SHEEP HUSBANDRY. 



559 



main in the sinuses feeding on the mucus secreted by the membrane, and 

 apparently creating no farther annoyance, until ready to assume their pu- 

 pa form in the succeeding spring. Figures 62 and 63 give the shape and 

 an upper and under view of the full-grown larva. 



THE "grub" or larva OF THE SHEEP GAD-FLY. 



The body consists of eleven rings, colorless in the young grub, but the 

 elevated portions gi'owing darker with age, and becoming a dark brown 

 when the full size is attained. There are round spots of a still darker 

 color on each of these bands. At the edges of the rings are a few short 

 hairs, and lower down some round darkish spots, as shown in fig. 62. — 

 Small red spines, as shown in fig. 63, cover the space between the rings 

 on the belly. The remainder of the body (with the exception of the poste- 

 rior stigmata) is white. The tentaculce, as well as certain appendages on 

 each side of the anus, the purposes of which have not been discovered, are 

 seen in fig. 63. 



The larva having remained in the sinuses through the fall and winter, 

 abandons them as the warm weather advances in the latter part of spring. 

 It crawls down the nose, creating even greater irritation and excitement 

 than when it originally ascended, drops on the ground, and rapidly bur- 

 rows into it. In a few hours its skin has contracted, become of a dark 

 brown color, and it has assumed the form of a chrysalis, as seen in fig. 64. 

 Or rather, this figure exhibits the shell of the chrysalis, af- 

 ter the escape of the fly ; and fig. 65 shows the upper ex- 

 tremity or head of the pupa, detached by the fly in its es- 

 cape. 



The experiments of Valisnieri go to show that the GEs- 

 trus ovis never eats — and this is the received opinion. — 

 The male, after impregnating two or three females, dies, 

 and the latter having deposited their ova in the nostrils 

 of the sheep, also soon perish. 



The larva in the heads of sheep may, and probably do 

 add to the irritation of those inflammatory diseases, such as catarrh, which 

 attack the membraneous lining of the nasal cavities ; and they are, as we 

 have seen, a powerful source of momentary irritation in the first instance, 

 when ascending to and descending from their lodging-place in the head. 

 But in the interval between these events — extending over a period of 

 several months — not a movement of the sheep indicates the least annoy- 

 ance at their presence, or reveals to the veterinarian whether they exist in 

 the sinuses or not. It would be very difficult to believe that all the local 

 irritation which these parasites could cause, would be suflScient to termi- 

 nate life, and, so far as my observation has extended, post-mortem exam- 

 ination discloses no lesions which would in anywise sanction such belief. 

 The larva?, moreover, are found, at the proper season, in the heads of near- 

 ly all sheep — the healthy as well as the diseased — and I never have been 

 able to ascertain that the number of them is greater, on the average, in the 

 heads of those sheep which were supposed to have fallen victims to their 

 attacks, than in the heads of perfectly healthy sheej) slaughtered for the 

 table. And to prove that the popular ideas on the subject are but vague 



(1039) 



SHELL 

 OF CHRYSALIS. 



