It is known that human individuals show different levels of attractiveness to mosquitoes. In this study, we investigated the chemical basis for low attractiveness. We recorded behaviors of Aedes aegypti toward the hands of human volunteers and toward the volatile chemicals produced by their bodies. Some individuals, and their corresponding volatiles, elicited low upwind flight, relative attraction, and probing activity. Analyzing the components by gas chromatography coupled to electrophysiological recordings from the antennae of Aedes aegypti, enabled the location of 33 physiologically relevant compounds. The results indicated that higher levels of specific compounds may be responsible for decreased "attractiveness." In behavioral experiments, five of the compounds caused a significant reduction in upwind flight of Aedes aegypti to attractive human hands. Thus, unattractiveness of individuals may result from a repellent, or attractant "masking," mechanism.

Summary1. Explaining variation in reproductive effort is fundamental to understanding diversity in male mating and life-history strategies, although relatively little is known about environmental influences on such variation and associated trade-offs. 2. Plasticity in reproductive effort was examined in male Yellow Dung Flies ( Scatophaga stercoraria ) reared under two larval density treatments. Relative testis size, thorax size and mate-searching effort were compared, and relationships between these traits examined to look for correlational evidence of predicted trade-offs in gonadal and mate-searching expenditure. 3. Males reared under high larval density conditions developed relatively larger testes than those reared at low density but no evidence was found for a corresponding reduction in mean mate-searching effort at the population level. 4. A negative relationship was found between testis size and mate-searching activity among males within the high larval rearing density treatment but not among those reared at low density. 5. Willingness to engage in struggles for possession of females increased in relation to body size among males reared at high larval density, and the opposite relationship was found among those reared at low density. 6. Plasticity in male reproductive effort in relation to environmental conditions may be more widespread among insects than has previously been appreciated.

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Summary 1. Offspring sex ratios in the yellow dung fly Scatophaga stercoraria were examined in the laboratory. 2. Previous work indicated that females using previously stored sperm to fertilise their eggs produced male‐biased sex ratios. This result may have been due to female influences or the effects of sperm storage per se. 3. This pattern was not reproduced in the study presented here. Females that were allowed to mate just prior to oviposition produced similarly male‐biased sex ratios to those females that used previously stored sperm to fertilise their clutch. 4. Captive‐reared females may have perceived a lack of males in the population and thus produced a male‐biased offspring sex ratio. Alternatively, gamete ageing or extra‐chromosomal sex ratio distorters may have produced the male bias.

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