PARASITES OF ANIMALS. 23 



or turning of the head, inclination to butt and push with the 

 head, fits of frenzy, and general appearance of suffering and 

 weakness, which sometimes goes so far as to render the animal 

 unable to rise, when death generally results. The gums, back 

 part of mouth, and lining of the nasal cavities are often red and 

 inflamed. Sometimes the larvae even force their way, by the 

 openings through which the nerves pass, into the brain, when 

 death quickly follows. 



These Iarva3, when full grown (Figure 24) are about an 

 inch long, and have lost more or less of the white color which 

 they had when younger, and each ring is crossed above by two 

 dark brown bands ; there are also small, round, raised brown 

 spots on the sides of each segment. The small end, or head, 

 has two sharp hooks and two papillae ; but the posterior seg- 

 ment bears two small brown plates, in which the spiracles 

 are placed, and this segment can be drawn into and con- 

 cealed by the next, which give a truncated appearance to 

 the posterior end. The under side is flattish, and each seg- 

 ment has a median row of minute brown spines, which are 

 directed backward. 



When full grown, they descend from the nostrils and fall 

 to the earth, where, in the soil or among the roots of grass, 

 they change to pupae in about two days. They remain in 

 this state from six weeks to two months, when a sort of lid is 

 forced off from one end of the pupae cases, and the mature 

 flies come forth, pair, and immediately proceed with the work 

 of perpetuating the race, for in the winged state they seem to 

 have no other instinct ; and not having any mouth, they take 

 no food. When not engaged in depositing the eggs, they 

 seem sluggish and lazy. The flies appear irregularly in June, 

 July, and August, and larvae of all sizes may be found at one 

 time in the same sheep. Each female produces several hun- 

 dreds of young. According to Mr. Riley, * Over 300 living 

 larvae have been taken from one female fly, by Mr. Cockrill. 



The amount of injury which these insects do to the sheep 

 has long been a matter of discussion among veterinary sur- 

 geons and others ; but it appears to be the universal testi- 



* First Annual Keport on the Noxious Insects of Missouri, p. 164, in Report of 

 of Agriculture, 1868. 



