The =young larva= when deposited on the nostril speedily makes its way up and takes refuge in the cavities of the turbinated bones and the frontal and maxillary sinuses, where it passes the winter feeding on the mucus and the purulent discharges determined by its presence. When mature it leaves the nose and assumes the chrysalis form in the soil. The =mature larva= is narrow anteriorly, broad behind: its upper surface is prominent and rounded, lower surface flat, and furnished at the anterior of each ring after the third, with a series of pointed tubercles or spines: the cephalic end bears the buccal organs directed downward, and bearing two great hooks connected with the hard framework of the pharynx and recurved downward, backward and outward; mouth small; antennæ thick and short placed above the buccal organs: the inferior part of the last ring projects beyond the upper portion and is furnished with two nodules with intervening spines: pentagonal patches of stigmata on the last ring: very small anterior stigmata between the first and second rings. The color is white with brown spines, stigmata and transverse striæ. Length seven lines to one inch. When dropped from the nostril in the course of summer they pass into chrysalis in one or two days; and after a residence of six or eight weeks in the soil emerge as the perfect fly. =Morbid Symptoms Caused by the Larvæ in the Head.= =Grub in the Head.= These bear a close relation to the number of larvæ present. If there are only two or three no trouble may result. If many there is muco-purulent discharge from the nose, sniffling breathing, frequent sneezing and snorting expelling mucus and even blood; shaking of the head; rubbing of the nose on the fore legs or other objects; weeping eyes; and occasional unsteadiness of the gait. In the worst cases the respiration becomes sighing, wheezing or even snoring; the mouth open; head pendent; appetite fails; a dull, apathetic condition ensues with grinding of the teeth rolling of the eyes, and, rapidly advancing emaciation. Fatal cases are not uncommon but most frequently the larvæ reaching maturity are dropped and health is promptly re-established. Septic poisoning from decomposition of dead larvæ and debris is a dangerous complication. _Lesions._ These consist in the presence of the larvæ in the sinuses, with violently congested, purple ulcerated mucous membranes and collections of pus. The mere presence of the grub is not conclusive as the majority of the sheep harbor two or three from October to June.