The pteromalid wasp Anisopteromalus calandrae (Howard) is a parasite of larval instars of several species of beetles. In our experiments we used larvae of the granary weevil (Sitophilus granarius L.) as hosts. Host size may differ considerably, older larvae being much larger than younger ones. Fitness of the wasps' offspring depends on host size but differences between small and large sons are probably less than between small and large daughters. A number of experiments were carried out to investigate the effect of host quality on sex ratio. Evidence was found for the hypothesis that in Anisopteromalus offspring sexratio depends on clutch size. On large hosts (better quality) the proportion of sons is small, on young hosts (lower quality) the proportion of sons is high but these differences are due to large numbers of female-eggs being deposited on old hosts, and far fewer on young hosts, per laying period of 24 hrs. On both categories the number of sons produced per laying period is about similar. In the course of time (i.e. with decreasing life expectancy) the initial difference between treatments disappears almost completely (i.e. more and more, sub-optimal, daughters are produced on young hosts). Evidence for a memory factor was obtained with experiments in which we offered different host sizes, either successively or simultaneously. A simple model is proposed to describe the succession of sex-ratio decisions made under different conditions of host quality. Results are compared with those obtained earlier with Lariophagus, another pteromalid parasite of granary weevils, which is supposed to change sex ratio independent of clutch size.

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Courting males of many species of chalcidoid wasps appear to produce sounds which can be recorded on tape. A few preliminary experiments were made with Nasonia vitripennis (Walk.) to investigate the biological functions, if any, of vibrating. Nasonia males can be silenced in a simple way, apparently without affecting their other courtship activities, by applying a small quantity of gum arabic on top of their thorax. Young, silenced males were as successful with courtship as untreated controls, but when older were definitely less successful than normal males of similar age. Courtship success of these older males increased significantly with a simultaneous playback of courtship sounds. RESUME CARACTÉRISTIQUES DES SONS ÉMIS PAR DES CHALCIDIENS MÂLES EN ACTIVITÉ SEXUEL‐LE (AVEC UNE NOTE SUR LEUR IMPORTANCE BIOLOGIQUE) Les mâles de la majorité des espèces de Chalcidiens émettent des sons pendant la parade. Ces émissions peuvent être enregistrées sur bande magnétique et analysées avec un oscilloscope et un sonographe. Les figures 1–3 représentent une série de sonogrammes illustrant ces émissions. Une éventuelle fonction biologique a été recherchée au cours d'expériences préliminaires avec Nasonia vitripennis. Les mâles de Nasonia peuvent être facilement rendus silencieux: une très faible quantité de gomme arabique appliquée sur le corselet entre les bases des ailes les rend silencieux sans modifier les autres activités de la parade. Les jeunes mâles silencieux et les mâles normaux ont le même succès sexuel, mais les mâles plus âgés traités sont nettement handicapés par rapport à leurs contemporains. Leurs performances sont cependant améliorées par émission simultanée par hautparleur, des chants de cour.

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