Biological Journal of the Linnean Society

doi: 10.1111/j.1095-8312.1996.tb01659a.xpmid: N/A

Fryer, Geoffrey

doi: 10.1111/j.1095-8312.1996.tb01659.xpmid: N/A

AbstractRecent claims that arthropods are monophyletic because all have jaws composed of a five-segmented coxa, that the groundplan of arthropod legs has no less than 11 segments, that crustaceans, chelicerates and insects share a ‘polyramous arthropod leg’, and that the labrum is formed from a pair of legs, are rejected on factual grounds. It is suggested that the earliest arthropod appendages were unsegmented. Putative homologies among mandibulate arthropods are considered. Striking as some of these are, a good case can be made for their convergent evolution, and the concept of the Mandibulata is rejected. Suggested separate ancestries of crustaceans and tracheates are compared. A realistic explanation of radiation from a common arthropod ancestor remains illusory. A polyphyletic concept of arthropod evolution from soft-bodied, segmented, haemocoele-possessing, non-annelid worms is elaborated. The degree of convergence demanded is amply matched by proven examples of the phenomenon. If the earliest arthropods lacked compound eyes, and these were acquired several times, as they have been at least twice in non-arthropods, several otherwise intractable problems are resolved. Sequence comparisons provide a powerful tool for determining relationships but seem powerless to establish whether arthropods are monophyletic, or polyphyletic in the manner envisaged here.

Johnson, Michael S.; Black, Robert

doi: 10.1111/j.1095-8312.1996.tb01660.xpmid: N/A

AbstractThe littorine snail Bembicium vittatum has direct development and associated high levels of genetic subdivision. In the Houtman Abrolhos Islands, Western Australia, this species occupies a wide range of habitats. Replication of habitat types across the three major groups of islands, which are separated by water gaps of 10 to 15 km, separates patterns of genetic divergence related to gene flow from those expected from localized selection related to habitat. Allozyme frequencies at 14 loci were examined in samples of A vittatum from 81 sites in the Abrolhos archipelago, at 72 of which descriptions of the physical and biological characteristics were made. Although shell shape and population density of B. vittatum varied with habitat, none of the allozymes showed such associations. Instead, there was spatial coherence of the allozyme variation, indicating connections among populations at two levels. On a larger scale, nearly half the interpopulation diversity was due to differences between island groups, including the association of rare alleles with island group. At a smaller scale, variation within island groups showed coherence in the form of isolation by distance, the extent and intensity of which were related to expected patterns of gene flow. This spatial coherence, independent of the mosaic of habitats, strongly favours the interpretation of patterns of genetic subdivision in B. vittatum in the Houtman Abrolhos as reflecting primarily the patterns of past and present gene flow. Contrasts with some results of an earlier study emphasize the importance of adequate sampling of sites and polymorphic loci.

Cook, L. M.; Graham, C. S.

doi: 10.1111/j.1095-8312.1996.tb01661.xpmid: N/A

AbstractMore than 300 samples of Macrolepidoptera have been collected over 24 years at a site in southern England on field courses run for university students. The samples were taken in mercury vapour light traps. They show that numbers have fluctuated markedly between periods of high abundance and periods of low abundance. Species richness in the samples is strongly affected by abundance. Evenness of distribution of numbers between species is higher in samples from woodland than in samples collected over grass, and higher earlier than later in the season. For a series of samples from the same population, MacArthur's overlapping niche and the broken stick resource apportionment models predict a weakly positive regression of the evenness J of a sample on species number, whereas the sequential breakage model predicts a negative regression. The latter implies the highest level of competitive interaction within the moth communities sampled. We find that the data agree with the sequential breakage model, rather than the other two. A weak positive regression was expected in view of the trapping method used but was not found. The fit of the sequential breakage model also implies that species abundance is log normally distributed, which it may be for many reasons. It is argued nevertheless that such comparisons may be of use for detecting competitive interaction, and that it is important to do so in order to judge the validity of predictions about effects of environmental change or human interference on the structure of communities.

Robinson, James V.; Allgeyer, Richard

doi: 10.1111/j.1095-8312.1996.tb01662.xpmid: N/A

AbstractThe apparent erratic variation in life history traits, coloration patterns, and behaviours that exists among species within the damselfly genus Ischnura is shown to be interpretable when the species are partitioned into three groups. One group consists of species whose males are missing a pair of stout basal spines on the penultimate segment of their accessory penes. These are the only ischnurans in which males, by tandem guarding females, prevent sperm displacement. The other two groups can be recognized by the relative frequency with which mating occurs: monandrous species mate infrequently, polyandrous species more often. Compared to polyandrous species, monandrous species contain smaller size individuals, have greater sexual size dimorphism, have shorter duration copulations, do not have male biased operational sex ratios at aquatic sites, and are more likely to contain monochromatic females. Females belonging to the monandrous species tend to develop a characteristic form of pruinescence at maturity that obscures their underlying colour, and mature at a younger age. We propose that copulation serves only for sperm addition in monandrous species, for both sperm addition and displacement in polyandrous tandem guarding ischnurans, and for contact guarding as well as sperm addition and displacement in polyandrous species that do not tandem guard.

Trigo, José Roberto; Brown, Keith S.; Witte, Ludger; Hartmann, Thomas; Ernst, Ludger; Barata, Lauro Euclides Soares

doi: 10.1111/j.1095-8312.1996.tb01663.xpmid: N/A

AbstractPyrrolizidine alkaloids (PAs) often serve as chemical mediators of plant-herbivore-predator interactions. Butterflies (Danainae and Ithomiinae) and moths (Arctiidae) usually acquire PAs from plant sources (larval host plants, flowers or withered leaves visited by adults—pharmacophagy) and thereby become chemically protected against predators; they also use PAs as pheromone precursors. Study by GC-MS of PAs in three species of Ithomiinae butterflies, their larval host plants and adult alkaloid sources showed three different acquisition patterns: (1) larvae of the primitive Tithorea harmonia sequester PAs from their food plant Prestonia acutifolia (Apocynaceae: Echitoideae), and adults may also acquire these alkaloids from plant sources; (2) larvae of the more derived Aeria olena feed on Prestonia coalita, in whose leaves no PAs were detected, but freshly emerged adults sometimes contain PAs and males intensively seek and sequester these alkaloids in plant sources; and (3) larvae of the still more advanced Mechanitis polymnia feed on several PA-free Solanum species, and adult males sequester the alkaloids from various plant sources. Males and females of all three species contain mostly two PAs, the diastereoisomeric retronecine monoesters lycopsamine and intermedine, stored in the N-oxide form. Larval host plants and adult plant sources showed a large array of PA structures, the most abundant and frequent being lycopsamine and its diastereoisomers intermedine, echinatine, rinderine and indicine, and the deoxy-analogues supinine and amabiline. Bioassays with wild caught and freshly emerged adults suggest that protection against predation by the orb weaving spider Nephila clavipes may be dependent on PA concentration and maybe some spider idiosyncrasies, but freshly emerged Aeria olena without PAs are also liberated by Nephila, suggesting other protective compounds. The role of this spider as a selective pressure for PA acquisition by ithomiines is not clear.