324 
The Warble-flies of Cattle 
is standing, but Ii. lineatum has been seen to oviposit all along the flank of 
a sickly recumbent animal, and in the region below the ischial prominence 
(Hadwen, 1915). Oviposition inflicts no pain on the animal and may be un¬ 
noticed, but if the victim becomes aware of the presence of the fly, great un¬ 
easiness and even terror are excited and “gadding” results. Wild-eyed with 
fright, and with tail horizontally outstretched, the animal gallops madly to 
escape its enemy and takes refuge in the nearest pond or river, where it stands 
trembling, knee-deep in the water. The terror is infectious, and often spreads 
to the whole herd. It is much more marked in the case of H. bovis than in 
that of H. lineatum. 
Young animals are always more parasitised than the older, but yearlings are 
usually more subject to attack than calves, the preference being: (1) yearlings, 
(2) calves, (3) older cattle 1 . There is, however, no evidence that eggs are laid 
more freely on the younger animals, and Hadwen considers the reduction of 
warbles on the older cattle a case of partial immunity through advancing age. 
Under normal conditions the eggs of H. bovis hatch in about four days 
(those of H. lineatum taking some days longer), and give rise to minute larvae, 
about 0*8 mm. in length, armed in every segment with small spines, and 
furnished with a median piercing tooth, flanked by a pair of well-developed 
mouth-hooks. The larva crawls down the hair and bores into the skin. In the 
case of H. lineatum it is alleged that the hair follicle is always entered. The 
process of penetration, which may occupy several hours, causes considerable 
uneasiness, and gives rise to certain skin lesions, but the symptoms usually 
disappear very speedily. 
The larva has now entered the host at a point (e.g. the lower part of the 
leg) far removed from its ultimate destination, which is invariably the back 
of the animal. Several months will elapse before the earliest indication of the 
formation of “warbles” in the dorsal integument. Taking mid-July as the date 
of maximum oviposition in England and mid-February as the time when the 
indications of newly-forming dorsal tumours are the most numerous, we have 
a rough indication of the average time elapsing between oviposition and the 
assumption by the larva of its final position, for it is reasonable to suppose 
that the earliest formed warbles arise from the earliest laid eggs and the latest 
from the latest. For about seven months, therefore, there is no external sign 
of the whereabouts of the larva, which is somewhere within its host. It is 
with regard to this period of the life-history that our knowledge is still in¬ 
complete, and especially with regard to the first portion of it, which is not 
surprising in view of the minute size—less than 1 mm.—of the larva on 
obtaining entrance. 
Now though it is not yet possible to trace accurately the course of a larva 
throughout these seven months, certain salient facts are well established. 
During the autumn and winter months (in England) the larvae, entirely 
lost sight of since their penetration of the outer integument, re-appear with 
1 Carpenter and Steen, 1908. 
I 
