=Symptoms.= It is easy for a careful observer to note the time at which the larvæ penetrate the sinus. During the hottest hours of the day the adult insects are continually hovering over the flocks, and on watching carefully one sees sheep suddenly become excited, tap with their feet, rub their faces against any hard, resisting object in the neighbourhood, plunge their nostrils into the dust, and snort violently. When the larvæ have penetrated the nasal cavities they produce frequent attacks of sneezing by irritating the mucous membrane, and cause an intense sero-mucous and afterwards a moderate muco-purulent coryza. As long as the larvæ remain of small size, the apparent results they produce are insignificant, as during the first months of winter; but when they are numerous, and have become of considerable size, they cause symptoms which might suggest an attack of gid or sturdy. Thus the bodily movements become spasmodic, the gait irregular, and the animals show attacks of vertigo. They stagger and fall, making convulsing movements, grinding their teeth and rolling their eyes, while frothy saliva escapes from the mouth, etc. Death may occur during such attacks, which, however, are happily very rare. Most commonly the animals are simply dull and somnolent. They feed badly, carry their heads low, and sometimes hide themselves under the mangers or in corners. In exceptional circumstances they bury their heads in the wool or carry them high in the air in walking, while they lift their front legs high, with a stepping movement. [Illustration: $1] =Diagnosis.= The diagnosis of parasitic invasion of the sinus in the sheep is rather difficult, because certain of the symptoms suggest gid. One never finds the signs of true gid, however, and, on the other hand, a certain amount of discharge and attacks of snorting always exist. Finally, gid (cœnurosis) only attacks young animals, whilst the larvæ of œstridæ are commonest in adults. To confound the disease with verminous bronchitis is still less likely, for although a discharge exists in both cases, this is accompanied by cough in bronchitis, and only by sneezing attacks in infection of the sinuses. Finally, in bronchitis, histological examination leads to the discovery of eggs or embryos of the strongyles. The diagnosis becomes very easy on post-mortem examination, the identification of the larvæ of œstridæ being extremely simple. =Prognosis.= The prognosis is only grave when infestation is very pronounced. Most commonly the parasites complete their development without producing disquieting symptoms; as summer approaches they are expelled and recovery occurs.