Are mosquitoes from lower latitudes smaller?

Body size is correlated with a large number of ecological and physiological variables and may be considered as an animal's most fundamental trait, in ̄uencing the organization of ecological communities and global biodiversity (Lawton, 1990). In mosquitoes (Diptera: Culicidae), body size has been shown to affect factors such as longevity (Nasci, 1987; Packer & Corbet, 1989), fecundity (Renshaw et al., 1994; Hurd et al., 1995) and size of the bloodmeal (Nasci, 1986). Many studies have examined the in ̄uence of latitude on body size, both between individuals of the same species, and between the different species in a community or biogeographical region. Work by Lindsey (1966) on poikilothermic vertebrates indicated that freshwater ®sh, marine ®sh, anurans and urodeles all show a tendency for there to be a higher proportion of small species in tropical latitudes, ful®lling Bergmann's Rule (Ray, 1960), although reptiles did not appear to show such a tendency. For insects, Schoener & Janzen (1968) demonstrated that the mean body length of Homoptera, Hemiptera, Formicidae and other Hymenoptera is greater in tropical than temperate samples, whilst Coleoptera and Diptera showed no clear correlation. More recently, attention has been focused on the relationship between insect body size and latitude. Formicine and myrmicine ants (Hymenoptera: Formicidae) in northern Europe show a decrease in body size with decreasing latitude (Cushman et al., 1993), as do bees of the family Andrenidae (Hymenoptera) in the eastern U.S.A. (Hawkins, 1995). In contrast, Australasian and Afrotropical butter ̄ies (Lepidoptera) tend to increase in size towards the tropics (Barlow, 1994), while North American bees from other families show no signi®cant relationship. The relationship between latitude and body size for several families of butter ̄ies has been examined recently across a number of biogeographical areas, with the authors failing to demonstrate any consistent pattern (Hawkins & Lawton, 1995). Many medical entomologists assume that tropical mosquitoes tend to be smaller than those from northern latitudes, and certainly some genera of mosquitoes with the smallest body size are predominantly tropical in their distribution, e.g. Haemagogus, Uranotaenia. Against this, however, some very large mosquitoes have a mainly tropical distribution, e.g. the Toxorhynchitini. We present data for latitudinal distribution and wing length (representing body size) of mosquitoes of North America, and the genus Anopheles in sub-Saharan Africa. Among Anopheles, intraspeci®c variation of body size is inversely correlated with needing multiple bloodmeals for completion of the gonotrophic cycle, strongly in ̄uencing vectorial capacity (Hurd et al., 1995; Hogg et al., 1996). Wing length is the most commonly used and readily available measure of body size in mosquitoes, showing good correlation with dry weight (e.g. Nasci, 1990; Renshaw et al., 1994); it was therefore adopted in this investigation. Intraspeci®cally, mosquito wing length can be inversely related to temperature and hence latitude (Le Sueur & Sharp, 1991). For our analyses, only female measurements were used, and species for which no wing length was available were excluded. When available, mean wing length was used but, where the data consisted of a range, the geometric mean of the two extremes was used. Wing lengths and latitudinal distributions of North American Culicidae were obtained from Carpenter & LaCasse (1955) and Darsie & Ward (1981). Data for Anophelinae were from Gillies & De Meillon (1968) and Gillies & Coetzee (1987). Latitudinal distributions were obtained from the distribution maps published in these monographs. Initial analyses were based on the mean wing length for all species of mosquito present in each 5° latitude band for North American species and 6° bands for African Anopheles, since Gillies & De Meillon (1968) and Gillies & Coetzee (1987) used 6° grid squares. Mean wing length in each latitudinal band was then plotted against latitude to determine if an overall relationship existed. The analysis was repeated for genera and subgenera containing more than ten species. No statistical analyses of these data were carried out as some mosquito species have a distribution that spans several latitudinal bands, violating the requirement for independence of data points. Second, the latitudinal mid-point of the distribution of each species was estimated from the published range and regressed against log-transformed wing length. Using this method, each species was represented only once, and regression equations and correlation coef®cients were calculated. The analysis was performed for all mosquitoes as well as for individual genera and subgenera. R