ATMOSPHERIC PRESSURE 237 



the spectrum, have shown that violet rays exercise an apparent 

 stimulating effect upon growth in the case of flesh-fly larvae and 

 silkworms and also with animals other than insects. This 

 subject, however, has not been followed up in recent years, but 

 is one which merits critical examination with modern technique. 



Atmospheric Pressure 



Observations on the influence of changes of atmospheric pressure 

 upon insects are too scanty to admit of any deductions to be 

 drawn. A few field observations have been recorded which 

 indicate that such changes do affect insect behaviour, and certain 

 early experimenters adduced evidence that insects are able to 

 survive in a partial vacuum even for many days. The evidence 

 appears to indicate, however, that ordinary barometric changes 

 which prevail in Nature probably exercise no very profound 

 influence upon insect life. Recent experimental investigations 

 on the subject are very few. According to Pictet (quoted from 

 Uvarov, 1929), a uniform reduced pressure of 710 to 728 mm. 

 maintained throughout the pupal instar of Pieris rapce greatly 

 shortens development, the duration of this stage being reduced 

 to nine days as compared with a normal period of fifteen days. 

 The emergence of the adults appears to take place mostly at times 

 of reduced barometric pressure, a diminution of only 1 mm. being 

 stated to be sufficient to induce eclosion, provided the insect be 

 in a condition ready for emergence. It would appear that a fall 

 in atmospheric pressure operates through creating a difference in 

 relation to the internal pressure within the insect itself. 



Experiments conducted by Back and Cotton (1925) with 

 reference to the effects of vacuum treatment upon insects affecting 

 stored products revealed considerable differences in their powers 

 of resistance. Adults and pupae succumbed more quickly than 

 larvae when subjected to a vacuum of 28 to 29 inches in a bell- jar, 

 and of 24 to 28 inches in a concrete vault, at a temperature in 

 both cases of 15-5° to 21-0°. Three days' treatment in a concrete 

 vault killed not only all the adults of twelve out of eighteen 

 species, but also the pupae of all the species present in that stage, 

 and all the larvae of several species. It required seven days' treat- 



