WATER-VAPOR UPTAKE BY DIAPAUSING EGGS OF A TROPICAL WALKING STICK

This study is the first to demonstrate the capacity of an arthropod eg g, that of a tropical walking stick Extatosoma tiaratum (Macleuy), to absorb water vapour from the air. This species diapauses both as an ea rly embryo and then again as a pharate first instar larva, and both st ages are capable of absorbing water vapour. Water vapour absorption oc curs at lower humidities and at a lower rate for an egg in early embry onic diapause (c. a(v) 0.30, 0.516 mg h-1 DELTA-a(v)-1) than in the di apausing pharate first instar (c. a(v) 0.60, 0.725 mg h-1 DELTA-a(v)-1 ) at 25-degrees-C. In addition to having the capacity to gain water at very low vapour activities, water is efficiently conserved as indicat ed by the low rate of water loss (0.015% h-1 in the early embryo and 0 .046% h-1 in the pharate larva at 25-degrees-C). Eggs that have been k illed lose water when held at a hydrating vapour activity, thus implyi ng that active uptake contributes to net absorption. Wax block experim ents suggest that water is absorbed over the entire chorionic surface. Eggs of five other insect species that were examined [Lymantria dispa r (L.), Bombyx mori (L.), Antheraea polyphemus (Cram.), Oncopeltus fas ciatus (Dallas) and Diaferomera femorata (Say)] lacked the ability to absorb atmospheric water.