Many studies have identified various host behavioural and ecological traits that are associated with parasite infection, including host gregariousness. By use of meta-analyses, we investigated to what degree parasite prevalence, intensity and species richness are correlated with group size in gregarious species. We predicted that larger groups would have more parasites and higher parasite species richness. We analysed a total of 70 correlations on parasite prevalence, intensity and species richness across different host group sizes. Parasite intensity and prevalence both increased positively with group size, as expected. No significant relationships were found between host group size and parasite species richness, suggesting that larger groups do not harbour more rare or novel parasite species than smaller groups. We further predicted that the mobility of the host (mobile, sedentary) and the mode of parasite transmission (direct, indirect, mobile) would be important predictors of the effects of group sizes on parasite infection. It was found that group size was positively correlated with the prevalence and intensity of directly and indirectly transmitted parasites. However, a negative relationship was observed between group size and mobile parasite intensity, with larger groups having lower parasite intensities. Further, intensities of parasites did not increase with group size of mobile hosts, suggesting that host mobility may negate parasite infection risk. The implications for the evolution and maintenance of sociality in host species are discussed, and future research directions are highlighted.

The blue biliverdin pigment in the haemolymph of Schistocerca gregaria Forskål (Orthoptera: Acrididae) was shown to be associated with all the stages and ages of the solitary-reared (solitarious) insects, irrespective of their origin or diet. The pigment was absent or present in very small quantities in the crowd-reared (gregarious) insects. The ratio of absorbance at 460 nm and 680 nm, the former representing the λmax for the carotenoid pigment and the latter for the biliverdin, provides a clear-cut index for partitioning the phases. The mean value of the index for the gregarious group fell in the range 3.69 to 4.78. That of the solitarious group ranged from 0.64 to 1.67. The absorbance ratio at 460/680 nm appears to represent a simple and convenient means of differentiating the two phases of the desert locust, Schistocerca gregaria.