333 
C. Warburton 
Thus in the case of both flies it has been demonstrated that direct entrance 
is obtained at the spot where the eggs are laid. That this is the only mode of 
entrance is not proved, though much negative evidence in the case of H. bovis 
tends to show that that fly, at all events, seldom or never obtains entrance 
by way of the mouth. It is perhaps worth noting, that muzzling experiments 
have not been applied to the case of H. lineatum, and that the eggs of that 
species are laid, as with Gastrophilus equi, in full view and much more accessible 
to the tongue than those of H. bovis , which are deposited singly and concealed 
at the base of a hair. Though in all probability the life-history is similar in 
the two cases, the possibility of licking in the case of H. lineatum cannot be 
altogether excluded. 
The Larva within the Host. 
The two last larval instars were well known to the earliest investigators, 
since they could be extracted from the open tumours. The antipenultimate 
instar was not properly described before the appearance of Brauer’s monograph 
(1863) though Clark (1815) evidently saw it, for he observes that “when young, 
the larva is smooth, white and transparent.” In 1884 Hinrichsen began a series 
of observations the results of which he did not publish till 1888, and his 
discoveries heralded a great advance in our knowledge of the bionomics of 
the warble-fly, though their interpretation was held to support a theory now 
discredited, namely that the parasite obtained an entrance to the host by 
way of the mouth. He found larvae which he doubtfully ascribed to H. bovis 
in the wall of the gullet and in the spinal canal, and he suggested that the 
cattle obtained the newly-hatched larvae from the grass and that these bored 
their way into the oesophageal wall where they spent about five months, 
proceeding thence by way of the spinal canal to their ultimate situation in 
the back. The lead given by Hinrichsen was followed by Curtice (1890). 
Horne (1894), Ruser (1896), Schneidemiihl (1897), Koorevaar (1898), J. P. 
Koch (1903), Jensen (1903), des Gayets and Vaney (1912), Glaser (1912-14) 
and Hadwen (1916), and though the “licking” theory had to be relinquished 
m 1915 when Carpenter proved direct penetration, Hinrichsen’s view of the 
passage of the larva from the oesophagus to the back was thoroughly confirmed. 
All these investigators agreed that larvae identical in structure with the 
earliest stage found in the warble tumours and little inferior to them in size, 
occurred with regularity in the oesophageal wall soon after the fly season, and 
in the spinal canal shortly before their appearance in the back. 
Koorevaar tried the experiment of introducing freshly obtained oesophageal 
larvae under the skin of a small dog, but the larvae were apparently not ripe 
for their final migration, and though on dissecting the animal a few weeks 
later he succeeded in accounting for every one of the 36 larvae employed, 
little was proved beyond their extensive wandering. Hadwen (1914-15) was 
more successful. He introduced twelve oesophageal larvae under the skin 
of the back of a calf born in September and therefore under no suspicion of 
having been struck by the fly on its own account. Four warbles resulted. 
Parasitology xrv 9<? 
