326 The Warble-flies of Cattle 
found between the periosteum and the dura mater, affords an easy passage 
to the posterior region of the back and gives ready access to any part of that 
area to which the warble tumours are always confined. 
The larva now selects the spot for its final changes, and, entering the hide 
from below, begins the formation of a “ warble.” It quickly undergoes an 
ecdysis and becomes fundamentally changed in form. No longer glassy and 
semitransparent, it becomes white and opaque, and well furnished on the 
ventral surface with minute spines. The mouth-hooks are lost and the posterior 
stigmata are markedly increased in size. The shape is somewhat tapering, being 
narrowed towards the posterior end. This is the penultimate larval instar, and 
it, together with the subsequent history of the insect, has long been perfectly 
well known. Migration having now ceased, a change takes place in the walls 
of the cavity in which the larva has established itself and a cyst is formed, 
but the movements of the spiny larva within cause inflammation in the walls 
of the cyst on the products of which the parasite lives. Its next procedure is 
to pierce a hole to the exterior towards which it advances tail first. Atmospheric 
air being now admitted growth, hitherto so slow, proceeds rapidly. The ad¬ 
mission of organisms from the exterior changes the nature of the pus formed 
in the cyst, and it now becomes dark and foetid. This penultimate larval 
instar not only perforates the external hide, but it keeps open and widens 
the orifice by the frequent insertion of its narrow posterior extremity. Another 
ecdysis then takes place, and the final larval instar is assumed. It is spindle- 
shaped, convex on the ventral surface but flattened dorsally, and furnished 
on either side with three longitudinal rows of tubercular ridges, and its spiracles 
are notably larger and more conspicuous. At first it is white, but it gradually 
assumes a darker hue. It lies in the cyst with its posterior stigmata applied 
to the external orifice. In extreme cases the back of a yearling may contain 
several hundred larvae in different stages of development, some nearly mature 
and others not yet in direct communication with the external air. It often 
happens that successive batches of tumours arise in the same animal, and this 
may be fairly assumed to be due to different dates of oviposition. 
The mature larva now emerges from the tumour through the orifice which 
has been formed and maintained during its later stages of development, 
though this orifice never approaches in size the normal width of the larva, 
and the exit is not made without difficulty. For a day or two before emergence 
the larva is observed frequently to protrude its two posterior segments through 
the aperture, apparently to widen it still further and to test the possibility 
of exit. When the time has arrived for the final effort these two segments are 
protruded and inflated to the utmost extent, while the next segment is con¬ 
stricted and passed through, and this process continues till emergence is com¬ 
plete. There is a great struggle to pass out the fourth segment (from the 
posterior end) but when this is accomplished the rest is fairly easy. Emergence 
is believed always to take place fairly early in the morning. In England most 
larvae emerge during May. The larva, now free, falls to earth and immediately 
