WARBLE FLY. 
103 
more than a bag of fluid, with a large proportion of the space occupied 
by breathing-tubes ,—a very important consideration relatively to avail¬ 
able methods of destroying 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. 4), 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. 5) to be that of a pair of short horny, 
somewhat bent cylindrical, or partially cylindrical, tubes, covered at 
the end (fig. 7) 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. 8 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 
Fig. 6. Fig. 7. Fig. 8. 
Fig. 6.—Full-grown maggot, under side, much magnified. Fig. 7.—Spiracle-tube 
(one of the pair), much magnified. Fig. 8.—Discs at extremity of spiracle, 
as seen with J-in. object-glass. 
the Trypeta pomonella, Walsh, described by Professor H. Comstock 
in the Report of the Department of Agriculture U.S.A. for 1882, 
p. 197. There it is noted that each of the spiracles he describes “is 
expanded into a plate, the free margin of which is fringed by a double 
