Seasonal and environmentally induced variation in the type and frequency of leaf trichomes of Wigandia urens (Ruiz & Pavón) Kunth (Hydrophyllaceae) was studied. Depending on the microsite, W. urens plants had smooth leaves with glandular trichomes or bristly leaves with both glandular trichomes and urticant trichomes (stinging hairs). Trichome density (number of urticant trichomes per unit leaf area) was higher in the dry season than in the wet season, and was significantly correlated with both temperature (r = 0.353, P < 0.05) and photosynthetic active radiation (r = 0.313, P < 0.05). Plants established in sun-exposed areas had trichome densities three times higher than those of plants established in shaded areas during the dry season, and 28 times higher during the wet season. At both exposed and shaded sites, trichome densities of the youngest leaves of young plants were higher than those of the youngest leaves of mature plants. In smooth and bristly leaves, transpiration rates decreased with increasing temperature during the day. However, smooth leaves had higher transpiration rates than bristly leaves at both exposed and shaded sites. In laboratory studies, trichome density was significantly (P < 0.01) reduced when small sun-grown plants (0.20-0.30 m tall) were either shaded or irrigated. In larger plants, also, irrigation significantly (P < 0.01) reduced trichome density relative to that of unirrigated controls.

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Summary 1. The effect of body size on the assortative mating and reproductive behaviour of the univoltine grasshopper Sphenarium purpurascens (Charpentier) was studied in Central Mexico. 2. Assortative mating by size was observed in the field. Evidence of positive assortative mating in relation to body size was found in laboratory experiments. Female fecundity and male success in contests were also correlated with body size. 3. Larger females had a higher number of eggs per pod. Larger males usually won fights and were able to take over females from other males, and to resist takeovers by other males while guarding. 4. Individuals of both sexes were observed copulating with more than one sexual partner in the field, suggesting polygamy. Male–male contests determined access to females, and males exhibited a postcopulatory prolonged mate‐guarding behaviour lasting up to 18 days. 5. In a 2‐year study, sex ratio was male‐biased at the beginning of the reproductive season and decreased to 1:1 by the end of the season, suggesting that the population is protandrous. 6. The results of this study indicate that assortative mating results from male–male competition and female availability, and suggests that body size is a potential target of natural and sexual selection.

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