Sexually transmitted diseases (STDs) of insects are known from the mites, nematodes, fungi, protists and viruses. In total 73 species of parasite and pathogen from approximately 182 species of host have been reported. Whereas nearly all vertebrate STDs are viruses or bacteria, the majority of insect STDs are multicellular ectoparasites, protistans or fungi. Insect STDs display a range of transmission modes, with 'pure' sexual transmission only described from ectoparasites, all of which are mites, fungi or nematodes, whereas the microparasitic endo-parasites tend to show vertical as well as sexual transmission. The distribution of STDs within taxa of insect hosts appears to be related to the life histories of the hosts. In particular, STDs will not be able to persist if host adult generations do not overlap unless they are also transmitted by some alternative route. This explains the observation that the Coleoptera seem to suffer from more STDs than other insect orders, since they tend to diapause as adults and are therefore more likely to have overlapping generations of adults in temperate regions. STDs of insects are often highly pathogenic, and are frequently responsible for sterilizing their hosts, a feature which is also found in mammalian STDs. This, combined with high prevalences indicates that STDs can be important in the evolution and ecology of their hosts. Although attempts to demonstrate mate choice for uninfected partners have so far failed it is likely that STDs have other effects on host mating behaviour, and there is evidence from a few systems that they might manipulate their hosts to cause them to mate more frequently. STDs may also play a part in sexual conflict, with males in some systems possibly gaining a selective advantage from transmitting certain STDs to females. STDs may well be important factors in host population dynamics, and some have the potential to be useful biological control agents, but empirical studies on these subjects are lacking.

Summary Angiosperm genome sizes (GS) range c. 2400‐fold, and as nucleic acids are amongst the most phosphorus‐ (P) and nitrogen (N)‐demanding cellular biomolecules, we test the hypothesis that a key influence on plant biomass and species composition is the interaction between N and P availability and plant GS.We analysed the impact of different nutrient regimes on above‐ground biomass of angiosperm species with different GS, ploidy level and Grime's C‐S‐R (competitive, stress‐tolerant, ruderal) plant strategies growing at the Park Grass Experiment (Rothamsted, UK), established in 1856.The biomass‐weighted mean GS of species growing on plots with the addition of both N and P fertilizer were significantly higher than that of plants growing on control plots and plots with either N or P. The plants on these N + P plots are dominated by polyploids with large GS and a competitive plant strategy.The results are consistent with our hypothesis that large genomes are costly to build and maintain under N and P limitation. Hence GS and ploidy are significant traits affecting biomass growth under different nutrient regimes, influencing plant community composition and ecosystem dynamics. We propose that GS is a critical factor needed in models that bridge the knowledge gap between biodiversity and ecosystem functioning.

Summary 1.Since parasite transmission is often density-dependent, group living is normally thought to lead to an increased exposure to parasitism. As a consequence, it is predicted that animals living in groups will invest more resources (energy, time, risk, etc.) in parasite defence than those living solitarily. 2. We tested this prediction by measuring basal immune parameters in the larvae of 12 species of Lepidoptera, grouped into six phylogenetically matched species-pairs, each comprising one solitary feeding and one gregariously feeding species. 3. Contrary to expectation, the solitary species in all six species-pairs had higher total haemocyte counts than the gregarious species, and in five out of six species-pairs the solitary species also exhibited higher phenoloxidase activity. Both measurements were positively correlated with each other and with the magnitude of the cellular encapsulation response. 4. The relationship between infection risk and group living was investigated with a dynamic, spatially explicit, host-pathogen model. This shows that when individuals aggregate in groups, the per capita risk of infection can be reduced if the lower betweengroup transmission more than compensates for the higher within-group transmission. 5. We conclude that the expectation that group living always leads to increased exposure to pathogens and parasites is overly simplistic, and that the specific details of the social system in question will determine if there is increased or decreased exposure to infection.

. 1. Non-linear allometries are those where a log -log scatterplot of trait size against body size deviates from simple linearity. These are found in many insects, including the horns of beetles, the forceps of earwigs, and the heads of certain castes of ant.2. Non-linear allometries are often associated with polyphenism that is itself related to behaviour: for example, the alternative mating tactics displayed by many species of beetle are widely associated with dimorphisms in horn size.3. This paper critically reviews the current techniques used to analyse these datasets. 4. Recommendations include the use of scatterplots and assessment of the goodness of fit of simple linear models as an initial screen for non-linear allometry. The use of recently developed algorithms for 'segmented' regression to analyse continuous allometric relationships, and a pragmatic approach to the analysis of discontinuous relationships that recognises that there is no simple way to distinguish between morphs in some cases, and that all of the proposed methods for doing so have some drawbacks.5. Worked examples of the analysis of two sets of data from animals that have been the subject of controversy regarding the nature of their allometric relationships are given: further worked examples are provided as online Supporting Information .Key words . Allometry , Allomyrina , analysis , dimorphism , non-linear allometry , polyphenism , switchpoint , Onthophagus . the detection and further analysis of non-linear allometries, with particular emphasis on dimorphic allometries, and makes recommendations for the best way to analyse a variety of different sorts of such non-linear allometric relationships. The use of these analyses is illustrated by two worked examples, and three further worked examples are provided as online Supporting Information . Figure 1 shows scatterplots of non-linear allometries from three insect species. It is obvious from a glance at this figure that there is substantial variation in these relationships, and the form of the relationship is an important consideration in deciding which analysis to use. The simplest and most important division is into what can be called continuous and discontinuous allometries. Continuous allometries are those where the allometric relationship can be regarded as a single, albeit not necessarily straight, line. These allometries can include simple curved relationships ( Fig. 1a ), ones with a switchpoint where the slope of the relationship changes abruptly [as is claimed, for example, for the horned beetle Onthophagus binodis ( Cook, 1987 ; see also Supporting Information Example 1) and for the fig wasp Sycoscapter australis ( Bean & Cook, 2001 )] and also those that show sigmoid patterns such as Onthophagus taurus ( Fig. 1b ). Discontinuous allometries are those that are divided into two (or more) discontinuous groups, which may be more or less separated from each other and which are not adequately modelled by a continuous line. Such discontinuous allometries are described from ma...