270 Prof. L. C. Miall and Mr. T. H. Taylor on the 
In the second larval stage the tracheal extensions are 
not dilated, but each is connected with an air-containing 
cylinder terminating in an end-plate, which forms a knob- 
like projection on the surface of the body. The pits with 
their appendages and filaments, if they are actually 
present, are difficult to demonstrate. 
III. Pupation. 
About the middle of March the larva is full grown and 
ready for pupation. It is still imprisoned within the leaf, 
but can move freely about in the blister, which is now a 
large open space lying between the leaf-veins and the 
epidermis. The palisade-cells are always the first to be 
attacked ; sometimes the spongy cells are excavated as 
well, but this comes later; in any case the epidermis with 
the cuticle is left intact. 
Before pupation the larva prepares an opening, through 
which it may afterwards emerge as a fly. To this end 
it pares down the epidermis in one place until only the 
external cuticle remains. This area, which may be called 
the “pupal blister,” is parabolic in plan, and although very 
small is easily distinguished from the rest of the larval 
blister by its different colour. The apex of the pupal 
blister is sharply defined, but the base gradually shades 
off. It now becomes possible to observe the movements 
of the larva through the transparent cuticle. The edge of 
the pupal blister is pierced; the larva turns over and lies 
on its back with the ventral surface pressing against the 
cuticle; the movements of the oral plates become slower 
and at length cease. Two short black rods now become 
visible in a strong light; these are the anterior spiracles 
projecting from the top of the prothorax. At first they 
lay behind the exposed part of the head, and pcinted 
upwards and backwards. The head is now completely 
retracted into the thorax, and the spiracles occupy the 
front extremity of the body, having swung round so as to 
point forwards. After a few preliminary trials they are 
passed through the slit in the cuticle, and become fully 
exposed to the outer air. They form a black speck on the 
apex of the pupal blister, and can easily be felt by the 
finger-tip. 
