Gustatory mechanisms and sugar-feeding in the mosquito, Culiseta inornata

ABSTRACT. Gustatory hairs on the legs and mouthparts of the mosquito, Culiseta inornata , were stimulated with sucrose solutions to determine their role in the finding and consuming of nectar by females. Stimulation of tarsi initiated probing, which continued without subsequent stimulation of tarsal hairs. Turning in response to tarsal stimulation depended on which leg was stimulated. Once gustatory hairs on the labella were stimulated, turning and probing were discontinued. Stimulation of the hairs on the external surface of the labella apparently initiates the labellar opening response, while stimulation of hairs on the oral surface of the labella maintains the labellar response. Electrophysiologically recorded activity of cibarial and pharyngeal pumps was used to assess the influence of the five relevant sets of gustatory hairs on initiation of sucking. None of the sets alone were capable of initiating sucking. Sucking was initiated when the hairs on the oral surface of the labella were stimulated simultaneously with sensory hairs on the tip of the labrum. The gustatory hairs on the oral surface of the labella exerted considerable influence in determining the amount of sugar or water imbibed.     

Gustatory salt rejection by Culiseta inornata

Gustatory salt rejection thresholds were determined behaviourally for the mosquito Culiseta inornata. The cation sequence of decreasing effectiveness in causing rejection for monovalent salts is K⁺ = Na⁺ > Li⁺ > Cs⁺ > Rb⁺. The anion sequence of decreasing effectiveness for sodium halides is Cl⁻ > Br⁻ > F⁻ > I⁻. The sequence for divalent cations is Sr²⁺ > Ba²⁺ > Ca²⁺ > Mg²⁺. A possible mechanism to account for the nonadditive effects of monovalent and divalent salts on rejection thresholds is discussed.     

Ultrastructure of osmoregulatory organs in larvae of the brackish-water mosquito,Culiseta inornata (Williston)

The ultrastructure of the Malpighian tubules, ileum, rectum, anal canal, and anal papillae of larvae of the mosquito Culiseta inornata was examined. The Malpighian tubules, rectum, and anal papillae have many of the ultrastructural features characteristic of ion transport tissues, i.e., elaboration of the basal and apical membranes and a close association of these membranes with mitochondria. The Malpighian tubules possess two cell types, primary and stellate. The larval rectum of C. inornata is composed of a single segment containing a homogenous population of cells. In this respect, the larval rectum of C. inornata is distinct from that of saline-water species of Aedes. The cells in the larval rectum of C. inornata, however, closely resemble those of one cell type, the anterior rectal cells, of the saline-water mosquito Aedes campestris with regard to cell and nuclear size, the percentage of the cell occupied by apical folds, and mitochondrial density and distribution. No similarities can be found between the rectum of C. inornata and the posterior segment of the saline-water Aedes, which functions as a salt gland. On this basis, we have postulated that the rectum of C. inornata does not function as a site of hyperosmotic fluid secretion. The ultrastructure of the anal papillae of C. inornata is consistent with a role in ion transport. The significance of these findings to comparative aspects of osmoregulatory strategies in mosquito larvae is discussed.     

Morphology of the abdominal skeleton and muscles of the mosquito,Culiseta inornata (Williston) (Diptera: Culicidae)

The morphology of the abdominal skeleton and muscles of the adult mosquito is incompletely known. The objectives of this study were to investigate these features in a common species, Culiseta inornata (Williston). Preserved specimens were stained lightly with methylene blue and studied with a dissecting microscope at 70 × and lower. The sclerites of the pregenital segements are best developed in segment II. The base of segment VIII in the male is narrow and semicircular in shape. This modification aids in rotation of the terminalia. Two new names are introduced for parts of the terminalia. Apodeme of sternum 9 is proposed for atrial plate of the female. Gonocoxital apodeme is a new term for a structure in the male. Both of these structures serve for attachement of muscles. Terms preferred for parts of the male terminalia are: (1) gonocoxite and gonostylus for the clasping organ; (2) paramere for the sclerotized plates on each side of and joined to the aedeagus; (3) sternum 10 for paraprocts. Sternum 10 is used because the occurrence of true paraprocts in the Nematocera is questionable. Thirty-four muscles are illustrated, and the origin and insertion of each is described. Eighteen of the muscles are newly described for the mosquito. The rotational muscles of the male terminalia were identified. The results are presented in 21 text figures.     

Effects of mating, nutritional condition, and mouthpart separation on ingestion and destination of sugar and water by the mosquito Culiseta inornata

ABSTRACT. 'Hungry' tethered female Culiseta inornata (Williston) (Diptera: Culicidae) with their mouthparts separated, respond in the sugar-feeding mode when the fascicle is immersed in 1 M sucrose, and in the drinking mode when the fascicle is immersed in water. Similarly treated animals that are 'not hungry' respond in the drinking mode to water on the fascicle, but when given 1 M sucrose on the fascicle take less than a normal amount and tend to put it in the midgut instead of the crop. 'Starving' animals respond in the sugar-feeding mode to sucrose on the fascicle, but when given water on the fascicle take slightly more than a normal amount, and about 50% put it in the crop. These results occurred whether the labellum was immersed in water, 1 M sucrose, or nothing. Differences in responses by tethered animals with separated mouthparts and non-tethered intact animals can be attributed to mouthpart deployment. These results do not agree with those of other workers who report that similarly treated female C. inornata require stimulation of both the fascicle and labellar chemoreceptors before they will ingest. These differing results raise interesting questions concerning the range of feeding behaviours available to different demes of this mosquito, and indicate how these behaviours are modified by differences in pre-test nutritional condition and mouthpart deployment.     

Cache Valley virus: experimental infection in Culiseta inornata

Experiments were conducted to examine the dynamics of Cache Valley virus in Culiseta inornata, the probable chief vector of the virus. Of about 1500 laboratory reared C. inornata exposed to viraemic suckling mice, 72 took a blood meal. A relatively high precentage (93%) of the latter mosquitoes became infected. The virus increased more than 100-fold in the experimentally infected mosquitoes. The increasing viral titres were noticed after 7 days and after 15 days. Peak titres averaged 105.0 (mean suckling mouse intracerebral lethal dose) SMICLD50/0.02 mL. The infected mosquitoes had peak titres until at least 35 days after the mosquitoes ingested blood from infected suckling mice. A single trasmission of virus by bite occurred 30 days after the viraemic blood meal. Transovarial transmission was demonstrated. In two experiments, 3.3 and 2.9% of infected mosquitoes transovarially transmitted Cache Valley virus to both male and female progeny. The minimum infection rate for the progeny was 2.05/1000 mosquitoes. This is the first reported experimental demonstration of transovarian transmission in a species of mosquito which overwinters as an adult. The role of transovarian transmission in the natural maintenance of Cache Valley virus remains undetermined.     

Ingestion and diet destination in Culiseta inornata: responses to water, sucrose and cellobiose

ABSTRACT. At ambient temperature, contact with free liquids containing sugar induces Culiseta inornata Williston (Diptera: Culicidae) females to adopt the nectar-feeding mode of behaviour. While in this mode, two distinct control mechanisms appear to operate: one controlling the amount of ingestion, the other diet destination. Mosquitoes were exposed to water and various concentrations of sucrose and cellobiose in water. Only small or taste amounts of water were imbibed, and it was directed to the midgut. Cellobiose solutions of less than 0.1M were treated as water. Cellobiose was phagostimulatory; as the concentration increased from 0.1 to 0.5 M the percentage of insects ingesting large amounts rose from 11% to 68%. Cellobiose diets were directed primarily to the midgut. Sucrose was also phagostimulatory. As the concentration increased from 0.025 to 0.2 M, the percentage of insects ingesting large amounts increased from 20 to 92. At lower sucrose levels, insects ingesting large amounts put it mainly into the crop, while insects ingesting smaller amounts put the diet into the midgut or both the crop and midgut. At sucrose concentrations of 0.4 M or more, most insects took large amounts and directed it to the crop. Different combinations of sucrose and cellobiose with a total concentration of 0.5 M all caused insects to take mainly large meals. At sucrose concentrations of 0.2M and above, the insects responded as they would to sucrose alone, directing the diet to the crop. At sucrose concentrations of 0.1M or less, diet destination patterns were more complex, with individual animals depositing the diet in the crop, the midgut, or in both. A behavioural model is presented which postulates different receptors for sucrose and cellobiose, control of diet destination via differential control of midgut and crop valves, and variation of threshold values according to endogenous factors.     

Experimental evidence for predation risk sensitive oviposition by a mosquito, Culiseta longiareolata

1. Females should choose to oviposit in habitats where risk of predation and competition are low. The ovipositional responses of a mosquito, Culiseta longiareolata, to a predator and to species sharing the same trophic level as this mosquito (controphic species) were assessed experimentally in outdoor artificial pools. 2. The predator, larval Anax imperator, which strongly reduced larval C. longiareolata survival, resulted in a 52% reduction of C. longiareolata egg rafts. The controphic species (primarily Daphnia magna), which had a small but statistically significant negative effect on the survival of C. longiareolata larvae, did not have a statistically significant influence on the number of egg rafts. 3. Laboratory trials indicated that only a small fraction of the reduced number of egg rafts seen in predator pools may be due to consumption of the egg rafts by A. imperator. 4. The experimental evidence indicates that the reduced number of C. longiareolata egg rafts found in the presence of A. imperator is due largely to oviposition habitat selection, i.e. C. longiareolata females choose pools with low risk of predation for their offspring.     

Oviposition habitat selection by the mosquito, Culiseta longiareolata: effects of conspecifics, food and green toad tadpoles

Abstract. 1. Ovipositing females should choose egg-laying sites which maximize the growth and survivorship of their offspring. Accordingly, they should avoid habitats with high densities of competitors and predators and should choose sites with adequate food. We tested experimentally the oviposition response of the mosquito, Culiseta longiareolata Macquart, to extra food and two species which are both potential competitors and predators of its offspring: conspecific larvae and green toad ( Bufo viridis ) tadpoles.
2. We conducted the experiment in outdoor artificial pools where wild C. longiareolata females were presented with choices of all combinations of the presence or absence of: (1) C.longiareolata larvae, (2) B.viridis tadpoles, and (3) extra food (a mixture of ground fish food and mouse chow).
3. Oviposition rates averaged approximately 4 times greater in response to extra food.
4. The presence of the tadpoles, particularly later in the experiment when they were larger, caused a significant reduction (approximately 50%) in oviposition rate.
5. We did not detect a statistically significant effect of conspecific larvae on oviposition habitat selection.
6. Our results indicate that Culiseta females choose habitats for oviposition based on food availability. They also tend to avoid habitats containing Bufo tadpoles, presumably to avoid predation and/or competition.     

Effects of a fire-retardant on oviposition habitat selection and larval development of the mosquito Culiseta longiareolata

1. Fire retardants are composed of fertilising salts that are commonly used for fighting wildfires. These chemicals have various effects on individual arthropod species and aquatic communities. 2. This study investigated the effects of four treatments of a prevalent fire retardant [FR CROS 134 (FR), applied at concentrations of 0, 3.8 × 10², 7.6 × 10² and 11.4 × 10² mg litre⁻¹] on oviposition habitat selection (OHS) by Culiseta longiareolata and Culex laticinctus mosquitoes in an outdoor mesocosm. Additionally, larval development of C. longiareolata was examined in response to this fire retardant. 3. The results demonstrate that OHS increased in both species, whereas survivorship (0–86.7%) and time to metamorphosis (5.4–23.1 days) of C. longiareolata were not affected by any of the treatments. However, cyanobacterial biomass increased and heterotrophic bacteria decreased in response to the addition of FR. 4. It was found that FR applications can have unintended consequences which can increase OHS by female mosquitoes. The growing use of fire retardants worldwide, and especially in Israel, justify evidence-based assessment and environmental management of their use. This study is a first critical step in filling the knowledge gap with respect to the impact of FR on aquatic ecosystems.     

A circadian rhythm in spontaneous locomotor activity in the larvae and pupae of the mosquito, Culiseta incidens

ABSTRACT. The spontaneous locomotor activity of the larvae of Culiseta incidens was monitored with acoustic actographs. Individuals, in water resting on the cones of the speakers, displayed an activity rhythm in LD 12:12 with peak activity in the light phase. A similar rhythm occurred in DD following rearing in either LD 12:12 or LL, with free-running periods of 23.5–24 h. No evidence was found of any consistent differences between the periods of the larvae and pupae. The amplitude of the pupal rhythm gradually increased as metamorphosis progressed. It is suggested that rhythmicity might also be found in those behavioural responses of mosquito larvae and pupae which have been the subject of previous studies.     

Predation-dependent oviposition habitat selection by the mosquito Culiseta longiareolata: a test of competing hypotheses

We investigated the mechanism underlying oviposition habitat selection (OHS) in the mosquito Culiseta longiareolata. The putative outcome of a trade‐off between the risk of predation and detrimental density dependence, OHS in this species presents an opportunity to test two competing alternatives: (1) a polymorphic scenario, in which a fixed proportion of females constantly avoid ‘predator pools’, while the remainder oviposits at random; and (2) a monomorphic scenario, in which all females oviposit in predator pools with a certain probability. We present a conceptual framework that demonstrates how a simple experimental design – whereby predator incidence in artificial pools is alternated between 0.25 and 0.75 – can distinguish between, or refute, the two scenarios. Given the proportional use, by ovipositing females, of predator‐free pools observed under each treatment, and a bootstrap estimate of the ratio of daily oviposition rates, we find the monomorphic scenario twice as likely as the polymorphic.     

Development of Amblyospora campbelli (Microsporida: Amblyosporidae) in the mosquito Culiseta incidens (Thomson)

The complete life cycle of Amblyospora campbelli (Kellen and Wills, 1962) (Microsporida: Amblyosporidae) requires a two-host system involving the mosquito host, Culiseta incidens (Thomson), and an obligatory intermediate copepod host. The parasite has dimorphic spore development producing meiospores (haploid condition) and binucleated spores (diploid condition), either as an exclusive infection or simultaneously (within females only). This is the 1st known report of concurrent spore development within an adult mosquito host, and, therefore, shows the Amblyospora campbelli system to be uniquely different from other Amblyospora spp. cycles previously described. The significance of dimorphic spore development is discussed. In females, diplokaryotic meronts may invade oenocytes, causing a benign-type of infection. A blood-meal is required to initiate sporulation of the binucleate spore. The binucleate spore contains the sporoplasm involved in transovarial transmission. A 2nd sporulation sequence, primarily in adipose tissue, may involve both males and females. In this sequence, repeated merogonic division greatly increased the density of diplokaryotic meronts and generally involved most of the body of the host. Production of meiospores, unlike that for the binucleate spore, appeared to be spontaneous (i.e. no obligatory blood meal). Survivorship of male and female larval mosquitoes was nearly equal. Adult females spread the parasite in three ways: transovarial, transovum, and by meiospore deposition.