102 
WABBLE FLY. 
with due pressure from behind, to force open and gradually enlarge 
the fine passage (see fig. 1) leading down in the early stage of attack, 
from the outside of the hide to the embryo swelling beneath. 
The power of pressure possessed by the maggots at this period of 
their life is enormous, from their capacity of inflating themselves with 
fluid until they are so hard that it is scarcely possible to compress 
them with the fingers, and likewise from their having (apparently) no 
power of discharging any of their contents. Thus they form living 
and growing plugs, quite capable of pressing back the tissues from 
around them, or from before the small hard tip; but not subject (so 
long as they continue inflated) to being themselves compressed. I 
had opportunities of watching this process of inflation both in the 
worm-shaped maggots and when they were slightly more advanced in 
growth to a club or lengthened pear-sliape. On placing them in fluid 
suitable for absorption (as in glycerine and water, in which they would 
live for as long as eighty hours, or until the spiracles sank completely 
beneath the surface) they became hard and shiny, and with little trace 
of the segments which are so clearly marked when the maggots are 
fully developed ; in fact, they were almost of a glassy smoothness, 
save for the short bands of minute prickles placed along a portion of 
the back. 
The size and shape of the perforation through the hide altered 
progressively with the growth of the maggot. At first this passage 
was very little larger at the lower than at the upper opening ; and, 
though the walls of the perforation had now become smooth and 
shiny, I could not distinguish the presence of any distinct lining 
membrane. With the enlargement of the passuge its shape became 
more cone-like (corresponding with the altering form of its tenant); 
and, on March 5th, I 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 4tli 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 
