Maculinea rebeli caterpillars spend their final instar as specific parasites of colonies of the ant Myrmica schencki. This paper describes 14 experiments in which 604 caterpillars were reared with six species of Myrmica. About 40% of caterpillars died during an initial period of integration: survival was significantly greater among the larger caterpillars, in M. schencki nests, and in nests that had ant-brood present. The growth of the survivors was extremely variable. After three weeks they had gained 2-18 times their initial body weight and appeared to belong to two distinct types: fast and slow developers. Only 11% of caterpillars died during the next 37 days of growth, before a fall in temperature to below c. 14-degrees-C caused them to overwinter. By then, the caterpillars weighed from 5-90 mg (mean 22 mg). Caterpillars of all weights lost 33% of their mass during winter, and 8% died, before rising temperatures caused growth to resume in spring. Of the 363 that survived winter, 44% died during a period of spring growth and 12% pupated after about seven weeks. The latter came mainly from the largest caterpillars before winter and pupal size (mean weight 81 mg) was also related to pre-winter caterpillar size. The remaining 44% of post-winter caterpillars continued to grow slowly and then entered another quiescent phase and were still alive after one year in the nest. A small number of these pupated in the second year, an average of 404 days after adoption. It is calculated that an average-sized Myrmica schencki colony of 350 workers can produce 4-5 adult butterflies; this laboratory result agrees well with field observations. The results are discussed in the light of a previous behavioural study.
