary pigment cells underlie the cornea and surround, more or less, the upper third of the crystalline cone. By giving rise to the cornea and by functioning as part of the pigment shield these pigment cells serve a double function. Interommatidial pigment cells extend from the cornea to the basement membrane and stabilise the ommatidium. In particular, the presence of cone cells, primary pigment cells as well as interommatidial pigment cells in the compound eye of S. coleoptrata is seen as an important morphological support for the Mandibulata concept. Furthermore, the phylogenetic significance of these cell types is discussed with respect to the Tetraconata.     

Ultrastructure of the larval stemmata,the stemmata nerves and the optic neuropils of the larval cotton bollworm,Heliothis armigera (Hübner) (Lepidoptera : Noctuidae)

Ultrastructure of stemmata (larval eyes), stemmatal nerves, and the optic neuropils of 5th-instar larvae of cotton bollworm, Heliothis armigera (Hübner) (Lepidoptera : Noctuidae), were examined with scanning and transmission electron microscopes. Six stemmata are on each side of the head. Each stemma consists of 7 retinula cells arranged into 2 tiers. Stemmata I and II have 4 distal retinula cells and 3 proximal cells, the other 4 stemmata (III–IV) have 3 distal cells and 4 proximal cells. Stemmata I and IV have a short proximal rhabdom and the rhabdomere of each proximal cell has its microvilli projecting in only one direction. On the other hand, each stemma (in stemmata II–V) has a long proximal rhabdom and the rhabdomere of each proximal cell has microvilli pitched in several different directions relative to the horizontal plane. An axon projects proximally from each retinula cell body. The stemmatal nerve is composed of the 42 retinular axons from all of the 6 stemmata on the same side of the head. Each stemmatal nerve projects to the ipsilateral optic neuropil. Axons from each stemma are in a fasicle (within the stemmatal nerve), which consists of 7 axons, 3–4 of them are thick and terminate synaptically in the proximal neuropil; the others are thinner and terminate in the distal neuropil. Organelles, particularly lysosomes, undergo ultrastructural transformations relative to ambient light levels. The functional significance of abovementioned structures are discussed in light of current knowledge.     

The retina of the phalangid,Opilio ravennae,with particular reference to arhabdomeric cells

Summary The retina of the phalangid, Opilio ravennae, consists of retinula cells with distal rhabdomeres, arhabdomeric cells, and sheath cells. The receptive segment of retinula cells shows a clear separation into a Proximal rhabdom, organized into distinct rhabdom units formed by three or four retinula cells, and a Distal rhabdom, consisting of an uniterrupted layer of contiguous rhabdomeres. One of the cells comprising a retinula unit, the so-called distal retinula cell (DRC), has two or three branches that pass laterally alongside the rhabdom, thereby separating the two or three principal retinula cells of a unit. The two morphologically distinct layers of the receptive segment differ with respect to the cellular origin of rhabdomeral microvilli: DRC-branches contribute very few microvilli to the proximal rhabdom and develop extremely large rhabdomeres in the distal rhabdom only, causing the rhabdom units to fuse. Principal retinula cells, on the other hand, comprise the majority of microvilli of the proximal rhabdom, but their rhabdomeres diminish in the distal rhabdom. It is argued that proximal and distal rhabdoms serve different functions in relation to the intensity of incident light.In animals fixed 4 h after sunset, pigment granules retreat from the distal two thirds of the receptive segment. A comparison of retinae of day- and night-adapted animals shows that there is a slight (approximately 15%) increase in the cross-sectional area of rhabdomeral microvilli in dark-adapted animals, which in volume corresponds to the loss of pigment granules from the receptive segment. The length of the receptive segment as well as the pattern and shape of rhabdom units, however, remain unchanged.Each retinula unit is associated with one arhabdomeric cell. Their cell bodies are located close to those of retinula cells, but are much smaller and do not contain pigment granules. The most remarkable feature is a long, slender distal dendrite that extends up to the base of the fused rhabdom where it increases in diameter and develops a number of lateral processes interdigitating with microvilli of the rhabdom. The most distal dendrite portion extends through the center of the fused rhabdom and has again a smooth outline. All dendrites end in the distal third of the proximal rhabdom and are never present in the layer of the contiguous distal rhabdom. Arhabdomeric cells are of essentially the same morphology in day- and night-adapted animals. They are interpreted as photoinsensitive secondary neurons involved in visual information-processing that channel current collected from retinula cells of the proximal rhabdom along the optic nerve. A comparison is made with morphological equivalents of these cells in other chelicerate species.     

Larval and adult eye of the Western Rock Lobster (Panulirus longipes)

A number of differences exists between the compound eyes of larval and adult rock lobsters, Panulirus longipes. The larval eye more closely resembles the apposition type of compound eye, in which retinula cells and rhabdom lie immediately below the cone cells. The adult eye, on the other hand, is a typical clear-zone photoreceptor in which cones and retinula cell layers are separated by a wide transparent region. The rhabdom of the larval eye, if cut longitudinally, exhibits a "banded" structure over its entire length; in the adult the banded part is confined to the distal end, and the rhabdom is tiered. Both eyes have in common an eighth, distally-located retinula cell, which possesses orthogonally-oriented microvilli, and a peculiar lens-shaped "crystal", which appears to focus light onto the narrow column of the distal rhabdom. Migration of screening pigment on dark-light adaptation is accompanied by changes in sensitivity and resolution of the eye. Retinula cells belonging to one ommatidium do not arrange into one single bundle of axons, but interweave with axons of four neighbouring facets in an extraordinarily regular fashion.     

Ultrastructure and central projections of extraocular photoreceptors in caddiesflies (Insecta: Trichoptera)

Summary Retained larval eyes (stemmata) were studied in the imagines of three species of Trichoptera: Phrygania grandis, Agrypnia varia, and Trichostegia minor. At the light-microscopic level the stemmata of all three species appeared to represent different stages of reduction with respect to size, shape and number of lenses. However, in all three species electron-microscopic studies showed units with monolayered rhabdoms, each formed by four retinula cells. By use of immunocytochemistry the presence of S-antigen was demonstrated in the retinula cells and their axons. This method also revealed the central projections of the axons of the retinula cells, which were found (i) to terminate either in the lamina accessoria or (ii) to penetrate this area to join the fibers of the outer chiasma of the optic lobes and then terminate in the medulla accessoria. The lamina accessoria and the medulla accessoria are the assumed remnants of the larval optic lobes. It is suggested that the imaginal stemmata might still be functioning photoreceptors.     

The compound eye of Parnara guttata (Insecta,Lepidoptera, Hesperiidae): Fine structure of the ommatidium

Summary The fine structure of an ommatidium of a skipper butterfly, Parnara guttata, has been studied using the electron microscope. Each ommatidium has nine retinula cells, which were classified into three groups: two distal, six medial and one basal retinula cells. The rhabdomeres of the distal retinula cells are localized in the distal part of the rhabdom, while those of the six medial retinula cells appear throughout most of the rhabdom. The rhabdomere of the basal retinula cell occupies only the basal part of the rhabdom. The rhabdomeres of four medial cells are constructed of parallel microvilli, while fan-like microvilli form the rhabdomeres of other two medial retinula cells. The distal and basal retinula cells have rhabdomeres consisting of both parallel and fan-like microvilli. This is the first time the construction of the rhabdomeres of the distal and basal retinula cells has been described in such fine detail for a skipper butterfly. Nine retinula cell axons of each ommatidium extend to the first neuropile of the optic lobe, the lamina ganglionaris. No difference was found in the number of retinula cells of an ommatidium or the shape of the rhabdom between the dorsal and ventral regions of the compound eye.     

The microanatomy of the compound eye of Munida irrasa (Decapoda: Galatheidae)

The compound eye of Munida irrasa differs in several respects from the typical decapod eye. The proximal pigment is found only in retinula cells. The eccentric cell is extremely large and expanded to fill the interstices of the crystalline tract area; thus, a typical "clear-zone" is absent. Six retinula cells course distally to screen two sides of the crystalline cone. There are approximately 12,500 ommatidia in each compound eye. There are several similarities to the typical decapod eye. Each ommatidium is composed of a typical cornea, corneagenous cells, crystalline cone cells, crystalline cone, crystalline cone tract and eight retinula cells. Distal pigment cells are present and surround the crystalline cone. The distal processes of the retinula cells also contain pigment. The retinula cell processes penetrate the basement membrane as fascicles composed of processes from adjacent retinulae.     

Fine structural description of the lateral ocellus of Craterostigmus tasmanianus Pocock, 1902 (Chilopoda: Craterostigmomorpha) and phylogenetic considerations

The lateral lens eye of adult Craterostigmus tasmanianus Pocock, 1902 (a centipede from Australia and New Zealand) was examined by light and electron microscopy. An elliptical, bipartite eye is located frontolaterally on either side of the head. The nearly circular posterior part of the eye is characterized by a plano-convex cornea, whereas no corneal elevation is visible in the crescentic anterior part. The so-called lateral ocellus appears cup-shaped in longitudinal section and includes a flattened corneal lens comprising a homogeneous and pigmentless epithelium of cornea-secreting cells. The retinula consists of two kinds of photoreceptive cells. The distribution of the distal retinula cells is highly irregular. Variable numbers of cells are grouped together in multilayered, thread-like unions extending from the ventral and dorsal margins into the center of the eye. Around their knob-like or bilobed apices the distal retinula cells give rise to fused polymorphic rhabdomeres. Both everse and inverse cells occur in the distal retinula. Smaller, club-shaped proximal retinula cells are present in the second (limited to the peripheral region) and proximal third of the eye, where they are arranged in dual cell units. In its apical region each unit produces a small, unidirectional rhabdom of interdigitating microvilli. All retinula cells are surrounded by numerous sheath cells. A thin basal lamina covers the whole eye cup, which, together with the distal part of the optic nerve, is wrapped by external pigment cells filled with granules of varying osmiophily. The eye of C. tasmanianus seemingly displays very high complexity compared to many other hitherto studied euarthropod eyes. Besides the complex arrangement of the entire retinula, the presence of a bipartite eye cup, intraocellar exocrine glands, inverse retinula cells, distal retinula cells with bilobed apices, separated pairs of proximal retinula cells, medio-retinal axon bundles, and the formation of a vertically partitioned, antler-like distal rhabdom represent apomorphies of the craterostigmomorph eye. These characters therefore collectively underline the separate position of the Craterostigmomorpha among pleurostigmophoran centipedes. The remaining retinal features of C. tasmanianus agree with those known from other chilopod eyes and, thus, may be considered plesiomorphies. Characters like the unicorneal eye cup, sheath cells, and proximal rhabdomeres with interdigitating microvilli were already present in the ground pattern of the Pleurostigmophora. Other retinal features were developed in the ancestral lineage of the Phylactometria (e.g., large elliptical eyes, external pigment cells, polygonal sculpturations on the corneal surface). The homology of all chilopod eyes (including Notostigmophora) is based principally on the possession of a dual type retinula.     

The eye of Dytiscus (Coleoptera)

The eye of Dytiscus (Coleoptera) has rhabdomeres at three different levels. The crystalline threads stretch from the ends of the crystalline cones only as far as the distal layer of rhabdomeres. There is one distal rhabdo-mere per ommatidium, and in this system the ommatidia are anatomically separate. Between the distal rhabdomere and the rhabdomeres of the next six retinula cells is a wide clear zone in which light entering by one facet could possibly reach deep rhabdomeres of a different ommatidium. Of the six proximal rhabdomeres, four have rhabdomere tubules which lie horizontal with reference to the normal posture, the other two having vertically oriented tubules. The eighth cell, with nucleus near the basement membrane, has a small rhabdomere. All eight retinula cells have axons and there is no other class of axons in the eye.     

Fine structural description of the compound eye of the Madagascar 'hissing cockroach'Gromphadorhina portentosa (Dictyoptera: Blaberidae)

The compound eyes of the wingless adults of the Madagascar ‘hissing cockroach’Gromphadorhina portentosa Sachum, 1853 were examined by light and electron microscopy. Each eye contains 2 400‐2 500 mostly hexagonal facets. However, irregularities affecting both shape and size of the ommatidia are relatively common, especially towards the margins of the eye. An individual ommatidium of this eucone type of apposition eye contains eight retinula cells, which give rise to a centrally‐fused, tiered rhabdom. The distal end of the latter is funnel‐shaped and accommodates the proximal end of the cone in its midst. Further below, the rhabdom (then formed by the rhabdomeres of four retinula cells) assumes a squarish profile with microvilli aligned in two directions at right‐angle to each other. Cross sections through the proximal regions of the rhabdom display triangular rhabdom outlines and microvilli (belonging to 3‐4 retinula cells different from those involved in the squarish more distal rhabdom) that run in three directions inclined to one another by 120°. Overall the organization of the eye conforms to the orthopteroid pattern and particularly closely resembles that of the American cockroach Periplaneta americana. However, since G. portentosa possesses fewer ommatidia, this could be a consequence of its inability to fly. On the other hand, the large size of the facets and the voluminous rhabdoms suggest considerable absolute sensitivity and an ability to detect the plane of linearly polarized light. Based on the pattern of microvillus orientations in combination with the crepuscular lifestyle G. portentosa leads and the habitat it occurs in, the prediction is made that this insect uses its green receptors for e‐vector discrimination in the environment of down‐welling light that reaches the forest floor.     

龟纹瓢虫成虫的复眼形态及其显微结构

利用光镜、组织切片法观察了龟纹瓢虫Propylaea japonica(Thunberg)成虫的复眼形态及其显微结构。结果如下:(1)头正前方观,复眼外形似半球,且后方稍向内合拢。每个复眼约包括630个小眼。(2)每个小眼是由1套屈光器(1个角膜和1个晶锥)、6至8个小网膜细胞及其特化产生的视杆和基细胞等几部分组成。晶体周围及小网膜色素细胞内均含有丰富的色素颗粒。(3)小眼整体纵切显示,其上、下段色素颗粒分布相对较多,中段分布较少。(4)明、暗适应状态对小眼的色素颗粒分布有影响,性别对其分布无明显影响。明适应状态下,其色素颗粒较均匀地分布于视杆两侧上下,暗适应状态时色素颗粒则主要分布在视杆部位的上侧,显示其具有一定的重叠眼性质;而在相同的明、暗适应状态下其雌、雄成虫复眼的色素颗粒分布间无明显差异。     

The ultrastructure of the compound eye of Munida rugosa (Crustacea: Anomura) and pigment migration during light and dark adaptation

The galatheid squat lobster, Munida rugosa, has compound eyes of the reflecting superposition type in which a distal cone cell layer and a proximal rhabdom layer are separated by an extensive clear zone. The eye is shown to have certain unique features. In all other reflecting superposition eyes, the clear zone is traversed by crystalline tracts formed by the cone cells. In M. rugosa a thin distal rhabdom thread, formed by the eighth retinula cell, connects the cones to the proximal fusiform rhabdoms. The cytoplasm of the other retinula cells also crosses the clear zone in a complex pattern. Fully light-adapted ommatidia are optically isolated by limited migrations of distal shielding pigments. A reflecting pigment multilayer lines each cone to facilitate the formation of a superposition image. This also shows a light-induced change which may limit the acceptance angle of the eye during light adaptation.     

Ultrastructure and migration of screening pigments in the retina of Pieris rapae L. (Lepidoptera,Pieridae)

Summary The retinal morphology of the butterfly, Pieris rapae L., was investigated using light and electron microscopy with special emphasis on the morphology and distribution of its screening pigments. Pigment migration in pigment and retinula cells was analysed after light-dark adaptation and after different selective chromatic adaptations. The primary pigment cells with white to yellow-green pigments symmetrically surround the cone process and the distal half of the crystalline cone, whilst the six secondary pigment cells, around each ommatidium, contain dark brown pigment granules. The nine retinula cells in one ommatidium can be categorised into four types. Receptor cells 1–4, which have microvilli in the distal half of the ommatidium only, contain numerous dark brown pigment granules. On the basis of the pigment content and morphology of their pigment granules, two distal groups of cells, cells 1, 2 and cells 3, 4 can be distinguished. The four diagonally arranged cells (5–8), with rhabdomeric structures and pigments in the proximal half of the cells, contain small red pigment granules of irregular shape. The ninth cell, which has only a small number of microvilli, lacks pigment. Chromatic adaptation experiments in which the location of retinula cell pigment granules was used as a criterium reveal two UV-receptors (cells 1 and 2), two green receptors (cells 3 and 4) and four cells (5–8) containing the red screening pigment, with a yellow-green sensitivity.     

Challenging limits: Ultrastructure and size‐related functional constraints of the compound eye of Stigmella microtheriella (Lepidoptera: Nepticulidae)

With a body length of only 2 mm, the nepticulid Stigmella microtheriella (Stainton, 1854) is one of the smallest moths known to date. We investigated the optical design of its lemon‐shaped compound eyes, which measure 83.60 μm in anterior–posterior and 119.77 μm in dorso‐ventral direction. The eyes consist of about 123 facets, each of the latter just 9.9 μm in diameter. Transmission electron microscopy reveals an optical design with features intermediate between apposition and superposition optics similar to that known from two other small species of moths (one Nepticulid and one Gracillarid). Size‐related evolutionary adaptations of the ommatidial organization include (1) the involvement of only five rhabdomeres in the formation of the distal rhabdom (2) the complete absence of a rhabdomere of the eighth (= basal) retinula cell, (3) the “hourglass” shape of the rhabdom with a characteristic narrow waist separating distal from proximal portion, and (4) the reduction to one single layer of tracheoles as an adaptation to the overall restricted space available in this minute eye. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.