60 PARASITES OF THE DOMESTIC ANIMALS 



Between the last appearances of H. lineatum and the first of H. hovis 

 there is usually a period of ten days when the cattle are immune from 

 attack of either species. H. hovis frightens cattle much more than 

 H. lineatum. The eggs take about a week to hatch; the larvae bore 

 through the skin in the coarser porous parts, taking several hours in the 

 process; at this stage they are rather less than 1 nun. long. The lesions 

 resulting from this penetration are caused partly l)y bacterial invasion 

 and partly by anaphylactic reactions; those produced by H. lineatum 

 being more severe. For the skin lesions I have proposed the name of 

 hypodermal rash. At this point there is a hiatus in the life history as it 

 is not positively known how the larvae reach the esophagus, where they 

 are subsequently found, most likely in the loose connective tissues under 

 the skin up to the region of the throat and into the esophagus where the 

 muscles bifurcate. Passing down the esophagus they follow the sub- 

 mucosa and are almost always found lying along the long axis of the 

 canal. Whilst in the esophagus small edematous swellings are found 

 surrounding the grubs, these are sterile and are anaphylactic in char- 

 acter, the exudate contains large numbers of eosinophilic leucocytes but 

 no pus cells. The earliest record made at Agassiz was on August 15th, 

 when a larva 3.4 mm. was found and several slightly larger. According 

 to Carpenter, continental observers have found them smaller than this. 

 H. lineatum makes its appearance in the backs of cattle about Decem- 

 ber 15th and H. bovis about a month later. The larvae at this time have 

 grown to about 1.5 cm. and are of the same size in the neural canal and 

 under the skin which they have just reached. At this age it is difficult 

 to separate the larvae of the two species, but Mr. F. C. Bishopp has, I 

 believe, discovered good distinguishing marks between the species. The 

 life histories overlap at this period making it difficult to follow the 

 migration, but in the latter part of the season (the middle of March) 

 the last larvae to leave the gullet are at the paunch end. They pass out 

 under the pleura and go to the neural canal either up the crura of the 

 diaphragm or up the posterior border of the ribs, entering the canal by 

 the posterior foramen, from there they descend the canal under the 

 dura mater, emerge again through the foramen and reach the back, 

 forming the characteristic swellings commonly called warbles. The 

 larvae follow connective tissue exclusively and no larvae have been dis- 

 covered in muscular tissue. The mature larvae leave the animals' backs 

 from the early part of the year up to the first days of July. The periods 

 for the two species have not been fully worked out, but judging from 

 what records we have of the pupal period and the time of year the flies 

 are about, H. lineatum begins to emerge in February and finishes about 

 May 1st. H. bonis begins about May 1st and ends approximately on 

 July 1st. The average pupal period for H. bovis is 32.5 days and for 

 H. lineatum a little less. The duration of the life of the flies is short 



