C. Warburton 323 
the general course of the life-history is so well established that further research 
is unlikely to revolutionise the views at present held, though certain lacunae 
remain to be filled up. 
Before 1892 the phenomena of oviposition were entirely unknown, and it 
was not till 1915 that the long discussed question as to how the parasite 
obtained an entry into its host was at length definitely settled. It follows, 
therefore, that the general accounts of the warble-flies which have from time 
to time appeared 1 , though some of them are quite admirable statements of 
the facts as then known, are too incomplete to be satisfactory in view of the 
important discoveries made since they were written. No such general account 
can claim finality, but whereas hitherto even the broad outline of the mode 
of life of these insects has been partly conjectural, at present the obscure 
points are matters of detail, and the time has therefore come when a fresh 
attempt may be usefully made to gather together the results of a research 
which has now proceeded for more than two hundred years. 
A convenient way will be to state briefly at the outset the facts for which 
the evidence will afterwards be discussed in detail. 
I. Bionomics of H. bovis and H. lineatum according to 
OUR PRESENT KNOWLEDGE. 
Domestic cattle, in Europe and N. America, are subject to attack by two 
species of Hypoderma (Diptera, Oestridae)— H. bovis and H. lineatum. Both 
species are widely distributed and are common in England, but H. bovis is 
the prevalent species in Ireland and H. lineatum in N. America. The flies 
appear during hot and sunny weather. The date of their appearance naturally 
varies with the climate and with the nature of the particular season. In 
England July is the month when most flies are observed, but individuals may 
appear much earlier and much later. In British Columbia, with a climate 
similar to that of England, H. lineatum has been taken in April, while in 
Ireland H. bovis has occurred as late as September. In the hot climate of 
Texas in 1892 H. lineatum was on the wing in the first days of March (Riley). 
H. lineatum is the earlier fly by about a month (Hadwen 1914). It follows, 
therefore, that the fly season may be a long one, though individual flies 
probably have a very short life. This is particularly the case where both species 
are present, as in England, where the season may in some years extend from 
April to September. 
Eggs are laid on the hairs of the cattle. H. bovis , which only oviposits 
on hot sunny days, attaches its eggs singly at the base of the hairs; H . lineatum , 
which seems to be less dependent on sunshine, attaches several eggs in series 
(1-14) to a single hair. In all about 800 eggs may be laid by a female of either 
species. 
The eggs are never laid in the region where the warble tumours sub¬ 
sequently appear. The legs are chosen by preference, especially if the animal 
1 Clark, 1815; Joly, 1846; Brauer, 1863; Schneidemiihl, 1897; Imms, 1906. 
