Water vapour uptake by diapausing eggs of a tropical walking stick

This study is the first to demonstrate the capacity of an arthropod egg, that of a tropical walking stick Extatosoma tiaratum (Macleuy), to absorb water vapour from the air. This species diapauses both as an early embryo and then again as a pharate first instar larva, and both stages are capable of absorbing water vapour. Water vapour absorption occurs at lower humidities and at a lower rate for an egg in early embryonic diapause (c. av 0.30, 0.516 mg h-1δv-1) than in the diapausing pharate first instar (c. av 0.60, 0.725 mgh-1δav-1) at 25dC. In addition to having the capacity to gain water at very low vapour activities, water is efficiently conserved as indicated 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 25dC). Eggs that have been killed lose water when held at a hydrating vapour activity, thus implying that active uptake contributes to net absorption. Wax block experiments suggest that water is absorbed over the entire chorionic surface. Eggs of five other insect species that were examined [Lymantria dispar (L.), Bombyx mori (L.), Antheraea polyphemus (Cram.), Oncopeltus fasciatus (Dallas) and Diaferomera femorata (Say)] lacked the ability to absorb atmospheric water.