DIPTERA. (103 
dant in the Mississipp1 Valley than in the Eastern States, at times grubs 
being found in the head of almost every sheep that died. While it is 
possible that the disease produced by this insect may be confused with 
the “ gid” or ‘“‘ staggers ” produced by an entozoan parasite, the Tenia 
cenurus, it is doubtiess true that many cases of death from these bots 
are assigned to other causes. And,on the other hand, undoubtedly 
many other diseases are cloaked by the popular verdict of ‘** grub in the 
head.” 
The flies which are represented in figure 48, 1 and 2, life size, make 
their appearance in June and July, and deposit living maggots in the 
nostrils of the sheep. 
This point may be considered as well established by competent 
observers, although the older authorities all speak of the deposition of 
anegg. It may be possible, however, that eggs are deposited in cases 
where the flies are particularly fortunate in finding their victim with- 
out delay, but in such cases the 
eggs doubtless hatch almost im- 
mediately after deposition. The 
larve at once commence to work 
their way up the nostrils and nasal 
passages, causing great irritation 
on their way until they reach the 
frontal sinuses, cavities located be- 
tween the two plates of the skull 
and lying one on each side of the 
central line of the head and be- 
tween and a little above the eyes. 
There they attach themselves by Fie. 48.—@strus ovis: 1 and 2. Adult fly. 3. 
the little hooks or tentacula, placed papa. 4. Full-grown larva, dorsal view. 5. 
each side of the head, to the mem- _— Same, ventral view. 6. Young larva. 1 and 2 
: . 1 natural size, the others enlarged (from Riley). 
| branes which line the cavities, 
feeding on the mucus which is always to be found in them. Until they 
attain their growth they are of a creamy white color with two brown 
spots placed side by side on the posterior segment. ‘These spots (fig. 
48,6,¢) are spiracles or stigmata, through which the worm breathes. 
The segment with these two spiracles is retractile, and can be drawn 
in and hidden at the worm’s pleasure. When full grown the grub 
) becomes darker, particularly toward the tail, the white of the first two 
) or three segments becoming dirty white on the fourth or fifth, and 
growing darker on each successive segment until the last, which is a 
very dark brown. (See 4 and 5 of fig. 48.) It has two small parallel 
hooks or tentacula at the head (a), and above these two very small 
tubercles, not very easily shown in the engraving. It also has small 
brown elevated round spots on each of the segments along the sides, 
which might at first be taken for spiracles, but which are not, and 
also two small corneous appendages (5, b, and 6, ¢ of fig. 48) on each 
side of the anus. The ventral region has a band of small elevated 
ee 
