The size relationship between insects and their hosts

.1 Regressions of adult insect size on host size were tested. The analysis was restricted to highly host‐specific insects that had thus been exposed to a narrow range of host size. 2 The phylogenetic regression method was used to analyse the data, so as to allow for the possible phylogenetic effects in cross‐species data. 3 Significant positive regressions were found in all the groups tested: female flower thrips, Meligethes species of pollen beetle, tephritid flies, and male and female fleas. They were also found by standard regressions within. and between Actornithophilus species of bird louse. 4 The regression of thrips size on pollen size was not significant, providing evidence against a hypothesis involving food size. 5 The regression of flea body size on host hair/feather length was significant, which is consistent with a hypothesis involving the size of spaces on the host. 6 The relationships may have implications for the understanding of evolutionary host shifts.