494 Observations on the Development of Ox Warble, 
presence of any distinct lining membrane. With the enlarge- 
ment of the passage its shape became more cone-like (corre- 
sponding with the altering form of its tenant) ; and on March 5, 1 
found for the first time a distinct pellicle or skin-like membrane 
covering the walls of the perforation, or passage, and continuous 
with the lining of the maggot-cell below. The amount of 
development of the warbles at a given date varied very much; 
warbles just beginning to form might be found together with 
those nearly three-quarters of an inch across, as measured on 
the flesh side of fresh hide on the 4th of March, but, when once 
started in growth, the progress was excessively rapid, and those 
who wish to watch the progress of the first stages have need to 
be on the alert. 
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 absence 
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 brcathivff-tubes, — a very 
important consideration relatively to available methods of de- 
stroying the creature. At the period, however, of its moult to 
its final stage, a change takes place respectively in the nature, or 
in the amount, of development of nearly the whole of both the 
internal and external structure of the maggot. The hard tips 
necessary, or at least serviceable, for forcing a passage up the 
hide, are no longer needed, and they are exchanged for a broad 
form of spiracle (Fig. 5), and the internal organs become suited 
to provide material for the development of the fly, which will 
presently form in the dry husk of the maggot which serves as 
the chrysalis case. 
One of the first and most remarkable of these changes is the 
complete alteration in the form of the spiracles. Up to this 
stage the general form continued (see Fig. 6) to be that of a pair of 
short horny, somewhat bent cylindrical, or partially cylindrical 
tubes, covered at the end (Fig. 8) with round or oval discs, which 
appear to have a definite narrow border, and across the centre 
of the disc to be of a sieve-like or spotted appearance. Fig. 9 
precisely represents the appearance when much magnified. These 
discs may amount to as many as about six-and-twenty on each 
spiracle, and appear to me to be placed each at the extremity of 
short cylinders. The structure is most elaborate and peculiar, 
and the only somewhat similar instance of this development in 
any maggot that I am aware of having been observed, is in the 
structure of the cephalic spiracles of the larva of the Trypeta 
