46 
and, on March 5th, I found for the first time a distinct pellicle or 
sldn-like membrane covering the walls of the perforation, or 
passage, and continuous with the lining of the maggot-cell below. 
The great change both in the appearance and the internal 
structure of the maggot took place when it was grown to about a 
third of its full size, when it assumed 
its well-known shape. Previously to 
this, whilst the work of forming its 
passage was still in progress, its chief 
characteristics externally were the ab¬ 
sence of everything that could obstruct 
its power of pressing onwards; and 
internally it was little more than a bag 
of fluid, with a large proportion of the 
space occupied by breathing tubes, —a very 
important consideration relatively to 
available methods of destroying the 
creature. 
From the earliest stages which I had 
opportunity of observing up to date of change mentioned in preced¬ 
ing paragraph, the general form of the tracheas and the spiracles at 
their extremities (that is, of the breathing-tubes and the breathing 
apparatus at their tips, see above figure) continued to be that of a 
pair of long tubes, each ending in a short, horny, somewhat bent 
cylindrical, or partially cylindrical, tube, covered at the end with 
round or oval discs, through which the air is drawn in. * 
Spiracles fully developed, end view, magnified. 
Breathing-tubes of maggot, 
magnified. 
At the period, however, of its moult to its final condition a change 
takes place respectively in the nature, or in the amount of develop¬ 
ment, of nearly the whole of both the internal and external structure 
* Figures of the apparatus, taken from my own dissections, greatly magnified, 
are given in various of my Annual Reports, notably in my Eighteenth, p. viii. 
of Appendix, but these minute microscopic details are not needed here. 
5 
