The structure of the mouthparts and distribution of cephalic sense organs of larval simuliids was investigated using the scanning electron microscope and methylene blue staining. The effect of water velocity, temperature and quality on larval feeding behaviour was studied in an artificial stream. Short, controlled, pulses of physical and chemical stimulants were injected into the water to observe their effect on larval feeding behaviour. Too rapid for the unaided eye, movements were described frame by frame from video recordings. Food is filtered from the water by the open cephalic fans. In alternation the fans are rapidly closed , swept by the mandible to remove food particles and opened again. The frequency of this endogenous behaviour pattern was modified by environmental factors that appeared to act mainly on the interval between fan beats. The interval between fan beats was found to be inversely related to water temperature and velocity and was also affected by water borne stimulants, being significantly shorter in unfiltered natural water than particle-free distilled water. Consequently fan cleaning frequency rose as water velocity and temperature were increased and when natural food was available. Larvae responded to pulses of a wide variety of chemical compounds with bursts of mandible and maxilla movements. Fan cleaning was inhibited when these mouthpart movements occurred but filtering continued. Short pulses of inert particles at a relatively high concentration caused a similar response but when a series of pulses was delivered bursts of mouthpart movements lengthened and the fans were often closed for longer than normal, inhibiting filtering. It is suggested that overstimulation of peripheral sense organs, responding to the physical and chemical qualities of food particles, initiates the inhibition of filtering. The temporary inhibition of feeding may regulate the rate of ingestion. A simple model of larval behaviour is proposed, recognising "food gathering" (filtering) and "food ingestion" (mouthpart movements) as its main components.

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Les variations de forme affectant le mentum et les ailes de Chironomus riparius (Insecte, Diptère), en réponse à un stress toxique, ont été analysées par morphométrie traditionnelle et par morphométrie géométrique. Afin de comprendre les causes et les mécanismes impliqués dans la genèse de ces variations phénotypiques, nous avons étudié : (1) la relation entre stress chimique, réponse cellulaire et variations de forme, (2) l'importance des variations de forme selon que le stress soit simple ou multiple, (3) l'évolution des variations de forme après la métamorphose des organismes ainsi que leur transmission à la génération suivante. Pour ce faire, nous avons exposé des larves de Chironomus riparius pendant l'intégralité de leur cycle larvaire, à du plomb (entre 3 et 500 mg/kg de MS), à du 4-nonylphénol (entre 0 et 200 mg/kg de MS), ainsi qu'à deux sédiments multi-contaminés provenant du milieu naturel : l'un caractérisé par une faible pollution d'origine urbaine, l'autre par une forte pollution d'origine urbaine et industrielle. Nous avons mesuré les effets du plomb sur la biochimie des organismes (réserves énergétiques, concentration en métallothionéines et dommages cellulaires). En outre, pour tous les essais, nous avons évalué les variations morphologiques (phénodéviations, asymétrie fluctuante et changement de la forme moyenne) affectant une pièce buccale du chironome : le mentum. Nous les avons également quantifiées, pour les essais portant sur les matrices provenant du milieu naturel, sur les ailes des adultes ainsi que sur le mentum des larves appartenant à la génération suivante. Enfin, dans la station dans laquelle le sédiment faiblement contaminé a été échantillonné, les variations phénotypiques affectant le mentum de la population en place, appartenant au genre Chironomus, ont été mesurées. Si le plomb affecte la biochimie de la larve de chironome, son action est, en revanche, peu visible au niveau de la forme du mentum. De même, nous n'observons que peu de variations morphologiques de cette pièce buccale après une exposition au 4-nonylphénol. Au contraire, nous notons une augmentation des phénodéviations du mentum chez les larves ayant crû dans le sédiment faiblement contaminé provenant du milieu naturel, ce qui suggère un effet toxique de cette matrice. Dans le sédiment fortement pollué, les faibles niveaux de phénodéviations et d'asymétrie fluctuante observés pourraient être expliqués par la sélection au cours du développement larvaire des individus présentant les phénotypes les plus stables. Cette hypothèse expliquerait, en outre, la ré-augmentation des instabilités de développement que nous observons chez la génération suivante placée dans un sédiment témoin. Aucune augmentation du niveau d'asymétrie fluctuante n'est détectée sur les ailes des adultes provenant de larves exposées. Enfin, nous remarquons que le niveau d'anomalies du mentum relevé dans le milieu naturel est similaire à celui observé sur la souche de laboratoire exposée au sédiment provenant de cette station. Les résultats obtenus mettent en évidence la faible réponse des biomarqueurs de forme étudiés en présence d'une pression toxique, qu'elle soit unique ou multiple. Ils soulignent, en outre, l'importance de la génétique de la population dans la réponse morphologique observée. Ces résultats suggèrent un intérêt limité de ces marqueurs ontogéniques pour révéler un stress chimique chez Chironomus riparius / Using traditional and geometric morphometrics, we analysed morphological variations of the mentum and the wings in Chironomus riparius (Insecta, Diptera) exposed to toxic stress. To understand both the causes and mechanisms involved in phenotypic changes, we studied: (1) the relationship between toxic stress, cellular response and shape variations, (2) the pattern of shape variations according to the type of stress: single vs toxic mixture, (3) the evolution of shape variations after metamorphosis and the transmission of the morphological defects to the next generation. To achieve this, Chironomus riparius larvae were exposed during their entire larval life cycle, first to sediment spiked with lead (from 3 to 500 mg/kg dry weight) or 4-NP (from 0 to 200 mg/kg dry weight), second to two sediments sampled in the wild. The first of the latter was characterized by light urban pollution and the other by heavy urban and industrial pollution. We measured the effects of lead on three biochemical markers (energy reserve content, concentrations of metallothionein and cellular damages). Furthermore, for all the bioassays, we assessed shape variations (phenodeviations, fluctuating asymmetry and mean shape changes) of the mentum, a mouthpart structure of the chironomid larvae. For both the sediments sampled in the wild, we also assessed the shape changes of imago wings and the shape changes of the mentum affecting the next generation. At the station where the sediment was characterized by light urban pollution, we also measured morphological variations of the mentum in the Chironomid population belonging to the genus Chironomus. Despite the adverse effects of lead observed at the biochemical level, only slight effects were observed in the mentum shape. Similarly, 4-NP exposure did not induce strong phenotypic defects. On the contrary, mentum deformities increased in Chironomid larvae exposed to slightly contaminated sediment sampled in the wild. This might have been due to the exposure to the toxic mixture. In the heavily contaminated sediment, the low levels of both deformities and fluctuating asymmetry observed might have been the result of selection acting during early developmental stages. This hypothesis could thus explain the increase of developmental instability observed in the next generation placed in a control sediment. In the imago from exposed larvae, no increases in fluctuating wing asymmetry were detected. We observed that the level of morphological variations observed in natural population was similar to that detected in the laboratory in our stock culture reared in the sediment from this station. These results underlined the weak response of the shape biomarkers studied to toxic stress, whether related to multiple toxic exposure or not. Moreover, they highlighted the importance of genetics in shape changes. Consequently, the use of these ontogenic biomarkers as indicators of toxic stress in Chironomus riparius seems to be of limited interest