268 ORDER DIPTERA 



eggs in siK'li places that the larva? would survive and the 

 habit gradually acquired of living within the host tissues. 

 They reach maturity without destruction of the host form 

 and the habit gradually becomes fixed. This habit is now 

 definitely fixed among all the species of the family and 

 they are restricted to mammals as hosts. Some infest the 

 alimentary canal, some the nasal passages, some the tissue 

 beneath the skin. The larva; as parasites have undergone 

 consideral)le modification from the primitive forms. They 

 are usually pro^'ided with rows of spines or sharp spurs that 

 assist their movements when they leave the host forms and 

 for those that live in the alimentary canal there are hooks, 

 modifications of the mouth parts, that serve to attach them. 

 They may feed to some extent upon the secretion of the host, 

 })ut most of the nutrition is absorbed through the body walls. 

 The Horse bot fly {(i astro philus equi) is one of the best 

 examples and probably represents most nearly the primitive 

 condition for the family. The adult occurs through the middle 

 and last of summer and the eggs are deposited on the hairs of 

 the horse, small yellow eggs glued very firmly to the hairs. 

 In depositing the eggs the female darts toward the animal 

 and thrusts the egg against the hair without alighting. The 

 abdomen is thrust forward under the body. A glutinous 

 secretion is discharged with the egg and binds and hardens 

 almost immediately. The eggs are very thick-walled with 

 chitinous walls and have a little operculum or cap at the end 

 of the egg which is easily detached after a certain stage in 

 the development. The time for hatching varies from three 

 days to forty, but usually ten to fifteen days after deposition. 

 Before that time they are hatched with a great deal of diffi- 

 culty, and the larvae are rather inactive. After fifteen to 

 twenty days the activity begins to diminish. The majority 

 die if not hatched in thirty to forty days. They do not 

 hatch without the assistance of moisture, friction, and 

 warmth to stimulate the larvse. With a little moisture and 

 friction the cap slips off easily and the hooks of the larva 

 fit it to attach itself immediately to the tongue or other part 

 of the body of the horse. The egg shells remain on the hair 



