Phylogeny and systematics of the Trogidae (Coleoptera: Scarabaeoidea)

Abstract. A cladistic analysis of the Trogidae using eighteen adult characters resulted in a monophyletic group from which the genera Glaresis Erichson, Afroglaresis Petrovitz and Cryptogenius Westwood are excluded. The Trogidae as here defined comprises three genera, Trox Fabricius (with two subgenera, Trox and Phoberus Macleay), Omorgus Erichson (with three subgenera, Omorgus, Afromorgus subg.n. and Haroldomorgus subg.n.) and Polynoncus Burmeister.     

Phylogeny of the families of Scarabaeoidea (Coleoptera) based on characters of the hindwing articulation, hindwing base and wing venation

A study is made of the articulation and base of the hindwings of Scarabaeoidea. The survey is based on an examination of over 250 genera from thirteen scarabaeoid families. Relationships among all families of Scarabaeoidea are examined here for the first time. The reconstructed phylogeny shows that the Scarabaeoidea is comprised of three major lineages: a glaresid, passalid and scarabaeid lineage. The glaresid lineage consists only of the Glaresidae. The passalid lineage is comprised of two major lines: a passalid line (containing Passalidae, Lucanidae, Diphyllostomatidae, Glaphyridae, Trogidae, Bolboceratidae and Pleocomidae) and a geotrupid line (containing Geotrupidae, Ochodaeidae, Ceratocanthidae and Hybosoridae). The scarabaeid lineage contains those taxa traditionally included within the Scarabaeidae (Aegialiinae, Aulonocneminae, Aphodiinae, Scarabaeinae, Orphninae, Melolonthinae, Acoma , Chasmatopterinae, Hopliinae, Oncerinae, Rutelinae, Dynastinae, Trichiinae, Cetoniinae and Valginae). Additional evidence, from other character suites, supporting this breakdown of the Scarabaeoidea is given.     

Glaresidae, archaeopteryx of the Scarabaeoidea (Coleoptera)

Abstract. Evidence is presented that Glaresidae are the most primitive livinj scarabaeoid group and as such represent the sister group of the rest of the Scarabaeoidea. This is based on a review of the states of seventy-two morphologica characters. The plesiomorphic states of many characters are unique to the Glaresidae or are shared with other primitive scarabaeoids; seven may be synapomorphic with other groups and two are autapomorphic for the family.     

皮金龟科的分类研究进展

皮金龟科Trogidae对分解动物尸体具有重要作用,全球已知3属320种。作者对其分类地位、分类研究历史和现状进行了概要总结;还就世界分类研究文献、分类系统和种类情况做了介绍,并对该科昆虫的地理分布特点进行了初步分析。     

Analysis of the ways of formation of the entomofaunistic complexes in the Northwest Caucasus based on the material on coleopterous insects (Coleoptera)

By the example of 14 coleopterous insect families, namely Gyrinidae, Haliplidae, Noteridae, Dytiscidae, Carabidae, Hydrophilidae, Staphylinidae, Lucanidae, Trogidae, Scarabaeidae, Elateridae, Alleculidae, Tenebrionidae, and Chrysomelidae, regularities and the basic sources of formation of the most typical landscapecoenotic complexes in the Northwest Caucasus are discussed. The total number of the species included in the material analyzed amounts to about 2000. The maximal species diversity (854 species) is registered for the deciduous forests and dry open woodlands. In total, 11 types of the chorological complexes and distributional ranges (chorotypes) of the regional beetle fauna are distinguished. Significant concordance of the chorologic patterns in some groups of beetles is established. In this respect, Carabidae and Tenebrionoidea seem to be the closest on the one hand, and also Elateridae and Scarabaeoidea, on the other. The zonal fauna of the Northwest Caucasus is formed basically by species possessing Boreal ranges, while the azonal fauna, predominantly by species with the Ancient Mediterranean ranges. Allocation of coleopterous insects within regional zoochorones is investigated as well. It is established, that in different zoochorones of the upland part of the region the majority of coenofaunas show significant similarity of the arealogical pattern, even though being composed frequently by different taxa. The fauna of agrarian landscapes of the Northwest Caucasus is also examined. It includes 382 beetle species. The overwhelming majority of them belongs to the ground beetles (229 species), leaf beetles (78), and Scarabaeoidea (30). This fauna reveals the maximal similarity with the coenofaunas of the lowland steppe and meadows. It is established that the major role in formation of the agrocenoses beetle fauna belongs to the natural plain and upland steppe and treeless fields, foothill broad-leaf forests and open woodlands, and also to the floodplain and lowland forests. The bulk of the species occurring in the agrocenoses possesses wide ranges, predominantly of the Boreal type. Some peculiarities of the regional endemism are discussed as well.     

Phylogeny of the family Trogidae (Coleoptera: Scarabaeoidea) inferred from mitochondrial and nuclear ribosomal DNA sequence data

Trogidae constitute a monophyletic and biologically unique family within Scarabaeoidea, being the only keratinophagous group in the superfamily. Traditionally, the family has been divided into three distinctive genera, Polynoncus Burmeister, Omorgus Erichson and Trox Fabricius. Although the taxonomy of the group is relatively well studied, changes to the existing classification have recently been proposed and the family as currently constituted has not been subjected to phylogenetic analyses. Here we present a molecular phylogeny for this cosmopolitan family based on three partially sequenced gene regions: 16S rRNA, 18S rRNA and 28S rRNA (domain 2). Included in the analyses are representatives belonging to four of the five extant genera (and three of the four subgenera) from all major zoogeographic regions, representing about 20% of the known trogid species diversity in the family. Phylogenetic analyses performed included parsimony and Bayesian inference. We deduce their historical biogeography by using trogid fossils as calibration points for divergence estimates. Our analyses resolved relationships between and within genera and subgenera that are largely congruent with existing phylogeny hypotheses based on morphological data. We recovered four well‐supported radiations: Polynoncus, Omorgus, Holarctic Trox and African Phoberus MacLeay. On the basis of this study, it is proposed that taxonomic changes to the generic classification of the family be made. The subgenera Trox and Phoberus should be elevated to genera to include the Holarctic and all the Afrotropical species, respectively, and Afromorgus returned to subgeneric rank. Estimates of divergence time are consistent with a Pangaean origin of the family in the Early Jurassic. The subsequent diversification of the major lineages is largely attributed to the break‐up of Pangaea and Gondwana in the Middle Jurassic and early Late Cretaceous, respectively.     

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.     

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.     

Intra-follicular visceral musculature in Omorgus freyi (HAAF) (Coleoptera: Trogidae) testes

The testicular follicles of the desert-adapted beetle Omorgus freyi (Haaf) (Coleoptera : Trogidae) are unusually large, relative to male size. The advance of the sperm-producing cells towards the efferent duct and, eventually, that of spermatozoa to the deferent duct, is apparently facilitated by the unique structure of these extended follicles. In contrast to the typical insect follicle, those of O. freyi and other scarabaeoid beetles have an internalized and elongated efferent duct. But, additionally, in O. freyi, the follicles are subdivided by longitudinal septa, radiating from the central efferent duct. A net of slow-supercontracting visceral muscles extends throughout the septa and efferent duct. We hypothesize that this unique structure is an adaptation maximizing reproductive potential by mobilizing large numbers of spermatozoa throughout the huge testes and transferring them to the female during the irregular short bouts of reproductive activity following the unpredictable rainfall.     

The ontogenetic development of refracting superposition eyes in crustaceans: Transformation of optical design

We have investigated, comparatively, the ontogenetic development of the compound eye in larvae of a mysid (Neomysis) and a euphausiid (Thysanoessa) species and found it to be close to identical in the two species. The larval eye is of apposition type with special adaptations for planktonic life. The elongated dioptric apparatus is devoid of screening pigment and instead has a proximal lens optically isolating the ommatidium. The pigmented retina is extremely compressed making the eye largely transparent and presumably suitable for a planktonic life. The presence of this specialized type of eye in the planktonic larvae of euphausiids was known before but it is intriguing to find exactly the same type in mysids, spending their entire larval life as embryos in the female marsupium. A possible explanation is offered if mysids earlier in evolution had planktonic larvae. Upon reduction of free-living larvae, the transparent type of eye may have been preserved because there is no selection pressure on the larva to change it. In late larval life, both species transform their eyes to a refracting superposition type typical for adult mysids and euphausiids. The process of transformation and the functional connection between transparent apposition and superposition is described.     

An apposition‐like compound eye with a layered rhabdom in the small diving beetle Agabus japonicus (Coleoptera,Dytiscidae)

The fine structure of the compound eyes of the adult diving beetle Agabus japonicus is described with light, scanning, and transmission electron microscopy. The eye of A. japonicus is mango‐shaped and consists of about 985 ommatidia. Each ommatidium is composed of a corneal facet lens, an eucone type of crystalline cone, a fused layered rhabdom with a basal rhabdomere, seven retinula cells (including six distal cells and one basal cell), two primary pigment cells and an undetermined number of secondary pigment cells that are restricted to the distalmost region of the eye. A clear‐zone, separating dioptric apparatus from photoreceptive structures, is not developed and the eye thus resembles an apposition eye. The cross‐sectional areas of the rhabdoms are relatively large indicative of enhanced light‐sensitivity. The distal and central region of the rhabdom is layered with interdigitating microvilli suggesting polarization sensitivity. According to the features mentioned above, we suggest that 1) the eye, seemingly of the apposition type, occurs in a taxon for which the clear‐zone (superposition) eye is characteristic; 2) the eye possesses adaptations to function in a dim‐light environment; 3) the eye may be sensitive to underwater polarized light or linearly water‐reflected polarized light. J. Morphol. 275:1273–1283, 2014. © 2014 Wiley Periodicals, Inc.     

Discontinuous gas exchange and water loss in the keratin beetle Omorgus radula: further evidence against the water conservation hypothesis?

Discontinuous gas exchange cycles are demonstrated in Omorgus radula (Erichson) (Coleoptera, Trogidae) for the first time, thus extending evidence for such cycles to another family of beetles. The closed, flutter and open phases of the cycle were clearly distinguishable in this species, and the duration of these phases was 221 ± 28, 1403 ± 148 and 755 ± 43 s (mean ± SE), respectively. No evidence for significant intraspecific mass scaling of VCO₂ or any of the components of the cycle was found. Although the prolonged F‐phase recorded here is unusual for many insects, it has previously been found in other scarabaeoid beetles, especially those from xeric environments. It has been suggested that such modulation of the discontinuous gas exchange cycle may result in a reduced VCO₂ and, consequently, reduced water loss. In O. radula VCO₂ (15.25 ± 1.49 μl/h) was considerably lower than that predicted from its body mass (0.207 ± 0.006 g). However, the small relative contribution of respiratory transpiration (6.5%) to total water loss indicated that reduced VCO₂ has little to do with water economy. Rather, it may be a consequence of generally low activity levels of these beetles. The low respiratory water loss, but distinct subterranean component in the adult life of O. radula, lend some credence to the hypothesis suggesting that regular use of subterranean habitats might have been responsible for the evolution of discontinuous gas exchange cycles. However, non‐adaptive hypotheses can still not be discounted.     

Specialized ommatidia for polarization vision in the compound eye of cockchafers,Melolontha melolontha (Coleoptera,Scarabaeidae)

Summary The superposition eye of the cockchafer, Melolontha melolontha, exhibits the typical features of many nocturnal and crepuscular scarabaeid beetles: the dioptric apparatus of each ommatidium consists of a thick corneal lens with a strong inner convexity attached to a crystalline cone, that is surrounded by two primary and 9–11 secondary pigment cells. The clear zone contains the unpigmented extensions of the secondary pigment cells, which surround the cell bodies of seven retinula (receptor) cells per ommatidium and a retinular tract formed by them. The seven-lobed fused rhabdoms are composed by the rhabdomeres of the receptor cells 1–7. The rhabdoms are optically separated from each other by a tracheal sheath around the retinulae. The orientation of the microvilli diverges in a fan-like fashion within each rhabdomere. The proximally situated retinula cell 8 does not form a rhabdomere. This standard form of ommatidium stands in contrast to another type of ommatidium found in the dorsal rim area of the eye. The dorsal rim ommatidia are characterized by the following anatomical specializations: (1) The corneal lenses are not clear but contain light-scattering, bubble-like inclusions. (2) The rhabdom length is increased approximately by a factor of two. (3) The rhabdoms have unlobed shapes. (4) Within each rhabdomere the microvilli are parallel to each other. The microvilli of receptor 1 are oriented 90° to those of receptors 2–7. (5) The tracheal sheaths around the retinulae are missing. These findings indicate that the photoreceptors of the dorsal rim area are strongly polarization sensitive and have large visual fields. In the dorsal rim ommatidia of other insects, functionally similar anatomical specializations have been found. In these species, the dorsal rim area of the eye was demonstrated to be the eye region that is responsible for the detection of polarized light. We suggest that the dorsal rim area of the cockchafer eye subserves the same function and that the beetles use the polarization pattern of the sky for orientation during their migrations.     

Phylogenetic analysis of Geotrupidae (Coleoptera, Scarabaeoidea) based on larvae

Abstract. Thirty‐eight characters derived from the larvae of Geotrupidae (Scarabaeoidea, Coleoptera) were analysed using parsimony and Bayesian inference. Trees were rooted with two Trogidae species and one species of Pleocomidae as outgroups. The monophyly of Geotrupidae (including Bolboceratinae) is supported by four autapomorphies: abdominal segments 3–7 with two dorsal annulets, chaetoparia and acanthoparia of the epipharynx not prominent, glossa and hypopharynx fused and without sclerome, trochanter and femur without fossorial setae. Bolboceratinae showed notable differences with Pleocomidae, being more related to Geotrupinae than to other groups. Odonteus species (Bolboceratinae s.str.) appear to constitute the closest sister group to Geotrupinae. Polyphyly of Bolboceratinae is implied by the following apomorphic characters observed in the ‘Odonteus lineage’: anterior and posterior epitormae of epipharynx developed, tormae of epipharynx fused, oncyli of hypopharynx developed, tarsal claws reduced or absent, plectrum and pars stridens of legs well developed and apex of antennal segment 2 with a unique sensorium. A ‘Bolbelasmus lineage’ is supported by the autapomorphic presence of various sensoria on the apex of the antennal segment, and the subtriangular labrum (except Eucanthus). This group constituted by Bolbelasmus, Bolbocerosoma and Eucanthus is the first evidence for a close relationship among genera, but more characters should be analysed to test the support for the clade. A preliminary classification at tribe level of Geotrupinae is suggested as follows: Chromogeotrupini (type genus Chromogeotrupes), Lethrini (type genus Lethrus), Taurocerastini (type genus Taurocerastes) and Geotrupini (type genus Geotrupes). Some ecological facts of Geotrupidae evolution could also be explained by the present results, such as those related to diet and nesting behaviour. Both coprophagy and male–female co‐operation in nesting appear as derived traits.     

北京萤火虫复眼的模型小眼设计与光线追迹

以实际的北京萤火虫复眼结构与生理参数与依据,基于已知的ＧＲＩＮ透镜特征,把小眼的整个光学系统分成几个部分被模型化。模型中,在晶体柱部分使用了一个径向梯度折射率透镜柱具有的抛物线轮廓折射率公式;在晶锥末端所接的半椭球ＧＲＩＮ透镜部分,使用了一个径向梯度和纵向梯度分布都存在的折射率公式．模型的尺寸与折射率的大小根据实际晶锥的测量值．通过对模型小眼的光线追迹,证明模型小眼基本上是一个平行光入射,年行光出射的望远镜系统．小眼光学系统的前半部分把入射的平行光束聚焦到中间焦点,而这个点的位置恰好位于实际晶锥的腰的部位,而光线再次经过晶锥近端以后又以大致平行光束从晶锥末端出射。此模型小眼的最大入射角为３０度,在这个入射角内其角放大率基本上是相同的。随着对平行入射光束与小眼光轴的角度的增加,所产生的中间像越来越远离晶锥光轴。     

A phylogeny of the families of Scarabaeoidea (Coleoptera)

Abstract. A study, based on examination of thirteen scarabaeoid families, was made of 134 adult and larval characters from the following character suites: 105 adult characters of the antennae, eye, epipharynx, mandible, maxillae, labium, tentorium, trochantin, procoxae, mesocoxae, mesothoracic spiracles, hind wing articulation, hind wing base, hind wing venation, hind wing folding, abdominal sternites, abdominal spiracles, male genitalia, ovarioles and karyotype; twenty larval characters of the antennae, fronto-clypeal suture, stemmata, labial palpi, maxillae, mandibles, legs, stridulatory apparatus, spiracles and ecdysial process; and nine adult and larval biological characters. In order to assess the reliability of different characters in resolving scarabaeoid family relationships, six data sets were subjected to cladistic analysis: the total evidence character set (134 characters), restricted adult character set (thirty-two characters, not including those of the wings), wing character set (seventy-three characters), larval character set (twenty characters), biological character set (nine characters) and re-coded Howden (1982) character set (thirty-nine characters). The complete character set and wing character set both produced phylograms with all nodes resolved; the restricted adult data set, larval data set, Howden (1982) data set and biological data set produced phylograms with diminishing levels of node resolution. The reconstructed phylogeny, from the preferred phylogram of the total evidence character set, shows that the Scarabaeoidea comprises three major lineages; a glaresid, passalid and scarabaeid lineage. The glaresid lineage consists only of the Glaresidae. The passalid lineage comprises two major lines; a glaphyrid line (containing Glaphyridae, Passalidae, Lucanidae, Diphyllostomatidae, Trogidae, Bolboceratidae and Pleocomidae) and a geotrupid line (containing Geotrupidae, Ochodaeidae, Ceratocanthidae and Hybosoridae). The scarabaeid lineage contains those taxa traditionally included within the Scarabaeidae (Aphodiinae, Scarabaeinae, Orphninae, Melolonthinae, Acoma, Chasmatopterinae, Hopliinae, Oncerinae, Rutelinae, Dynastinae, Trichiinae, Cetoniinae and Valginae).     

New fossil evidence of the early diversification of scarabs: Alloioscarabaeus cheni (Coleoptera: Scarabaeoidea) from the Middle Jurassic of Inner Mongolia,China

Abstract Scarabaeoidea are known from the Lower Jurassic and may have originated in the Triassic based on fossil evidence and phylogenetic research. However, the early diversification of Scarabaeoidea remains unclear due to the lack of high‐quality fossil evidence. Here we describe an exceptionally well‐preserved new fossil of Scarabaeoidea, Alloioscarabaeus cheni gen. et sp. nov from the Middle Jurassic Jiulongshan Formation of Inner Mongolia, China. Based on a morphometric analysis using 17 landmarks of the hind wing of Alloioscarabaeus and 10 scarabaeoid families, we found that Alloioscarabaeus cheni gen. et sp. nov clearly does not belong to any of the known scarabaeoid families and, consequently, is a new family, Alloioscarabaeidae fam. nov., was erected. The discovery of Alloioscarabaeus brought further evidence for the early diversification of major scarab lineages which could allow more detail in the palaeobiogeography of the Scarabaeoidea and Northeast of China which might be one of the originating places or an important radiation place during the evolution of Scarabaeoidea. Alloioscarabaeidae were very likely not good diggers and might have fed on decaying organic materials. Based on the evidence we have now, we tend to believe that most families and some subfamilies of Scarabaeoidea were present in the Jurassic period.     

Feinstruktur des Komplexauges der Roten Waldameise,Formica polyctena (Hymenoptera,Formicidae)

Zusammenfassung Die Analyse der Feinstruktur des Komplexauges der Ameise Formica polyctena ergab, daß der dioptrische Apparat der insgesamt 750 Ommatidien aus einer lamellierten Cornealinse und einem euconen Kristallkegel besteht. Zwei Hauptpigmentzellen umgeben schalenförmig den Kristallkegel, 8 Nebenpigmentzellen schirmen das Ommatidium in seiner ganzen Länge von der Cornea bis zur Basalmembran ab. Jedes Ommatidium besteht in seinem distalen Teil aus 8 Retinulazellen, 2 gegenüberliegenden schmalen und je 3 gegenüberliegenden großen Sehzellen. Weiter proximal tritt eine 9. Retinulazelle hinzu. Die Mikrovillisäume der Sehzellen verschmelzen zu einem zentralen Rhabdom. Im distalen Teil sind die Mikrovilli in 3 Richtungen angeordnet. Es wird im besonderen die Orientierung der Mikrovilli zur Augenlängsachse und zur Ommatidien-Symmetrieachse untersucht. Auch die 9. Sehzelle bildet Mikrovilli. Das Rhabdom endet an 4 basalen Pigmentzellen.Auf den Mikrovillisaum folgt im Querschnitt des dunkel adaptierten Ommatidiums ein Kranz von großen intrazellulären Vakuolen. Die anschließende cytoplasmatische Zone der Retinulazellen enthält viele Pigmentgranula und Mitochondrien; multivesikuläre und multilamelläre Körper sowie Golgiapparate treten nur selten auf. Die funktionelle Bedeutung des Ommatidienaufbaues und die Verteilung der Organellen bei Dunkeladaptation werden diskutiert.

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The fine structure of the complex eye of the ant Formica polyctena (Hymenoptera, Formicidae)

Summary The fine structure of the compound eye of the ant Formica polyctena was investigated. The eye consists of a total of 750 ommatidia, each containing a dioptric apparatus of a lamellated cornea lens, a eucone-type crystalline cone, and 8 long pigment cells which surround the ommatidium for its total length, i.e. from the cornea to the basal membrane. Each ommatidium has in its distal portion 8 retinula cells—6 large plus 2 small ones. The retinula cells are arranged in such a way that 3 pairs of large cells, and the one pair of the small cells lie opposite each other. Further proximally, a 9th retinula cell is encountered. The fused, centrally located rhabdom is built up by the microvilli of all nine retinula cells. The rhabdom ends at 4 basal pigment cells. In dark adapted ommatidia, a ring of large intracellular vacuoles is to be seen immediately peripherally to the rhabdom. The outer, cytoplasmic zone of the retinula cells contains many pigment granules and mitochondria; multivesicular bodies, onion bodies and Golgi apparatus are of relatively rare occurrence. The functional significance of the ommatidial structure and the arrangement of the cell organelles in the dark adapted condition are discussed.

Für technische Hilfe danke ich Frau Langer und Frl. Müller, Herrn Prof. H. Markl für die Durchsicht des Manuskripts.     

Pollen morphology of the Phyllanthus species occurring in the continental United States

Twelve native United States species have been examined. The pollen grains could be divided into five pollen types based on sexine ornamentation, aperture structure, sexine/nexine ratio and sexine 1/sexine 2 ratio. Within these types it was often possible to distinguish the grains of the different species. Two groups of types have been recognized based on the similarities in ornamentation. One group, consisting of the Phyllanthus amarus type, the P. tenellus type and the P. urinaria type, is characterized by the Tilia structure, whereas the second group of types, comprising the P. caroliniensis type and the P. liebmannianus type are microreticulate. The postulation that different subgenera show different pollen types is true in general, but there is one exception: P. niruri, taxonomically belonging to the subgenus Phyllanthus, has pollen grains of the P. caroliniensis type, a type also comprising species from the subgenus Isoclades.     

Sex ratio and gamete size across eastern North America in Dictyostelium discoideum,a social amoeba with three sexes

Theory indicates that numbers of mating types should tend towards infinity or remain at two. The social amoeba, Dictyostelium discoideum, however, has three mating types. It is therefore a mystery how this species has broken the threshold of two mating types, but has not increased towards a much higher number. Frequency‐dependent selection on rare types in combination with isogamy, a form of reproduction involving gametes similar in size, could explain the evolution of multiple mating types in this system. Other factors, such as drift, may be preventing the evolution of more than three. We first looked for evidence of isogamy by measuring gamete size associated with each type. We found no evidence of size dissimilarities between gametes. We then looked for evidence of balancing selection, by examining mating type distributions in natural populations and comparing genetic differentiation at the mating type locus to that at more neutral loci. We found that mating type frequency varied among the three populations we examined, with only one of the three showing an even sex ratio, which does not support balancing selection. However, we found more population structure at neutral loci than the mating type locus, suggesting that the three mating types are indeed maintained at intermediate frequencies by balancing selection. Overall, the data are consistent with balancing selection acting on D. discoideum mating types, but with a sufficiently weak rare sex advantage to allow for drift, a potential explanation for why these amoebae have only three mating types.