SECT. 6] OF THE EMBRYO 905 



And a good deal of evidence exists, indicating that insect eggs, 

 although terrestrial, need a humid environment for proper develop- 

 ment. Thus Harukawa obtained the following figures on the Oriental 

 peach-moth : 



Relative humidity Percentage of eggs hatching 



and it is known that the Trinidad froghopper, Tomaspis saccharina, 

 cannot hatch at all under 90 per cent, humidity. Again, Peterson has 

 shown that Aphid eggs are very dependent on their normal humidity 

 for proper hatching, which is easily stopped by a small decrease in 

 the water-content of the environment {Aphis avenae and Aphis pomi) 

 and Dampf, Hoffman & Varela have shown the same thing for various 

 grasshopper's eggs. (See also Tchang and Andersen.) 



It remained for Bodine to show in 1929 that the water-content of 

 the whole system in grasshopper and other orthopteran eggs rose 

 during development. This process was evidently closely associated 

 with the egg's metabolism for increase of temperature accelerated the 

 water-intake. We may therefore have to picture the insects as solving 

 the problem of terrestrial embryonic life, not by providing enough 

 water in the eggs from the maternal body, as the sauropsida do, but 

 by inventing a sort oi deliquescent egg which should absorb atmospheric 

 moisture. In this connection, Peacock has made some suggestive 

 experiments on the eggs of the saw-fly Pristiphora pallipes which 

 normally inserts them in pockets artificially contrived in gooseberry- 

 leaves. It seems very probable that water is absorbed by the eggs 

 from the plant, for Peacock found that if the stalk of the gooseberry- 

 twig was immersed in a weak solution of eosin, the dye would pass 

 into the leaves and thence into the eggs. On the other hand, if the 

 eggs were removed from the pockets at the beginning of development 

 they hatched as usual, being able, apparently, to pick up from the air 

 all the moisture they required, and swelhng normally. Swelling, in 

 fact, seems to be a regular occurrence in the development of the eggs of 

 all Tenthredinidae, Cynipidae,and Formicidae. Kerenskihas shown 

 that the eggs of the scarab beetle, Anisoplia austriaca, double their wet 

 weight in a fortnight and that this increase can be at the expense of 

 distilled water, no dissolved substances being taken up, and develop- 

 ment proceeding normally. As insect eggs are so small, they no doubt 

 make use of " micro-climates ", for small crevices etc. may have 

 a very different humidity from the main climate in which they exist. 



58-2 



