Invertebrate hosts of parasites surpass by far the function of a vector in the literal sense, i.e. of a host that gives a direction to the parasite’s transmission. The biology of the vector mediates, first, the geographical and the local dispersion of the parasite and determines thereby an important part of the epidemiology of the disease. Second, the vector as an intermediate host (IH) provides a vital part of the substrate for the parasite’s development and propagation. Third, the vector population influences, by its seasonally changing density and other features, the population dynamics of the parasite.
Invertebrates develop r-selective populations (ecologists name them r-strategists) as a rule, whereas vertebrates form K-selective populations. The characteristics of the two populations are diametrically opposite (tab. 6.1, page 242). Cyclically transmitted parasites often combine, during their life cycles, a K- and a r-strategist as hosts, e.g. a mammal and a mosquito. The evolutionary adaptation to such vastly different hosts (in all respects) has marked selective advantages. To judge this function adequately, knowledge of the arthropod structures on which they are based is essential. In order to describe simply the development of the parasite stages in its invertebrate host we have to be familiar with its anatomy. Comprehension of the character of invertebrate defence reactions requires consideration of invertebrate physiology.