The Bot-Flies 



the flies appear and are immediately attracted to cattle, laying their 

 eggs upon the legs, especially just above the hoof, which explains 

 the southwestern name "heel fly." The eggs are occasionally 

 laid on other parts of the body but the neighborhood of the hoof 

 is preferred. They are attached to the hair by means of a clasp- 

 ing projection and usually from four to six are laid together. The 

 animal licks its legs and the larva at once hatches and is carried 

 down into the oesophagus, the walls of which it penetrates by 

 means of its strong spines. It then molts and becomes smooth 



and for several 

 months wanders 

 through the con- 

 nective tissues 

 of the cow, 

 between the 

 skin and the 

 flesh, penetrat- 

 ing gradually 

 along the neck 

 and ultimately 

 reaching a point 

 beneath the skin 

 on the back of 

 the animal. The 

 larva then molts 

 again, becomes 

 more spiny, and 

 bores a hole 



Fig. 86. — Hypoderma lineata, eggs. (After Riley.) 



through the skin, placing its anal spiracle near the orifice in order 

 to get air. During its earlier life it probably breathes by an en- 

 dosmotic method as do the larvae of the parasitic Hymenoptera 

 and in fact much as do the aquatic larvae of certain other insects. 

 The larva now develops rapidly, living upon the pus and 

 bloody serum which is produced by the irritation of its spiny skin. 

 It molts again and is then more than an inch long and yellowish- 

 white in color. It works its way out of the minute orifice which 

 it enlarges and drops to the ground where it contracts and hardens, 

 the larval skin becoming the protection for the pupa which is 

 formed within. In three to six weeks the adult fly escapes by 

 pushing off the circular cap at one end of the puparium. 



156 



