Extreme resistance to desiccation in overwintering larvae of the gall fly Eurosta solidaginis (Diptera, Tephritidae)

During winter, larvae of the goldenrod gall fly Eurosta solidaginis are exposed for extended periods to severe low ambient temperatures and low humidities within plant galls. The resistance of these larvae to desiccation at various temperatures and humidities, the transition (critical) temperature, and the effects of treatment with organic solvents on the larval rates of water loss and on changes in osmolality during desiccation were examined. The water loss rates of the flesh fly Sarcophaga crassipalpis under desiccating conditions were also measured. The water permeability of the cuticle of E. solidaginis larvae was very low (0.038 mu g h(-1) cm(-2) Pa-1 at 20 degrees C and 4% relative humidity) compared with that of larvae of other species. The value for E. solidaginis is equivalent to that of the very drought-resistant larvae of the tenebrionid beetle Tenebrio molitor (0.038 mu g h(-1) cm(-2) Pa-1 at 30 degrees C). In contrast, the permeability of larvae of the flesh fly Sarcophaga crassipalpis at 20 degrees C and 4 % relative humidity was 0.331 mu g h(-1) cm(-2) Pa-1. The thermal dependence of the cuticular permeability increased with temperature by approximately 0.0010 mu g h(-1) cm(-2) Pa-1 degrees C-1 in the interval between 4 and 40 degrees C. At the transition temperature of 40 degrees C, the thermal dependence of the permeability increased abruptly to 0.0400 mu g h(-1) cm(-2) Pa-1 degrees C-1. Larvae treated with hexane and acetone remained remarkably resistant to water loss. However, treatment with chloroform:methanol increased the water loss rate approximately 25-fold. During desiccation at 4 degrees C and 4 % relative humidity for 21 days, E, solidaginis larvae showed a mass loss of 18.5+/-4.4 % (mean +/- S.E.M., N=6), Animals dried under the same conditions over the same period showed haemolymph osmolality of 851+/-75 mosmol kg(-1) (N=4). Larvae freshly removed from the galls showed a haemolymph osmolality of 918+/-67 mosmol kg(-1) (N=3). A higher osmolality in the dried compared with the fresh larvae would have been expected. The present observation suggests that important ions in the haemolymph may have been excreted or rendered osmotically inactive during desiccation.