Background: One of the major problems concerning dengue transmission is that embryos of its main vector, the mosquito Aedes aegypti, resist desiccation, surviving several months under dry conditions. The serosal cuticle (SC) contributes to mosquito egg desiccation resistance, but the kinetics of SC secretion during embryogenesis is unknown. It has been argued that mosquito SC contains chitin as one of its components, however conclusive evidence is still missing.

Insects have been extraordinarily successful in occupying terrestrial habitats, in contrast to their mostly aquatic sister group, the crustaceans. This success is typically attributed to adult traits such as flight, whereas little attention has been paid to adaptation of the egg. An evolutionary novelty of insect eggs is the serosa, an extraembryonic membrane that enfolds the embryo and secretes a cuticle. To experimentally test the protective function of the serosa, we exploit an exceptional possibility to eliminate this membrane by zerknü llt1 RNAi in the beetle Tribolium castaneum. We analyse hatching rates of eggs under a range of humidities and find dramatically decreasing hatching rates with decreasing humidities for serosa-less eggs, but not for control eggs. Furthermore, we show serosal expression of Tc-chitin-synthase1 and demonstrate that its knockdown leads to absence of the serosal cuticle and a reduction in hatching rates at low humidities. These developmental genetic techniques in combination with ecological testing provide experimental evidence for a crucial role of the serosa in desiccation resistance. We propose that the origin of this extraembryonic membrane facilitated the spectacular radiation of insects on land, as did the origin of the amniote egg in the terrestrial invasion of vertebrates.

Despite its vector importance little attention is given toDengue and yellow fever are considered two of the most important viral diseases transmitted by arthropods (Gould & Solomon 2008). The mosquito Aedes aegypti, vector of urban yellow fever and also the main dengue vector, is widely distributed worldwide, being usually found between latitudes 35° N and 35° S (OPAS, 1995). Ae. aegypti embryos tend to enter dormancy at the end of embryogenesis, being able to keep its viability inside the egg for six months or more, according to the environmental relative humidity (Christophers 1960, Kliewer 1961, Clements 1992. When the surroundings become favorable to larval development, e.g. flooding of larvae habitat, dormancy is interrupted and the larva hatches (Christophers 1960, Clements 1992. Little attention has been given to the influence of temperature on Ae. aegypti embryonic development, when compared to other mosquito species (Rosay 1959, Christophers 1960, Trpiš et al. 1973, Clements 1992, Gillooly & Dodson 2000. The precise time Ae. aegypti takes to complete its embryogenesis at distinct temperatures is not known (Christophers 1960, Consoli & Lourenço-de-Oliveira 1994. In the present work the influence of distinct constant temperatures on Ae. aegypti embryogenesis, egg viability and the time course of larval hatching were analyzed.To induce Ae. aegypti larval eclosion from eggs previously stored in our laboratory for one month, preliminary tests were performed utilizing conditions described as effective stimuli in the literature (Gander 1951, Barbosa & Peters 1969, Novak & Shoroyer 1978. In all cases mosquitoes from the Rockefeller strain were used. The conditions used were: (i) dechlorinated water; (ii) deaerated dechlorinated water; (iii) 0.3 mM ascorbic acid in Ae. aegypti larval hatching, in our laboratory conditions, is the 0.3% (w/v) YES, the rate of egg hatching being 80% on average (data not shown).We observed that the embryonic dormancy state of Ae. aegypti is facultative, as expected for a multivoltine species (Forattini 2002), and could be bypassed by adding YES at the end of embryogenesis. Preliminary experiments have been performed in order to time this event, by adding YES at distinct hours after egg laying. Synchronous egg laying was performed during 20 min, as recently described (Rezende et al. 2008). After this period, eggs were kept at the distinct temperatures employed in the experiments. The precise moment of hatching of the first larvae was then observed for each temperature (see Table for the values). For all further experiments, YES was added 2 h before this moment. In this way, it was possible to monitor Ae. aegypti eclosion immediately after completion of embryonic development. For each experiment at a constant temperature, triplicates from the same egg laying were utilized. These eggs were kept on moist Whatman # 1 paper until YES was added. From this moment on, larvae hatching was observed every 30 min, until no more hatching occurred. Three experiments were performed for each tested ...