The intrinsic rates of increase of insects of different sizes

ABSTRACT.

• 1 A negative relationship between intrinsic rate of increase, r, and body size has only clearly been shown using data for species drawn from a number of phyla and covering several orders of magnitude in size. Analyses for more closely related species are equivocal.
• 2 Data for ninety-one species of insects, from nine orders, were used to examine the correlation between intrinsic rate of increase and size.
• 3 Intrinsic rate of increase was negatively correlated with both length and weight across orders, but no relationship could be shown within orders.
• 4 Generation times were positively related to body size, but there was no relationship between net reproductive rate (R_Q) and size.
• 5 These results support the hypothesis that documented relationships between species size and colonization success in insects could be a consequence of the scaling of intrinsic rate of increase with size.

     

The plasma membrane and generative cell organization in pollen of the mimosoid legume,Acacia retinodes

Summary Cytological changes associated with the final maturation, and dehiscence of the 16-grain compound pollen (polyads) have been followed in anthers at female and male phase of flowering. InAcacia retinodes, the transition from female to male phase takes approximately 24 h. The spherical generative cell at female phase is connected with the vegetative cell plasma membrane by a junction zone. This is sited adjacent to a germinal aperture, comprising wall ingrowths and membrane labyrinths. By male phase, the generative cell has elongated into a spindle-shape, and its surface is characteristically scalloped. The membrane labyrinths, especially those at the apertures, now contain masses of coated vesicles, implicated in the transport and secretion of proteins. Two-dimensional projections indicate that the generative cell and vegetative nucleus are closely associated forming a male germ unit.     

The Tangled Bank: The maintenance of sexual reproduction through competitive interactions

The maintenance of sexual reproduction is discussed using a model based on the familiar Lotka-Volterra competition equations. Both the equilibrium and the stability conditions that allow a sexual population to resist invasion by a single asexual clone are considered. The equilibrium conditions give results similar to previous models: When the cost of sex, within phenotype niche width, and environmental variance are low, the sexual population coexists with the asexual clone and remains at a high density. However, the asexual clone is never completely excluded. Analysis of the stability conditions shows a different picture: The introduction of an asexual clone considerably reduces the stability of the community. However, owing to its larger total niche width, the sexual population exists partly in a “competitor-free space” where the asexual clone has almost no influence on the outcome of the interactions. Therefore the asexual clone is less stable than the sexual population and has a higher probability of extinction. In contrast, the sexual population does not become extinct, since the extreme phenotypes remain at a stable, though low, density, and the central phenotypes, where stability is low, are recreated every generation through recombination. I therefore conclude that the ecological conditions under which sexual reproduction is favored over asexual reproduction are fairly easily attained and are more general than previous analyses had suggested.     

The reconciliation behavior of golden monkeys (Rhinopithecus roxellanae roxellanae) in small breeding groups

Observations were made following 130 spontaneous aggressive incidents in two small breeding groups of captive golden monkeys (Rhinopithecus roxellanae roxellanae). Participants were observed both during the first 10 min following these incidents and during matched control observations. An increased contact rate was observed between opponents following the aggressive incident. Post-conflict contacts were characterized by a number of behavior patterns: open mouth, rapid grooming, embrace, and crouching. Adult males played an important role as mediator in agonistic disputes among females: males intervened in 93.6% of female fights. It is speculated that this intervention behavior is related to the species' organization into one-male units.     

Condition, growth and reproduction in female red lechwe (Kobus leche leche Gray 1850)

Information on condition, growth and reproduction was collected from a sample of 155 female lechwe in the Linyanti Swamp, northern Botswana. Condition varied significantly with age, reproductive state and season and indications were found of relationships between body mass and the attainment of puberty in young females and body mass and the fertility of adult females. There were also indications that the population was under nutritional stress because of high water levels and that this had caused reductions in the growth rate and fertility of young females. It is suggested that the sensitivity of young females to adverse environmental conditions could be exploited to optimize population monitoring for conservation purposes by using them as an indicator class, rather than monitoring the entire population. Because of relationships between condition, growth and reproduction, it is also suggested that long-term monitoring of condition should form part of any effort to understand population dynamics.     

Distribution of salmon gonadotrophin releasing-hormone in the brain and pituitary of the sea bass (Dicentrarchus labrax)

Summary The distribution of salmon gonadotrophin-releasing hormone (sGnRH) was studied in the brain and pituitary of two-year-old immature sea bass (Dicentrarchus labrax) by means of an enzymoimmunoassay (EIA) for sGnRH and immunocytochemistry. The EIA for sGnRH is a competitive assay using a tracer made of sGnRH coupled to acetylcholinesterase from an electric eel. The separation of free and bound tracer is achieved by coating the plates with mouse anti-rabbit IgG monoclonal antibodies. Displacement curves generated by sGnRH and extracts from pituitary and different brain regions showed a good parallelism allowing the assay to be used for sGnRH measurements in this species. Although all parts of the brain contained measurable levels of sGnRH, the highest concentrations were found in the pituitary, the olfactory bulbs and the telencephalon. These data were confirmed by immunocytochemistry. Cell bodies were found in the olfactory bulbs, ventral telencephalon, preoptic region and mediobasal hypothalamus. Immunoreactive fibers could be observed in all parts of the brain including the optic tectum, the cerebellum (corpus and valvula), the vagal lobe, the medulla oblongata and the rostral spinal cord. In most cases, these fibers do not form well defined bundles; however, there was clearly a continuum of immunoreactive fibers, extending from the olfactory bulbs to the pituitary, and along which all the cell bodies described above were located. In the ventral telencephalon and the preoptic region, clear pictures of varicose positive fibers contacting immunoreactive perikarya could be observed. These data indicate that sGnRH is most likely an endogenous peptide in the brain of the sea bass, although the presence of other forms of GnRH cannot be excluded at this point. This study also demonstrates that the general organization of the GnRH systems in the sea bass is highly similar to what has been described in most freshwater teleost species, and provides basis for further studies on the neuroendocrine control of gonadotrophin release in this commercially important species.     

Morphology and life cycle of Ceratium rhomvoides nov. sp. (Dinophyceae) from the lake Plußsee (Federal Republic)

A new species of freshwater dinoflagellates, Ceratium rhomvoides nov. sp., is described from the Plußsee, northern West Germany, where it occurs regularly. C. rhomvoides nov. sp. is distinguished from closely related taxa by various aspects of the morphology of the vegetative cell and the cyst and by its sexual reproduction and seasonal distribution.     

Sexual reproduction and life cycle of Ceratium furcoides (Dinophyceae) in situ in the lake Plußsee (F.R.)

The life cycle of Ceratium furcoides (Levander) Langhans which features sexual reproduction is described for the first time in situ from the lake Plußsee (northern West Germany). Anisogamic copulation occurs during the bloom period in July–August by incorporation of the male gamete by the female gamete, resulting in an overwintering planozygote. The planozygote develops into a benthic hypnozygote which germinates in spring to establish a planomeiocyte. The morphology and structure of the stages of the life cycle are described using light and scanning electron microscopy.     

Observations on egg production rates and seasonal changes in the internal morphology of Mediterranean populations of Acartia clausi and Centropages typicus

Egg production rates in wild populations of Acartia clausi and Centropages typicus, sampled biweekly in the Gulf of Naples from October 1985 to July 1987, showed marked seasonal fluctuations with maximum values in early spring that proceeded the annual maxima for adult female densities in summer. A positive correlation between chlorophyll a concentrations and egg production was evident only during the early spring phytoplankton bloom. A strong diminution in egg deposition occurred later in spring and continued throughout the summer notwithstanding high chlorophyll concentrations. In winter, when population abundances for adult females were lowest, egg production rates were always higher than in summer. Differences in egg production rates coincided with pronounced morphological changes between summer and winter populations of both species. The most striking of these changes consisted, in winter, in the presence of a dark brown fluid-like mass of granular material that seemed to freely bathe the gonads. The presence of this substance only during periods of elevated egg production suggests that it may enhance egg production rates when the adult population reaches minimum annual levels. Such a mechanism of self-regulation may operate to dampen the effects of environmental variability thereby contributing to maintain a conservative structure in coastal copepod communities.