338 
The Warble-flies of Cattle 
IV. H. BOVIS AND H. LINEATUM DIFFERENTIATED. 
The Flies . 
Though diagnoses of flies already well known would be out of place in 
the present paper, it may be useful to give the characters by which the two 
species can be distinguished at the various stages of their life-history. 
Many writers have indicated the most obvious differences between the 
imagos (e.g. Hadwen, 1916) which may be thus tabulated: 
H. bovis 
H. lineatum 
Average length 
14 mm. 
12-7 mm. 
(pinned specimens) 
Thorax 
Dense yellow hairs in front, 
Uniform covering of mixed 
hiding longitudinal markings, 
black and white hairs, leaving 
and contrasting strongly with 
longitudinal marks visible 
black mesothoracic region 
(hence lineatum) 
Wings 
Veins dark brown; alulae with 
Veins black (or nearly), alulae 
reddish brown border 
uniformly white 
Legs 
Comparatively smooth, with few 
Rougher, and more hairy,— 
especially tarsi 
hairs 
Abdomen 
Terminal hairs yellow 
Terminal hairs orange-red 
The Eggs. 
The eggs of both flies are long-oval, white bodies about 1 mm. long, provided 
with a bi-lobed foot or clasper for attachment to a hair. “ The egg of H. lineatum 
is relatively longer and more parallel-sided than that of H. bovis, while the 
latter has a shorter and broader foot” (Carpenter and Steen). In situ the 
eggs are easily recognised because of the different habit of oviposition of the 
two flies. H. bovis deposits them singly at the base of a hair: H. lineatum 
attaches several eggs to the same hair. 
The Larvae. 
There is a difficulty as regards the nomenclature of the larval instars, 
because we are still uncertain as to their number. This has led to confusion 
in the past, for different authors allude to the same instar by different numbers, 
according to the state of knowledge at the time. We now know the first-stage 
larva and also three final stages in the case of both flies, and in the case of 
H. lineatum Laake has recently shown that the oesophageal larvae hitherto 
considered as all of the same instar, present two different forms, the younger 
spiny, the older smooth. This gives for lineatum five instars at least and it 
is likely that a revision of all the oesophageal larvae of H. bovis will reveal 
a similar state of things. Add to this that there is no certainty that moults 
do not occur between penetration and arrival in the gullet, and it is clear 
that any numbers applied to the later instars are purely provisional. The only 
safe way, however cumbersome, seems to be to allude to the later instars 
as ultimate, penultimate, antepenultimate, without allotting definite numbers 
to the stadia. These three instars all occur in the hide, but the first of them— 
the antepenultimate as we term it—is the form in which the larva leaves the 
gullet to take up its final position, and includes, therefore, the larger oesophageal 
