32 PROTECTION OF PLANTS, 1917-18 



the mouth and alimentary canal, it was generally supposed by some writers that 

 warble larvae penetrated through the skin. Bracy Clark (pp. 408-409) was 

 strongly of this opinion, although he was wrong in his belief that the eggs were 

 simply laid on the skin. At a later date Ormerod^'* stated of bovis that the 

 larvae made direct entrance through the skin. Reaumur, quoted by Osborn^^ 

 (p. 92) one of the earliest writers, went a step further by asserting that the adult 

 punctured the host's skin with its ovipositor and introduced the egg beneath. 

 He was. no doubt, partly drawn to this inference by interpreting the great terror 

 of the animals as being symptomatic of pain experienced when the operation 

 of oviposition by the fly was being effected. 



Course of Larva Within the Host. 



In an experiment undertaken by Hadwen and Bruce'^ on November 17, 

 1914, four warble larvae were dissected from the gullet of a cow and introduced 

 beneath the skin of a calf three weeks old in the region of the knee. The larvae 

 were about 1 to 1.5 cm. long. On December 7th two of the larvae made their 

 appearance on the calf's back, puncturing the skin just behind the scapula. In 

 tracing the course which the larvae had taken, the authors found that the track 

 was distinctly visible, passing from the knee to the radius and hence up to the 

 elbow-joint, whence it could be followed under the scapula. Evidently these 

 larvae had not re-entered the gullet. 



In making their way from the gullet to the back of the host, the larvae first of 

 all gradually work their way to the posterior end of the gullet where the muscle 

 fibres are widely separated. Making their way beneath the refiexed pleura of the 

 oesophagus and diaphragm it is supposed that, passing downwards and outwards, 

 the larvae gain the cartilages of the ribs. Thence they proceed along the posterior 

 border of the ribs between the intercostal muscles until they reach the upper in- 

 tercostal space. By passing between the external intercostal muscle and the 

 levator costarnm and thence between the longissimus dorsi and the transversalis 

 costarum, they finally reach the skin of the back. The larvae invariably adhere 

 to the connective tissue between the muscles. 



Hadwen and Bruce (loc. cit) also found evidences that larvae may enter the 

 spinal canal by either the posterior or intervertebral foramen, and actually en- 

 countered several larvae in the spinal canals of different animals. They quote 

 Carpenter (p.ll) as having found larvae outside the muscular coat of the gullet 

 in the thoracic region close to the diaphragm. 



Seasonal Appearance. 



The two species do not appear on the wing synchronously, and it is interesting 

 to note in this respect that lineatum first makes its presence known as an adult 



