116 =Mr. O. H. Latter on the Prothoracic Gland of 
places where the cocoons touched the glass numerous fine 
silky threads could be seen which had not become agglu- 
tinated to the main mass of the cocoon. Six other larvae 
of like age were placed in other glass vessels and provided 
with fragments of blotting-paper which had previously 
been soaked in solution of sodium carbonate and then 
dried. Five of these made cocoons incorporating the 
paper fragments and pupated. Hach of these cocoons 
presents a very large number of loose silk fibres on the 
external surface, and especially in the immediate neigh- 
bourhood of the alkaline scraps of paper, but on the inner 
surfaces and where the external surface touches the glass 
the texture is smooth and horny as usual. 
These experiments conclusively prove that the formic 
acid secreted by the larva is utilised not only for defen- 
sive purposes during larval life, but also for giving a 
special character to the silk of the cocoon, whereby the 
latter is rendered extremely tenacious to any foreign sub- 
stance brought in contact with it and also extraordinarily 
hard, waterproof, and completely closed against ail 
aggressors. When the exposed position of the cocoon on 
a tree trunk is considered it will at once be seen that this 
remarkable relation between the silk of this species and 
formic acid constitutes a wonderfully complete adaptation 
to the specific needs. 
lt would be interesting to endeavour to decide which 
of these two functions of the prothoracic gland is phylo- 
genetically the older. Personally, I am inclined to think 
that the defensive is the more ancient ; for the gland is 
as fully developed and relatively of the same size in the 
freshly-hatched larva as in that ready to pupate. This 
evidence is certainly not conclusive; and, indeed, the 
question can hardly be settled until more is known of the 
function of the prothoracic glands of other larvee—a 
point to which I hope to direct my attention this vear. 
Passing now to the structure of the prothoracic glands 
in this species. By way of preliminary I wish to enter 
an emphatic protest against the use of such terms as 
“neck” or ‘‘chin”’ glands in reference to these struc- 
tures. The word ‘“ neck” assuredly refers to the region 
between the head and thorax—a region which I have not 
yet seen in any lepidopterous larva; and to many minds 
an idea of dorsal position is associated with the word, 
whereas those glands are ventral, while ‘‘ chin,” though 
