B. N. SONI                                        377

been noted in the two neighbouring localities of Naini Tal and Ranikhet during
April and May respectively. It may be mentioned that the bulls examined at
Mukteswar were brought from Garhwal (altitude 4,000 ft.) about four to six
months previously, and that one of the oxen, which showed oesophageal larvae
at Naini Tal, was brought from Muzaffarnagar (Western U. P.), while the
bulls at Ranikhet were imported from Lucknow only a few days before they
were slaughtered. On the basis of the available evidence concerning the
length of the different stages in the life-cycle of H. lineatum, the oesophageal
larvae appearing in the end of March and early April should be the progeny of
adult flies that were on the wing in the latter part of January—a conclusion
which would seem difficult to uphold, in view of the climatic conditions in
Garhwal during this month of the year.

    IV. THE GRUBS IN THE SUBCUTANEOUS TISSUES

In order to determine with some exactitude the average length of time
taken by the larvae to mature after their first appearance on the animal's
back, infected bulls were kept under constant observation at Mukteswar. The
duration of this period was determined by the interval between the first
appearance of the warble tumour and the escape of the mature larva from it.

The following figures (Table I) were derived from eighteen tumours
observed on four different bulls :—

                            TABLE I

At Mukteswar, the average period of development of H. lineatum larvae in
the subcutaneous tissues of the animal's back was, therefore, nearly fifty-one
days.

A perusal of the available literature on warble-flies shows that Laake
[1921] is the only worker who has recorded the occurrence of spineless forms of
H. lineatum larvae in the subcutaneous tissues of cattle. Laake's observations
in this respect, however, have been recently disputed by Knipling [1935],
according to whom the apparent moulting of an oesophageal larva to a spineless
form, as observed by Laake [1921], was merely a process of separation of the
outer layer of its cuticle, together with the spiny armature, this process being
particularly noticeable in disintegrating larvae. Further he observes that
such larvae, being flaccid, are devoid of the characteristics of truly moulting