498 Observations on the Development of Ox Warble, ^c. 
skin of the maggot becomes furnished within with a powerful 
coat of muscles, extending over it like basket-work. A maggot 
at this stage, besides the power of contraction and expansion, 
which may be observed in protruding and withdrawing the 
mouth-end with the regularity of pulsation, has a power of 
dragging itself along at a rate of three times its own length in 
two minutes, and has a very definite method of progression. 
The mouth-end is somewhat raised, and the creature appears to 
move with as settled a purpose in any given direction as other 
grubs or caterpillars. Externally, in this stage the skin of the 
Fig. 14. — Muscles wWnrfSlcin Fig. 15. — PricJclesof Maggots, 
of Maggot, magnified. much magnified. 
maggot is furnished with a much larger amount of prickles, 
arranged in more numerous bands, than are noticeable in the 
previous stages. The prickles are now strong enough to cause 
an unpleasant sensation when the maggot crosses the hand, and to 
play an important part in its locomotive powers in its cell, and 
in the effect on the tissues caused thereby. The visceral con- 
tents are now thick, and obviously formed of the filthy hiatter, 
which is caused by the perpetual irritation of the suction of the 
mouth-end of the maggot at the bottom of the sac. It is also 
now furnished with a small curved caudal aperture, placed nearly 
between the spiracles (see Fig. 5), from which some slight 
amount of discharge of contents can take place. 
These are the main differences connected with the moult to 
the final form of the maggot, and, following on these alterations 
in its structure, and especially on the power of keeping up a 
constant irritation by means of the muscular expansion and 
contraction of its prickly skin, we find the lining membrane of 
the cell increasing in thickness, until it becomes well defined as 
a tough wall round the perforation, continuous with the upper 
part of the cell. Fig. 16 shows a cell drawn in section, and 
