28 



of the fly in Transactions Linnean Society, 1797, vol. Ill, i)> 315, as 

 follows : 



Early symptoms. — The moment the fly touches this part (the nose) of the sheep, 

 they shake their heads and strike the ground violently with their fore-feet. At the 

 same time holding their noses close to the earth they run away, looking about them 

 on every side, to see if the fly pursues ; they also smell to the grass as they go, lest 

 one should be lying in wait for them. If they observe one they gallop back, or take 

 some other direction. As they can not, like the horses, take refuge in the water, they 

 have recourse to a rut, dry dusty road or gravel pits where they crowd together during 

 the heat of the day with their noses held close to the ground, which renders it diffi- 

 cult for the fly conveniently to get at the nostril. 



This description of the action of the sheep when attacked by the fly 

 is correct in all but one or two minor points. The sheep's actions when 

 running indicate that they are taking every means to shake off and 

 dodge a single rather than a number of pursuers. If this quotation 

 had also described the sheep huddled under buildings, along fences, 

 under rock ledges and shade trees, holding their noses close under their 

 fellows, it would have completed a picture familiar to every farmer or 

 flock-master. 



The fly only works during the heat of the day, while early in the 

 morning and late in the evening the sheep seem to enjoy feeding in 

 freedom from its annoyance. 



Pathology.— The young larva deposited in the nostril of the sheep im- 

 mediately begins its migrations upward into the dark passages of the 

 nose. It progresses by means of its hooks and spines. By firmly fix- 

 ing the hooks into the mucous membrane it is enabled to draw up the 

 rear part of its body after it, and by pushing upon the spines of the 

 abdomen it holds itself in place while it thrusts out its head for a new 

 hold. The only appearance of limbs that the larva has is the two rows 

 of prominences along each side of the abdomen, as shown in Fig 13. In 

 this method of progression lies one cause of irritation to the sheep, viz., 

 the hooks sink into the mucous membrane and not only irritate it, but 

 cause minute points of hemorrhage which are afterwards indicated by 

 very minute black dots scattered over the surface of the nares or in- 

 ternal nose. Whoever has felt the tickling and itching of a fly at the 

 entrance to, or a foreign body in the nose, can imagine some of the sen- 

 sations which induce the sheep to make such attempts to escape its foes. 



As the larva grows in size it finds its way farther into the recesses of 

 the nose, and by following the grooved passage (see Plate III, Figs. 1 and 

 2) penetrates into the furthest chambers. Fig. 1, w, n, shows the young 

 larva; wandering over the turbinated bones and in the main passage. 

 One of these larva^, advanced in size, is shown at i in Figs. 1 and 2 fol- 

 lowing the direction of the channel marked by the straw o, o, which 

 emerges into the frontal sinus of the head, Jc. The larva3 may also wan- 

 der among the windings of the superior turbinated bones g, and finally 

 growing to such a size that they can not escape, become entrapped 

 there. The same may happen when they wander through the small ori- 



