﻿THE CATTLE GEUBS OR OX WAEBLES' 53 



ing of larA'se along the ribs, on the diaphragm, and in the neural 

 canal indicates that they pass from the gullet across the diaphragm 

 to the ventral ends of the ribs and thence follow the connective 

 tissue beneath the pleura up to the back. Some probably work their 

 way between the muscles and pass directly up the diaphragm from 

 the gullet to the back. A number of these no doubt gain entrance to 

 the neural canal, later pass out through the posterior foramen, and 

 then go up to the connective tissue beneath the skin along the back. 

 Both of these routes are probably followed in migrating from the 

 gullet to the back. 



The writers' observations indicate that the migration from the 

 gullet to the back is rather rapid. The first larvse begin to appear 

 in the subdermal tissues of the back very soon after the maximum 

 size has been reached in the gullet. As soon as the number of larvae 

 are observed to decrease in the gullet they are observed to appear 

 on the back, with practically no increased growth. 



Unfortunatelj^ it has not been possible carefully to dissect ani- 

 mals infested only with H. hovis so as to determine just where the 

 larvse occur from the time they enter the host until they appear 

 on the back. In New York, where both species are present, the neu- 

 ral canals of 140 slaughtered animals were examined and all the 

 larvee found were H. hovis. Thirty specimens were located, with 

 a maximum of four in one canal. Larvse were found in this situa- 

 tion during the months of October to March inclusive. The size 

 ranged from 6.5 to 14 millimeters, the larger ones being found later 

 in the season. 



At Dallas, Tex., where H. lineatuvi onlj^ occurs, about 75 beeves 

 were examined rather carefully as they were dressed. Only one 

 Hypoderma larva was found in the neural canal, a third-stage speci- 

 men located in the central portion of the second lumbar vertebra 

 and measuring 14 millimeters in length. 



This observation indicates that the larvse of H. hovis enter the 

 neural canals of cattle more frequently than do those of H. lineatum. 

 It seems unlikely, also, that all larvse found in this situation are 

 en route directly to the back, as the larvse found during October 

 were only from 6.5 to 9 millimeters long, and never have such small 

 larvae been encountered in the subdermal tissues of the back. Fur- 

 thermore, the larvse did not begin to appear under the skin of the 

 backs of cattle in that locality for over three months after these 

 larvae were taken in tlie neural canals. 



PREPARATION FOR THE EGRESS OF FULLY DEVELOPED LARV^ 



The appearance of third-stage larvae under the skin on the back 

 is frequently accompanied by considerable local inflammation, in- 

 dicated by edema of the connective tissue and sometimes marked 

 swellings and soreness. 



Certain cattle are more prone to such manifestations and show 

 swellings as large as 10 inches in diameter around each larva as 

 it comes to the siibciitan<'Oiis tissue of the back. From one to three 

 days after the appcai-ance of one of these swellings a niinuie hole 

 is cut by the laiva near the center, after which I he swelling soon 

 subsides. Although no conchisive evidence is at hand, the writei's 

 arc of the opinion that the holes are cut through the skin ahnost 



