212 DIPTEEA OR TRUE FLIES. 



abdomen, and with yellow and black marks on the thorax. The 

 head of these CEstridas is large, but the mouth is very rudiment- 

 ary. The Warble-fly itself cannot bite hke the Gad-fly ; it pur- 

 sues the cattle to lay its eggs. The Ox- Warble fly appears in 

 the summer from June to August and September, delighting in 

 hot still days, when we can hear its low duU 

 " burr " in the air. It is quite a mistake to 

 fancy the AVarble-fiies make no noise. I have 

 frequently heard the low hum produced by 

 CEstrus ovis, and taken the insect on the wing. 

 The ox-pest lays its eggs either on the skin or 

 just beneath its upper layer, and it is these 

 eggs that hatch into the so-called "Ox-hots'' 

 (fig. 105). At first the young larvae are white 

 and thread-like, burrowing under the hypo- 

 "^"fEnTar^eST^' dermis, and live there, feeding upon the blood. 

 They are sometimes quite red when examined 

 in this condition. For some little time they show no signs of 

 their presence externally, but a few months later a small swelling 

 appears over the " hot." After the larva has motdted a second 

 time it becomes banded with prickles, which cause irritation 

 and inflammation of the immediate surrounding parts, and thus 

 the swelling or warble is produced. We notice these outward 

 signs of attack coming about January. The inflammation set 

 up by the hot produces matter, which eventually causes the 

 warble to burst at the top, a small round orifice appearing. 

 The " hot " lies beneath this, in a cell formed in the hypodermal 

 tissues, and when full-grown squeezes its way out and falls to 

 the ground, where, wriggling under some tuft of grass or stone, 

 it pupates in its old larval skin. H. lineata is much like E. 

 bovis, but somewhat smaller. 



Prevention and Treatment. — Smearing the back and loins of 

 beasts with some greasj^ substance, such as cart-grease and 

 paraffin, keeps the flies off. As Warble-flies, it seems, are very 

 susceptible to changes of temperature, and will not enter shade or 



