(ESTRUS BOVINUS. 205 



this fly, that I have seen one of them meet the herd when 

 almost driven home, and turn them back, regardless of the 

 stones, sticks, and noise of their drivers ; nor could they be 

 stopped till they reached their accustomed retreat in the 

 water/' 



Further on Mr Clark says : " When young the larva is 

 smooth, white, and transparent; as it enlarges it becomes 

 browner ; and about the time it is full grown it is totally of 

 a deep brown colour. 



"The larva having attained its full growth and size, effects 

 its escape from the abscess by pressing against the external 

 opening, which occasions its enlargement by the points 

 pressed upon being absorbed, and the skin also may 

 give way and extend itself under their impression. When 

 the opening has obtained the size of a small pea, the 

 larva writhes itself through, a ring or segment at a time, till, 

 arriving at the narrower part of its body, it falls out, and 

 tumbles to the ground, and, seeking a convenient retreat, be- 

 comes a chrysalis. 



"After leaving the abscess, and preceding their change, they 

 contract themselves into a much smaller space, and assume a 

 different figure. They never change or throw off their skin. 

 apparently, but the same serves them through their whole 

 growth, and at length also forms the shell of the chrysalis, 

 as in the other species of this genus. 



" If we examine the sac which enclosed the larva beneath 

 the skin, it appears formed of condensed cellular membrane, 

 and is rough on the inside. The pus secreted is mostly of a 

 yellow colour, and sometimes flows from the opening, and 

 dries about the orifice. After the exit of the caterpillar the 

 abscess disappears, and the wound in the skin closes up and 

 is healed in a few days : but although the skin heals up on 

 the exit of the larva, we may remark, that the union is not 



