Whether the spotted or sieve-like appearance of the discs on 
the spiracles is caused by microscopic hairs placed to preserve 
the entrance free, or by other structures, she could not 
ascertain on account of the excessive minuteness of the 
organs; but the use of such an apparatus to guard the 
entrance of the breathing tubes, when acting in a passage 
which is formed in the living hide, is obvious. 
Up to the time when the moult takes place to the final form, 
these spiracles are buried up to their disc-covered tips in the tail 
end of the maggot; but then they are cast off entirely with the 
moulted skin, and in the newly-exposed skin beneath, we find 
the first appearance of spiracles of the well-known kidney 
shape, but with the surface more radiated, and of a paler 
chestnut colour than in their later condition. In spring, Miss 
Ormerod had frequent opportunities of observing and securing 
both the moulted skin containing the early form of spiracle, and 
the proprietor maggot, bearing the new kidney-shaped pair. 
The newly-formed spiracle is furnished on the upper surface, 
with numerous cylindrical pores or openings, which are the 
termination of cylindrical passages which are connected lower 
down, and which appear to be ramifications of a small number 
of upright passages opening from the 
great trachea below, and passing 
upward through the spiracle, thus 
giving communication to the outer air 
by means of the cylindrical branch- 
lets with their open extremities. 
The 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, has a power of dragging 
itself along at a rate of three times 
its own length in two minutes, and has a very definite mode 
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 maggot is furnished with a much 
larger amount of prickles, arranged in more numerous bands, 
v 
bfl 
c 
**.» 
o 
V 
s 
o 
O 
Fig. ii.—Spiracles and tracheae of 
young maggot, much magnified. 
