138 PHYSIOLOGICAL REGULATIONS 



While many kinds of insects are highly protected at their sur- 

 faces from rapid evaporative losses, others are known to be more 

 vulnerable to such losses. This has given rise to the question, are 

 insects able to choose environments that tend to minimize evapora- 

 tion? There is no doubt that some species frequent atmospheres of 

 a particular humidity ; indeed, structures that detect differences of 

 humidity have been described in spiders (Blumenthal, '35) and in 

 mealworm beetles (Pielou, '40). The clearest case of preference 

 for high humidity is in the isopod crustacean Porcellio (Gunn, '37) ; 

 it is indifferent to atmospheres of 65 or more per cent relative 

 humidity, but in the region of half saturation distinguishes differ- 

 ences as little as 6 per cent. In general the animal keeps moving 

 rapidly in dry air, becoming quiescent only when it arrives in a 

 moist region ; in addition it orients away from dry air if it suddenly 

 comes into a boundary region. Some other species shun wet air 

 (locust, Kennedy, '37; mealworm beetle, Pielou and Gunn, '40). 



In the cockroach (Blatta) the preference for moist air is mani- 

 fest only after previous desiccation (Gunn and Cosway, '38). The 

 correlation between behavior and water deficit then constitutes 

 regulation in the sense of all the studies here considered. For, in 

 the humid air the rate of water loss is believed to be diminished; 

 hence the modification of behavior tends to preserve the body's 

 water content. 



To some extent the ability of insects to move toward bodies of 

 water has been investigated, and positive attractions have been 

 demonstrated in several species (Turner, '24; Krijgsman, '30; 

 Hertz, '35). This behavior which in some is dependent upon sight, 

 is an integral part of success in gaining water to compensate for 

 deficits. 



Amid the diverse studies of insects in relation to water, data are 

 insufficient to furnish an equilibration diagram, but enough to sug- 

 gest that each of the constituent compensations exists. Neither 

 turnover, variability of water content in an individual, nor modi- 

 fication ratio can be stated. Certainly behavior toward humidity of 

 environment contributes to maintenance of content in some species ; 

 the complete story requires the correlation within one species, of 

 this mode of adjustment with the others. 



§ 48. Phascolosoma 

 Does an animal that lives in the ocean have an easier time main- 

 taining its water content than others? The marine gephyrean 



