54 ONERA A. M. HAWKES 



were invariably damp, or even saturated with moisture. The 

 brown of the cocoons was so striking, that a special cause was 

 at once sought. Now, the only difference in the environment 

 of these solitary larvae and those in the large cages was moisture : 

 hence, there seemed a possibility that moisture might be a factor 

 in cocoon colour. In all my experiments the larvae were reared 

 exclusively on Privet (Ligustrum vulgare). 



The forests in which the Philosamias live are very moist at 

 certain seasons, but I have been unable to obtain any satisfactory 

 information as to the degree of moisture of these forests, and as 

 to whether the moist seasons coincide with the periods when these 

 larvae spin their cocoons. 



In order to test the effect of moisture, the following tj^pical 

 experiments were made. 



A. Experiments on unfinished cocoons 



1. A larva began to spin a cocoon in a dry room. At 10 p.m. the 

 first threads were a pale brown. The larva was removed from the 

 cage and placed in a dried box. The next morning, at 8 a.m., the cocoon 

 was complete and a pale brown colour. This expe-riment was repeated 

 several tunes and it was found that if the orignal silk were brown, the 

 cocoon was broAvn, but not a deep brown. When subsequently ex- 

 posed to moisture, cocoons made under these conditions, became a 

 deep red-brown. 



2. A larva began a cocoon of white silk at 9 a.m. At 10 p.m., the 

 cocoon was white but incomplete; the cocoon was then removed to a 

 dry box containing CaCl2. By 8 a.m., the following morning, a good 

 cocoon was complete, the colour being a dirty white. The cocoon was 

 left in the cell and the pupa died of desiccation. 



3. A larva at 9 p.m. had completed a pure white platform. The 

 larva and the platform were then placed in a damp cell with a dozen 

 broken privet leaves. The following day at 10 a.m., the platform. had 

 become brown, but it was deserted by the larva, which had spun a 

 complete new brown cocoon. On the evening of the same day, the 

 leaves were removed from the box, the box dried and CaCl2 introduced. 

 After ten days the colour was unaltered, but the pupa had died of des- 

 iccation. This larva finally spun a dark cocoon although it was dis- 

 turbed by me and as a result deserted its original cocoon. 



4. A larva began to spin brown silk in a damp cell. The leaves which 

 produced the moisture were at once removed, the cell dried, and CaCl2 

 introduced. The cocoon was finished and was a pale brown colour. 

 Three months later, when the cocoon was placed in a very damp box, 

 it became much darker. 



