Male Firefly

might allow sister-sister discrimination. The locations of the cues used in discrimination are not known. Egg-stage switches indicated that eggs or the comb, or their combination, are adequate for discrimination of a sister's comb from a nonsister's. For several reasons (7) we suspect that discrimination cues reside in the comb, not in brood members themselves, and are odors applied by the foundress (8). If foundresses do odormark combs, then brood discrimination may be based on the same cues as those used by Polistes females in adult kin recognition (9). The reproductive status of preworker brood depends largely on their order of eclosion. Early emergents are workers, whereas later females are more likely to be potential queens (males do not appear until later in the summer). For this reason, age-biased responses to foreign brood could be expected and, in fact, such a bias appeared (Table 1); eggs were more often destroyed than were younger larvae, and older larvae and pupae were seldom destroyed. Also, the proportion of brood destroyed increased in older combs for both nonsister and sister switches (10). Desertions were uncommon in later nonsister switches as compared to NS-E switches (Fig. 1); only one NS-L and one NS-P female deserted (neither female was relocated). Response to foreign brood on the basis of relatedness might be important for Polistes females in at least two natural contexts: (i) cooperative colony-founding associations of sisters (11) and (ii) usurpation of a single foundress's comb by another female (12, 13), usually one of unknown origin (13). Subordinate foundresses in associations tolerate their dominant sisters' brood and thereby probably increase their own inclusive fitness. In contrast, usurpers show age-specific brood destruction patterns very similar to those observed in our nonsister experimental switches (13). Older brood are spared and are later, as workers, exploited by the usurper to rear her own offspring. By destroying brood the usurper gains three probable advantages: (i) cells are emptied for her own eggs, (ii) she removes a competitor's genes, and (iii) brood eaten by herself or fed to older larvae are a valuable food resource. JEFF E. KLAHN Department of Zoology, University of Iowa, Iowa City 52242 GEORGE J. GAMBOA Department of Biological Sciences, Oakland University, Rochester, Michigan 48063