Development and ageing of the salivary glands of adult male Aedes aegypti (L.) and Aedes togoi (theobald) mosquitoes (Dipteria : Culcidae)

The salivary glands of adult male Aedes aegypti and Aedes togoi (Diptera : Culicidae), varying in age from less than 1 day after emergence to 42 days or 33 days respectively, were examined by light microscopy. Following emergence, the trilobed glands rapidly accumulate secretory product and attain their full size within about 48 hr. The amount of secretion in fully developed glands shows marked individual variation, but it is predominantly located in the posterior parts of the glands. Ageing changes begin to appear after about 8 days (A. aegypti) or 10–11 days (A. togoi), and although there is wide individual variation in their extent, in general, they become progressively more severe until at the age of 4–5 weeks all mosquitoes are substantially affected. Since other tissues, including ganglia, also regress with age, it is unlikely that salivary gland changes are the prime cause of death.     

Development and ageing of the salivary glands of adult female Aedes aegypti (L.) and Aedes togoi (Theobald) mosquitoes (Diptera : Culicidae)

Histological sections of the salivary glands of adult female Aedes aegypti and Aedes togoi (Diptera : Culicidae), varying in age from less than 1 day to 48 days after emergence were examined by light microscopy. Following emergence, the glands underwent morphological differentiation and also enlarged as secretory product accumulated, especially in the posterior regions. Development was complete within about 3 days (A. aegypti) or 7 days (A. togoi). Ageing, which began between 10 and 15 days after emergence in both species, predominantly affected the posterior regions of the glands. Regression, involving loss of stored secretion and cytological evidence of reduced protein synthesis, was accompanied by degenerative changes including substantial cytoplasmic vacuolation and increased prominence of large cyst-like structures. Although, in general, both regression and degeneration increased with time, there was marked variation between one gland and another in the speed with which ageing changes progressed.     

Peripheral cells in the salivary glands of female Aedes aegypti and A. togoi mosquitoes

A new type of cell, the peripheral cell, is described. These cells are located at the perimeter of the simple tubules which form the distal zones of the lateral lobes of the salivary glands of female Aedes aegypti and A. togoi. They may represent degenerate secretory cells which are segregated so that their altered secretory product cannot be discharged during blood feeding.     

Physiological effects of carbohydrates on survival, metabolism, and flight potential of female Aedes taeniorphynchus

Starved Aedes taeniorhynchus and Aedes aegypti females 2 days after emergence were fed ad lib. a mixture of 10% test carbohydrate solution and 5% sorbose solution (as a phagostimulant) and maintained for survival. Also, starved A. taeniorhynchus females 5 days after emergence were fed, with a micro-pipette, 2 μl/female of a mixture of 20% test carbohydrate and 5% sorbose. These females were analysed for glycogen accumulation 18 hr after feeding, and they were flown tethered on a flight mill immediately as well as 24 hr after feeding. Twenty-eight carbohydrates including sugar alcohols were tested.     

Laboratory Evaluation of Controlled-release Repellent Treated Pulp Fabric on Human Volunteers Against Mosquito Vectors

Three controlled‐release personal‐use pulp fabric impregnated insect repellent formulations of 5% N, N‐diethyl‐3‐methylbenzamide (DEET), and 10% and 15% neem oils were evaluated in an environmental chamber on volunteers for their repellent efficacy against three mosquito species, Culex pipiens pallens, Aedes aegypti and Ochlerotatus togoi. The 5% DEET formulation showed significant repellency in pulp fabric (5 mm in width) against Culex pipiens pallens and Aedes aegypti (P < 0.05), providing an average repellency of 88.0%, 66.3%, and 46.8% of Cx. pipiens pallens, Ae. Aegypti and O. togoi bites, respectively, during the 6 hours of exposure period. Against night‐biting mosquitoes Cx. pipiens pallens, the DEET formulation provided mostly complete protection for at least 4 hours after the application. In pulp fabric of 10 mm in width, the 5% deet formulation showed significantly the highest repellency among the repellents against O. togoi (P<0.05), providing an average repellency of 52.3% during the 6 hours of exposure period. However, the pulp fabrics treated with 10% and 15% neem oil were less effective than 5% DEET against three mosquito species. This study demonstrated the potential of 5% DEET as pulp fabric repellent against both day‐ and night‐biting mosquitoes.     

Laboratory culture of Aedes (Finlaya) togoi (Theobald), 1907 and measurements of its susceptibility to insecticides

The literature on Aedes togoi is reviewed. This oriental mosquito occasionally acts as a vector for filarial or virus diseases and is sometimes a nuisance from its bites. Eggs from Taiwan (Formosa) were brought to London in 1958. A laboratory colony was reared from these eggs and maintained by methods similar to those commonly used for Aedes aegypti. Various aspects of the bionomics were studied, including the lengths of stages, mortality during development and the effects of copulation, feeding and salt content of water on oviposition. The levels of susceptibility to DDT, BHC and dieldrin were measured by the World Health Organisation (W.H.O.) method and compared with those of colonies of A. aegypti originating in Taiwan and the Pescadores.

---

Zusammenfassung Eine Laborzucht von Aedes togoi wurde erfolgreich eingerichtet und die Kulturtechnik beschrieben. Die Ergebnisse der vorläufigen Beobachtungen über ihre Biologie werden ebenfalls mitgeteilt. Die Kulturtechnik unterscheidet sich nicht sehr von den gewöhnlich für Aedes aegypti angewandten. Jedoch wurde die Dauer eines Generationszyklus viel länger als die von Aedes aegypti befunden. Bei einem Teil der Weibchenpopulation konnte Autogenie beobachtet werden. Kopulationen wurden nicht in allen benutzten Käfigtypen durch Augenbeobachtung festgestellt, durch Sektion der Weibchen jedoch in allen Käfigformen bis zu den kleinsten von 19×19×19 cm nachgewiesen. Unbegattete Weibchen legen nachweislich ebenso gleichmä£ig Eier ab wie begattete, aber die produzierten Eier waren alle steril. Die von befruchteten autogenen Weibchen abgelegten Eier waren lebensfähig und entwickelten sich zu normalen Imagines. Erwachsene Weibchen, denen 2-und 5%ige Salzlösungen, 16%ige Zuckerlösung und frisches Leitungswasser geboten wurden, vermieden Eiablagen in die Gläser mit Salzlösungen, obwohl die Larven der Stammzucht in brackigen Felsentümpeln lebend gefunden wurden. In Hongkong wurden die Larven dieser Art in Gewässern mit einer Salinität von 0,046 bis 1,85% Chlor gefunden, annähernd dem höchsten Prozentgehalt in Seewasser. In der vorliegenden Untersuchung ertrugen die Larven Salzlösungen bis zu 5% und die Eier entwickelten sich ganz normal zu Erwachsenen in Salzlösungen von 1, 2, 3, 4 und 5%. Die Entwicklung wurde in höheren Konzentrationen nur schwach verzögert befunden. Es bestanden keine signifikanten Unterschiede zwischen den Sterblichkeitsraten bei verschiedenen Konzentrationen, nur in der höchsten Konzentration (5% Salzlösung) zeigte sich eine höhere Mortalität.Die Empfindlichkeit von Aedes togoi und zwei Stämmen von Aedes aegypti wurde mit den W.H.O.-Prüfwannen gemessen. Die Imagines wurden mit DDT und die Larven mit DDT, Dieldrin und gamma-BHC geprüft. Der vorliegende Stamm von Aedes togoi entstammte einem Gebiet, das von überständigem Antimalaria-DDT-Gebäudesprückstand bedeckt war; so konnte die Möglichkeit einer Resistenzentwicklung bei diesen Mücken erwartet werden. Unglücklicherweise ist kein Stamm von Aedes togoi verfügbar von Orten, die von DDT-Begiftung frei sind; deshalb kann seine mögliche Resistenz genen DDT nicht abgeschätzt werden. Jedoch erwies er sich als ebenso empfindlich wie normale Stämme von Aedes aegypti aus dem südlichen Taiwan und Pescadores. Die männlichen Imagines von Aedes togoi sind viel stärker DDT-empfindlich als die Weibchen und beide Geschlechter normaler Stämme von Aedes aegypti. Im Larvalstadium ist Aedes togoi viel toleranter gegen DDT als normale Stämme von Aedes aegypti. Die Resulte sind in 12 Tabellen und in den Figuren 1 und 2 aufgeführt.

     

A comparative study of flight performance and fuel utilization as a function of age in females of Florida mosquitoes

Sugar-fed females of 7 species of Florida mosquitoes were flown for 4·5 hr on a flight-mill system twice a week during their life span and analysed for flight performance (speed of flight during the first hour, the distance flown at the end of 4·5 hr flight period, and per cent reduction in flight speed of the fourth hour as compared to the first hour) and for utilization of haemolymph sugars, glycogen, and triglycerides reserves.Species-specific differences occurred in 50 per cent survival age and flight performance. Aedes taeniorhynchus, Mansonia titillans, Culex nigripalpus, Psorophora confinnis, and A. sollicitans exhibited maximum flight potential during 2 to 8 weeks after emergence, as compared with only 1 to 2 weeks in A. aegypti and Anopheles quadrimaculatus. This suggested that the former group of mosquitoes possessed a greater potential for dispersal by searching flights during their life span when compared with the latter group. Glycogen was the only energy reserve utilized during sustained tethered flight in all species. The energy requirements as calculated from depletion of glycogen during sustained flight for 4·5 hr in different species varied from 0·06 to 0·09 cal/1000 m or 16 to 32 cal/hr per g. The considerably lower values of energy expended, on the average of 16 to 22 cal/hr per g in C. nigripalpus, P. confinnis, A. sollicitans, and A. aegypti, compared with 30 to 32 cal/hr per g in A. taeniorhynchus and M. titillans, is most probably due to the substantially lower speed of the former group of mosquitoes, 1000 to 1500 m/hr, than in the latter group of mosquitoes, 1500 to 2000 m/hr. A. sollicitans and M. titillans started to show wing abrasion and distinct signs of senescence during the last 2 to 3 weeks of their life span.Non-flown females of different species maintained on sugar ab lib. achieved maximum levels of glycogen and triglycerides reserves during the second week and they maintained these levels for the major portion of their life span, after which the levels of these reserves showed a distinct decline. It is suggested that this stabilization and decline of the energy reserves control longevity and flight potential in each mosquito species.     

Release of egg development neurosecretory hormone in Aedes aegypti and Aedes taeniorhynchus induced by an ovarian factor

Ovariectomized Aedes aegypti do not synthesize vitellogenin after a blood meal, unless an ovary from a blood-fed donor is implanted. Decapitation, however, prior to implantation inhibits vitellogenin synthesis. A female ovariectomized and decapitated 6 hr after a blood meal, synthesizes vitellogenin if an ovary from a blood-fed donor is implanted. On the other hand, females that are fed on blood and immediately decapitated can not be stimulated to synthesize vitellogenin with implanted ovaries removed from blood-fed donors. These experiments led to the hypothesis that the blood meal stimulates the ovary to secrete a corpus cardiacum stimulating factor, that in turn promotes release of egg development neurosecretory hormone stored in the corpus cardiacum.Injection of 20-hydroxy-ecdysone or ovarian extract prepared from ovaries removed from unfed females does not release egg development neurosecretory hormone. Thus corpus cardiacum stimulating factor is not 20-hydroxy-ecdysone, and ovaries removed from unfed females do not store it.The rate of inactivation of egg development neurosecretory hormone released from the corpus cardiacum after a blood meal was investigated by implanting an ovary into females that were blood fed for various intervals than decapitated and ovariectomized. Seventy per cent of implants grow when the operation is done 18 hr after feeding, and 30% when the operation is done between 18 and 24 hr after feeding, indicating that egg development neurosecretory hormone is stable for the first 18 hr after a blood meal.Aedes taeniorhynchus females ovariectomized 24 hr after adult emergence do not synthesize vitellogenin. When such a female is implanted with an ovary removed from a sugar-fed or blood-fed Aedes aegypti donor vitellogenin synthesis is initiated, and the implant grows. Decapitation prior to implantation inhibit vitellogenin synthesis and implants do not grow. These results indicate that corpus cardiacum stimulating factor is not species specific.     

Effects of mosquito larval feeding behavior on Bacillus sphaericus efficacy

Three species of mosquito larvae, representing three genera, were exposed from 10 min to 2 hr to the pathogen Bacillus sphaericus. Culex quinquefasciatus rapidly ingested the bacterium with resulting high mortality. Anopheles albimanus ingested it at a much lower rate initially with correspondingly low mortality. At the longest time interval they had accumulated approximately the same number of bacterial cells as C. quinquefasciatus but achieved a lower mortality. Aedes aegypti ingested and accumulated bacterium at the same rate as C. quinquefasciatus but was resistant to all time intervals. Utilizing ¹⁴C-labeled bacteria, we demonstrated that these differences were attributable to larval behavior in the case of A. albimanus but not in the case of A. aegypti.     

Sporulation and toxin production by Bacillus thuringiensis var. israelensis in cadavers of mosquito larvae (Diptera: Culicidae)

Laboratory experiments with 4th-instar larvae of Aedes aegypti and Anopheles albimanus (Diptera: Culicidae) demonstrated that the entomocidal bacterium, Bacillus thuringiensis var. israelensis, can grow vegetatively, sporulate, and produce toxin in cadavers of mosquito larvae. In A. aegypti, spore counts rose from 2 × 10²/cadaver 4 hr after treatment to 1.4 × 10⁵/cadaver approximately 72 hr later, whereas in A. albimanus spore counts per cadaver increased from 2.2 × 10³ between 4 and 24 hr to 3.2 × 10⁵ at 72 hr post-treatment. Bioassays of larval cadavers indicated that toxicity associated with sporulation of B. thuringiensis var. israelensis reached a maximum level approximately 72 hr after treatment. These results demonstrate that under appropriate conditions B. thuringiensis var. israelensis can use the substrates available in larval cadavers for growth and sporulation.     

A laboratory evaluation of the microsporidian Vavraia culicis as an agent for mosquito control

The susceptibility of the mosquitoes Aedes aegypti, Aedes taeniorhynchus, Anopheles albimanus, Culex pipiens quinquefasciatus, Culex salinarius, and Culex tarsalis to infection by the microsporidian Vavraia culicis was determined. Using 18-hr exposures to 5 × 10³, 1 × 10⁴, 5 × 10⁴, and 1 × 10⁵ spores/ml, C. salinarius, C. tarsalis, and A. albimanus were found to be significantly more susceptible than A. aegypti. The most severe infections were observed in C. salinarius and C. tarsalis, although heavy infections of approximately 1 million spores per adult were recorded at the higher dosages in all species tested except A. aegypti. Production trials indicated that up to 5.4 × 10⁸ spores could be routinely produced in individual corn earworms, Heliothis zea. Inactivation of the spores by sunlight was measured by observing the subsequent incidence of infection and spore production in A. albimanus. These two measurements provided an LT₉₀ of 5.5 and 3.3 hr, respectively.     

Nutrient limitation results in juvenile hormone-mediated resorption of previtellogenic ovarian follicles in mosquitoes

Juvenile hormone (JH) is a central hormonal regulator of previtellogenic development in female Aedes aegypti mosquitoes. JH levels are low at eclosion and increase during the first day after adult emergence. This initial rise in JH is essential for female reproductive maturation. After previtellogenic maturation is complete, the mosquito enters a ‘state-of-arrest’ during which JH synthesis continues at a slower pace and further ovary development is repressed until a blood meal is taken. By examining the relationships between juvenile hormone, follicular resorption and nutrition in A. aegypti, we were able to define a critical role of JH during the previtellogenic resting stage. The rate of follicular resorption in resting stage mosquitoes is dependent on nutritional quality. Feeding water alone caused the rate of follicular resorption to reach over 20% by day 7 after emergence. Conversely, feeding a 20% sucrose solution caused resorption to remain below 5% during the entire experimental period. Mosquitoes fed 3% sucrose show rates of resorption intermediate between water and 20% sucrose and only reached 10% by day 7 after emergence. Follicular resorption is related to JH levels. Ligated abdomens separated from a source of JH (the corpora allata) showed an increase in resorption comparable to similarly aged starved mosquitoes (16%). Resorption in ligated abdomens was reduced to 6% by application of methoprene. The application of methoprene was also sufficient to prevent resorption in intact mosquitoes starved for 48 h (14% starved vs. 4% starved with methoprene). Additionally, active caspases were localized to resorbing follicles indicating that an apoptotic cell-death mechanism is responsible for follicular resorption during the previtellogenic resting stage. Taken together, these results indicate that JH mediates reproductive trade-offs in resting stage mosquitoes in response to nutrition.     

Postemergence growth of the ovarian follicles of Aedes aegypti

Growth of the ultimate follicle to the resting stage in Aedes aegypti is linear and reaches maximum development about 60 hr after emergence. Decapitations and ligations at various times after emergence indicate that growth of the follicles is under the control of factors from the head and thorax. Release of head factor occurs within one day after emergence and is relatively sudden. The thoracic factor is released gradually over a 2 to 3 day period. Near normal growth of follicles in isolated abdomens after topical application of juvenile hormone (JH) indicates that the thoracic factor is JH from the corpora allata and demonstrates the feasibility of using this system as a JH assay. When ecdysone was injected simultaneously with JH the follicles failed to grow.     

PERMEABILITY OF THE OVARIAN FOLLICLE OF AEDES AEGYPTI MOSQUITOES

The passage of tracers of various molecular weights into resting and vitellogenic ovarian follicles of Aedes aegypti mosquitoes was studied ultrastructurally. The outermost layer of the follicular sheath (the basement lamina) is a coarse mechanical filter. It is freely permeable to particles with molecular weights ranging from 12,000 to 500,000 (i.e. cytochrome c, peroxidase, hemoglobin, catalase, ferritin, immunoglobulin (IgG)-peroxidase, iron dextran and Thorotrast) that have dimensions less than 110 A. Molecules as large as carbon (300–500 A) are totally excluded. Whereas proteins and polysaccharide tracers permeate the basement lamina with apparent ease, certain inert particles (e.g. Thorotrast, Fellows-Testager Div., Fellows Mfg. Co., Inc., Detroit, Mich.) penetrate more slowly. With respect to the tracers tested, resting follicles are as permeable as vitellogenic follicles. The follicle epithelium of resting or vitellogenic follicles is penetrated by narrow intercellular channels. Our observations suggest that these spaces are lined with mucopolysaccharide material. After permeating the basement lamina, exogenous tracers fill these channels, while the bulk of material accumulates in the perioocytic space. Within 3 hr after imbibing blood, the pinocytotic mechanism of the oocyte is greatly augmented. Pinocytosis is not selective with regard to material in the perioocytic space, since double tracer studies show that exogenous compounds are not separated, but are incorporated into the same pinocytotic vesicle. During later stages of vitellogenesis, 36–48 hr after the blood-meal, the pinocytotic mechanism of the oocyte is diminished. Simultaneously, the intercellular channels become occluded by desmosomes, and the vitelline membrane plaques separate the oocyte and follicle epithelium.     

Chromosomal differentiation in two species of Aedes and their hybrids revealed by giemsa C-banding

Giemsa C-banding patterns in two species of mosquitoes, Aedes aegypti and Aedes mascarensis, their hybrids and backcross progeny revealed differences in the sex chromosome pair. In A. aegypti, the female determining or the m chromosome in both males and females shows a conspicuous band in the centromere region and another band in one arm. The male determining or the M chromosome is devoid of any bands. Progeny of crosses involving A. aegypti females and A. mascarensis males showed interesting albeit unexpected results. The intercalary band was suppressed in both sons and daughters. When such F₁ sons were backcrossed to A. aegypti females, a proportion of males developed into intersexes. These intersex progeny differed from the normal males in terms of their banding pattern. In the reciprocal cross (A. mascarensis female × A. aegypti male), the F₁ and the backcross progeny yielded the expected C-banding patterns. The implications of the reversible expression of the intercalary band on the A. aegypti m chromosome and its relevance to genetic regulation are discussed.     

Germination of Bacillus thuringiensis var. israelensis spores in the gut of Aedes larvae (Diptera: Culicidae)

In laboratory experiments, germination and growth of Bacillus thuringiensis israelensis in the gut of Aedes aegypti and A. vexans larvae (Culicidae: Diptera) was observed. The number of spores and vegetative cells in the gut of living larvae and in cadavers was estimated by plaing homogenized larvae on selective agar plates. The number of spores per gut increased in the first 40–140 min of exposure to a maximum, and decreased in the subsequent time, demonstrating spore germination in living larvae, moribunds, and in cadavers. Twenty-four hours after the death of the larvae, a minimal amount of spores, but an increased number of vegetative cells, was found in cadavers. In A. aegypti larvae, germination and growth of B. thuringiensis israelensis in the larval gut was photographically documented.     

Dissemination and Transmission of the E1-226V Variant of Chikungunya Virus in Aedes albopictus Are Controlled at the Midgut Barrier Level

Emergence of arboviruses could result from their ability to exploit new environments, for example a new host. This ability is facilitated by the high mutation rate occurring during viral genome replication. The last emergence of chikungunya in the Indian Ocean region corroborates this statement since a single viral mutation at the position 226 on the E1 glycoprotein (E1-A226V) was associated with enhanced transmission by the mosquito Aedes albopictus in regions where the major mosquito vector, Aedes aegypti, is absent.We used direct competition assays in vivo to dissect out the mechanisms underlying the selection of E1-226V by Ae. albopictus. When the original variant E1-226A and the newly emerged E1-226V were provided in the same blood-meal at equal titers to both species of mosquitoes, we found that the proportion of both variants was drastically different in the two mosquito species. Following ingestion of the infectious blood-meal, the E1-226V variant was preferentially selected in Ae. albopictus, whereas the E1-226A variant was sometimes favored in Ae. aegypti. Interestingly, when the two variants were introduced into the mosquitoes by intrathoracic inoculations, E1-226V was no longer favored for dissemination and transmission in Ae. albopictus, showing that the midgut barrier plays a key role in E1-226V selection.This study sheds light on the role of the midgut barrier in the selection of novel arbovirus emerging variants. We also bring new insight into how the pre-existing variant E1-226V was selected among other viral variants including E1-226A. Indeed the E1-226V variant present at low levels in natural viral populations could rapidly emerge after being selected in Ae. albopictus at the midgut barrier level.     

Ultrastructural changes in intersegmental cuticle during rotation of the terminal abdominal segments in a mosquito

The terminal abdominal segments of male Aedes aegypti rotate 180° within 24 hr after adult emergence, rotation occurring in the intersegmental membrane between abdominal segments VII and VIII. The ultrastructure of this rotating membrane is compared with non-rotating intersegmental membranes at different developmental stages. The deposition of cuticle in both the rotating and non-rotating intersegments appears ultrastructurally similar, and follows the sequential pattern described for other insects. Shortly after adult emergence, however, disruptive changes occur in the membrane cuticle that are more pronounced in non-rotating intersegments. This disruption occurs initially 1 hr after adult emergence and becomes maximal within 3 hr. Disruption appears to occur by the addition of fluid to the cuticle and results in a ten-fold increase in cuticle thickness in non-rotating intersegments but only a two-fold increase in thickness in the rotating intersegment. While in the disrupted condition, the non-rotating intersegmental membranes become extensively folded whereas the cuticle in the rotating intersegment becomes stretched. During rotation, strain forces in the rotating intersegment result in a reorientation of microfibers in the cuticle from parabolic to parallel. This reorientation is presumably brought about by plastic flow.     

Dynamics of vitellogenin uptake in Aedes aegypti as demonstrated by trypan blue

Trypan blue has been shown to be a reliable indicator of the micropinocytotic uptake of vitellogenin by developing oöcytes. Trypan blue was injected into the mosquito Aedes aegypti to determine at what times after the blood meal vitellogenin was taken up. Histological sections examined by light microscopy showed that trypan blue began to be sequestered from 2 to 5 hr after the blood meal. Any association between dye and ovariole ended from 39 to 42 hr after the blood meal, in which period no dye was incorporated into spheres of yolk protein. Of the times investigated in this experiment, the greatest amount of dye was seen in the oöcyte at 24 hr after the blood meal. The onset and conclusion of trypan blue uptake correspond with the related events in the synthesis of vitellogenin by the fat body. The presence of trypan blue in occasional interfollicular spaces suggests that the route of entry of vitellogenin in Aedes aegypti is indeed an interfollicular one.     

Occurrence of heat-dissociable ribosomal RNA in insects: the presence of three polynucleotide chains in 26 S RNA from cultured Aedes aegypti cells

When RNA extracted from a mixture of cultured mosquito (Aedes aegypti) and hamster (BHK) cells is heated at 60 °C for five minutes the 26 S mosquito RNA but not the 28 S BHK RNA is converted to 18 S products. These products are not separable from each other or from pre-existent 18 S RNA on 2.4% acrylamide gels and have molecular weights near 0.7 × 10⁶. The large ribosomal RNA from insects belonging to ten different orders shows a similar conversion, although this property is absent in two species of aphid.A. aegypti 26 S RNA dissociates over a narrow temperature range. The reaction equilibrium favours dissociation and is dependent on ionic strength, showing a 6 deg. C change in T_m′ (the temperature of 50% dissociation) with tenfold change in salt concentration. Although the T_m of 26 S RNA from Drosophila melanogaster and A. aegypti is markedly different, reflecting the difference in base composition, the T_m′ of the two RNA species was virtually the same.High molecular weight ribosomal RNA from Escherichia coli, BHK cells and A. aegypti cells was terminally labelled with [³H]isonicotinic acid hydrazide. The specific activities of the large RNA species show the presence of one, two and three polynucleotide chains in 23 S, 28 S and 26 S RNA, respectively. A. aegypti 26 S RNA contains a small, heat-dissociable “IRNA” similar in relative amount and mobility to that found in BHK cells.