146 INJURIOUS AND USEFUL INSECTS 



view would not be effected if the horse were to lick up and 

 swallow the egg as soon as it is laid. Four or five days are 

 required to develop the larva within the egg, and until that is 

 accomplished the egg, if swallowed, would merely pass with 

 the food along the alimentary canal of the horse and be cast 

 out. To guard against this, the egg itself is securely fixed to 

 a hair ; it is only the larva issuing from the egg which leaves 

 the spot, and the larva, once admitted to the stomach of the 

 horse, has the means of staying there. The larva does not 

 hatch out until the moment arrives when it can enter the 

 horse, for if exposed for even a few minutes to the air, it would 

 dry up and perish ; it lies curled up within the egg until it is 

 warmed and moistened by the horse's tongue. That is the 

 signal for escape. A little lid at one end of the egg-shell is 

 raised, the larva becomes free, adheres to the tongue, and 

 enters the mouth. Bracy Clark, a veterinary surgeon, re- 

 marked this curious stratagem more than a century ago. In his 

 full and interesting history of the bot-flies, which is printed in 

 the third volume of the "Transactions of the Linnean Society" 

 (1797), he tells us that he had often clipped off hairs with 

 eggs adhering to them, placed them in his hand, and moistened 

 them with saliva. If they were sufficiently advanced to contain 

 fully formed larvae, they hatched in a few seconds. Horses 

 not only lick themselves, but lick one another, and it is 

 probable that they often transfer to their own mouths the 

 larvae hatched upon their companions. In spite of the 

 elaborate, and so to speak, ingenious contrivances for securing 

 the safety of the parasites, the difficulties are so great that many 

 fail to reach their destination. Bracy Clark suspects that 

 " near a hundred are lost for one that arrives at the perfect 

 state of a fly." Once swallowed by the horse, and set free 

 from the risk of being crushed by the teeth, or detained in 

 the mouth, the larva has a fair chance of fixing itself in the 

 stomach. The means of doing this must now be explained. 



The larva of the bot-fly is a maggot, and agrees in all 

 essentials with the larva of the blow-fly. It has the retractile 

 and greatly reduced head, the pair of strong hooks projecting 

 from the mouth, the transverse rows of backward-pointing 

 spines, and the terminal pair of spiracles. The spines, acting 

 together with the muscles of the body-wall, enable it to move 

 through the contents of the stomach in the same fashion that 



