Water is an important factor in the ecology of many insects (see Hadley (1994) for a general review of across all arthropods). However, water is often overlooked in experimental studies in favour of other environmental variables, particularly temperature and photoperiod. The source of this bias is probably two-fold, firstly temperature and photoperiod are easier to manipulate in a laboratory setting than water. Secondly, when researchers talk about water and insects they usually mean liquid water but for insects, the sources, significance, and function of liquid water depend on the context. This also results in a complicated (but what isn’t?) relationship and compounds to the first challenge.
I work on insect egg development so let us explore the second challenge using insect egg development as examples. For insect eggs buried in the soil, the main determinant of water availability is the water potential of the substrate. Water potential is the capacity of a substrate to hold water, or in other words its affinity to water. Particle type, particle size, mineral content, and bulk density (the amount of air space) are some of the many variables that determine the water potential of a substrate. Contrast this to eggs laid above ground where water potential is not important. For example, an egg on a leaf could receive water from precipitation that is trapped around the egg or the leaf, perhaps due to the surface tension of water. For either case, metabolism is another potential source of water, but this requires development to have started. An egg currently in desiccation-induced dormancy may not be metabolically active for effective production of metabolic water. I won’t go into the importance of water for insect dormancy responses here – but it is important.
But what about water vapour?
Eggs above ground are surrounded by water vapour. Water vapour could come from the air or transpiration from the leaf, to stick with the leaf example. Water vapour is not liquid water but has been shown to be effective in insect development. Yoder and Denlinger (1992) is the first paper describing the role of water vapour for egg development of a stick insect, Extatosoma tiaratum. In the paper, they also found no evidence for water vapour uptake in other insect species examined. There hasn’t been a more recent examination of this relationship in insects since this paper (or have there? I’d like to know), but see Yoder et al. (2004) for an example in ticks. Moreover, where liquid water is involved it is almost impossible to isolate the presence of water vapour, especially within a small enclosed space so it is not inconceivable for eggs to uptake water vapour even in the presence of liquid water.
Early in my PhD I received a stick insect from my supervisor (who only ever seems to give me insects as presents ). This stick insect, a Sipyloidea nelida I think (left picture), eventually matured and parthenogenetically produced some eggs (unless a male stick insect snuck into the cage for a rendezvous but highly unlikely). I said I was going to try to hatch the eggs but I never did, for two years. During that time those eggs have been sitting in the laboratory, either at 25 or 30°C and about 30% RH, out of sight out of mind. When I did have a look several months later, one of the eggs had hatched, and I’m certain I have not intentionally sprayed them. I’ll save you from some grizzly photos of mummified nymphs.
I opportunistically collected more Sipyloidea eggs from Western Australia and kept them together. I think I sprayed these eggs only once after I found the first hatched egg and kept the PCR tubes I stored them open, but I can’t remember exactly when that was so I never directly took them out and sprayed them. A year after collecting them and months after getting sprayed, I checked the eggs again while cleaning the lab and found 5 more nymphs had hatched. Did they hatch after one indirect spray of water? Or did they receive enough water from the water vapour in the air? I have incubated the remaining eggs under various water conditions but in any case, I can’t answer those questions, unfortunately. I had eggs from only three females, each from a different location and from two species.
It would be awesome if this genus of stick insect was able to meet its water requirements for egg development from water vapour alone. It’s probable that the ability to uptake water vapour is a more widespread trait than currently known since it has only been looked at in a few distantly related taxa. This trait would also be ecologically interesting for this parthenogenetic genus because this genus is widely distributed across Australia, including arid and semi-arid zones. Independence from the presence of liquid water would be a neat adaptation to the dry and unpredictable climate of the Australian bush, which would potentially explain their widespread distribution. These eggs have a morphological feature at the anterior end of the egg which looks like part of a dandelion fluff. The role of this morphological feature is to collect water, presumably liquid water but an alternative explanation is that it allows for the condensation and collection of water vapour. Perhaps a project for another student at another time?
For experiments, manipulating water potential in the laboratory is not impossible. Vermiculite is one option, I’ve never used it so I can’t say how accurate it is. Extremes of the wet/dry spectrum are easy enough. Desiccants such as Drierite will remove water vapour for as long as it is active. Water vapour is commonly measured by the relative humidity, but can also be quantified as the dew point. Relative humidity is the amount of water vapour in the air compared to the total amount possible at a given temperature. Dew point is the temperature at which water droplets will form. Most papers will refer to relative humidity (if at all) but it may not be as descriptive as dew point. Like water potential, water vapour is not easy to manipulate in the lab and has the extra complication of requiring chemicals (there may be other ways of doing this). Super-saturated aqueous solution of different salts will produce different relative humidity. Conveniently, salt (NaCl) will produce a relative humidity of 75%, there are tables of this information, try Wexler and Hasegawa (1954).
Although water is experimentally more challenging to manipulate and control in a laboratory setting than commonly measured variables, and influences individuals in many ways and forms, water in any phase should not be overlooked in studies because the presence of water can also be dependent on other variables or vice versa. A humid day may feel hotter than a dry day of the same air temperature because of the lack of evaporation. After all, water is only one of the many interlinked environmental variables that, in combination, affect the ecology, behaviour, and physiology of individuals.
UPDATE: none of the eggs have hatched at 30°C regardless of water availability. The eggs looks like they have started developing but maybe something is not quite right for them to hatch.
Hadley, N. F. 1994. Water relations of terrestrial arthropods. Academic Press, Inc., San Diego.
Wexler, A., and S. Hasegawa. 1954. Relative humidity-temperature relationships of some saturated salt solutions in the temperature range 0° to 50°C. Journal of Research of the National Bureau of Standards 53:19-26.
Yoder, J. A., J. B. Benoit, and A. M. Opaluch. 2004. Water relations in eggs of the lone star tick, Amblyomma americanum, with experimental work on the capacity for water vapor absorption. Experimental and Applied Acarology 33:235-242.
Yoder, J. A., and D. L. Denlinger. 1992. Water vapour uptake by diapausing eggs of a tropical walking stick. Physiological Entomology 17:97-103.
 Number of insects I have received from him, excluding ones collected on joint field trips = 29 and expecting more at the time of writing– FYI cake is also acceptable .
 Number of baked goods I have received, excluding at parties = 0