48 
not the slightest trace of a dead larva. Subsequently I was surprised to 
find part of a mine and a cocoon lower down on the same twig. When 
I examined this leaf a few days previously there was neither mine nor 
egg upon it. It appeared strange, but it was not till later that I con¬ 
nected the two events. On a later occasion-—August 2nd, 1901— 
another larva was missing from a mine, and this one I traced into the 
petiole of the leaf, and found it about a quarter of an inch below the 
base of the leaf. The next day this larva had mined down round the 
leaf-stalk, and was brought up against a small leaf bud. The follow¬ 
ing day (August 4th) the larva was mining at the base of the petiole 
of another leaf, as though about to enter the leaf. On August 5th 
the larva had continued the mine down the twig instead of ascending 
into the leaf. August 6th, the larva was dead in the mine. It had 
mined down the stem below the water in which I kept the twig, and 
was drowned. As above mentioned, the eggs are often laid on very 
small leaves—-too small for a larva’s wants—but I fancy these leaves 
continue to grow while the larva mines. If they do not do so the 
larval habit of travelling from one leaf to another must be of common 
occurrence. However, I have seen hundreds of larvae, and have never 
observed this travelling in a state of freedom. A larva will always 
avoid crossing the mine of another, but where there are two or more 
in a small leaf this becomes inevitable. Should a larva, in crossing 
another mine, come in contact with its inmate it is certain to kill it, 
not purposely, but incidentally, in the course of its operations. There 
is no hope for the victim once its delicate skin comes in contact with 
the intruder’s jaws. 
Pro-thoracic Shield.— The dark brown ovoid patch on the larval 
prothorax is w T orthy of notice. It may be connected with a pro- 
thoracic shield, but I am scarcely inclined to think its colour is. I 
view it rather as of sematic significance. First of all, it only occurs 
on those larvae which, by their position, are exposed to the light, and, 
therefore, more likely to be seen by their enemies. Secondly, it is 
situated not immediately behind the head but on the posterior portion 
of the prothorax. This mark is absent in the first instar, but clearly 
visible in the second, whilst in the third it appears in full force. The 
colour may simply be due to the access of light, light being, as Wood 
remarks (Tutt, Brit. Lep., i., p. 170), “ the most general and potent 
factor in the production of pigmentation,” and, certainly, I think light 
is in this case the direct cause, for if we take a young larva mining 
the underside of a leaf and turn the leaf so as to expose the underside 
to the light, the colour in the patch will appear in the third instar. I 
have proved this by direct experiment, and have also noticed one case 
(September 13th, 1901), where a larva mining on the underside bore 
the black patch. The leaf in which it mined was so situated that the 
undersurface was exposed to the light. The first time I noticed this 
patch I passed the larva by, as I thought it was diseased. When a larva 
dies in the mine dark patches often appear, the result, perhaps, of a 
growth of some micro-fungus. Therefore, I fancy this dark patch 
may be something of the nature of a warning colour, like the black 
sunken patch in the larva of Stauro/ms fat/i, as mentioned by Poulton 
(Colours of Animals, p. 281), a warning, for instance, for a parasite 
that the larva is moribund, and therefore useless for its purpose. 
