Y. Higa tanks, ant traps, used tires and plastic buckets around human dwellings [10–12]. Many domestic animals are also available for blood meals, a situation often seen in tropical countries, but Ae. aegypti is highly anthropophilic and feeds in the daytime. Its preference for humans as a host is an important factor for transmission [13, 14]. In addition, the species prefers dark places with moisture for blood feeding and resting [10]. The indoors and urban areas apparently provide environments which Ae. aegypti prefers over the outdoors and suburban/rural areas. Thus, the species is usually abundant in the indoors and urban areas in tropical countries [15, 16], and the close association with humans contributes largely to the effective transmission of dengue viruses. Aedes albopictus Ae. albopictus originates in the Orient and is distributed throughout tropical and temperate areas [2, 17]. Unlike Ae. aegypti, Ae. albopictus eggs have the ability to diapause during the winter season in temperate zone Asia [18, 19]. Larvae breed from a wide variety of natural and artificial habitats such as bamboo stumps, tree holes, discarded tires and flower vases [2, 3, 11, 20]. Since hosts of Ae. albopictus blood feeding include not only humans but also a wide range of amphibians, reptiles, birds and mammals in forests and domestic environments, the species has been regarded as a bridge vector of zoonosis pathogens [2, 3, 13, 21–24]. In fact, it has been reported that the species experimentally transmits many viruses and that medially important arbo- viruses were isolated from Ae. albopictus caught in the wild [2, 23]. Recently, it was reported that Ae. albopictus was involved in chikungunya epidemics in Italy, India and else- where [25–27]. In the domestic environment, Ae. albopictus prefers vegetation and feeds and rests outdoors [28–32]. Because of the preference for vegetation, the density of Ae. albopictus is usually high in rural and suburban areas [11, 33–39]. Biting usually takes place at dawn and dusk, although nighttime biting is sometimes observed [3, 40]. The geographic distribution of Ae. aegypti and Ae. albopictus overlaps in Asia, Americas, the South Pacific and some parts of Africa. However, the niche characteristics of the adult are somewhat different as described above. Recently, Lambrechts et al. (2009) reviewed the role of Ae. albopictus in past dengue epidemics and compared its dengue virus vector competence with that of Ae. aegypti [41]. They noted that although Ae. albopictus is overall more susceptible to dengue virus midgut infection, rates of virus dissemination from the midgut to other tissues are signifi- cantly lower in Ae. albopictus than in Ae. aegypti. With regard to biological and ecological characteristics and rates of virus dissemination, vectorial capacity is much higher for Ae. aegypti than for Ae. albopictus. Although Ae. albopictus 19 is definitely responsible for epidemics of DF in Japan and other countries [23, 42], Ae. aegypti is a primary vector of DF/DHF worldwide. SPATIAL DISTRIBUTION OF DENGUE VECTORS Distribution along the north-south gradient The fact that Ae. aegypti is distributed in tropical regions and Ae. albopictus from tropical to temperate regions shows that the geographical distribution of the two species differs along the north-south gradient. In Vietnam, where the climate is both tropical and subtropical, the difference is very clear. Higa et al. (2010) showed the pronounce effect of the north-south gradient on the spatial distribution of Ae. aegypti and Ae. albopictus in Vietnam, and they indicated that the distribution and survival of these mosquitoes was strongly affected by climatic factors such temperature, humidity, and precipitation [43]. Ae. albopictus was highly abundant in northern Vietnam, while Ae. aegypti was abundant in the south. In the central part of the country, the two species co-existed. The change of response to climatic conditions was possibly one of the factors in the sequential change of infestation of Ae. aegypti and Ae. albopictus along the north-south environmental gradient in Vietnam [43]. In Japan, like Vietnam, Ae. aegypti was previously established in the Ryukyu Archipelago where the climate is subtropical, while Ae. albopictus was very common on the mainland where the climate is temperate [17]. The ecological mechanisms of dengue vectors along the north-south gradient can be defined as follows. In a region where one species dominates over the other, climatic conditions highly favor the former species. For example, Ae. albopictus is distributed in temperate regions as well as tropical regions, and thus it is more adaptive to cooler climates than Ae. aegypti. Moreover, the eggs and adults of Ae. aegypti are resistant to desiccation which makes Ae. aegypti more adaptive to a hot and dry environment than Ae. albopictus [3, 43–46]. In such regions, the distribution of the less adaptive species is marginal or seasonal, and the adaptive species is superior to the other in all stages. Therefore, the adaptive species dominates over the other and expands its distribution. On the other hand, in the region where Ae. aegypti and Ae. albopictus co-exist, the climate may be moderate for both species and one species does not dominate over the other in all the stages. In such regions, habitat heterogeneity along the urban-rural gradient and interspecific competition may be more important than climate as a factor in spatial distribution [36, 46]. In Vietnam, the number of reported cases of DF/DHF is associated with the distribution of Ae. aegypti, indicating that this species plays a significant role in the country [47].