428 



NATURAL HISTORY OF ARTHROPODS. 



ability to contract either end into an elongate cylindrical form, which not only serves 

 them in their egress but also to bore into the ground. A few days before they are 

 ready to emerge they begin to enlarge the opening by this expansion and contraction ; 

 when they have enlarged it sufficiently, a ring-like contraction of the body, that begins 

 at the posterior part and progresses toward the head, enables them in a few minutes 

 to free themselves, which they usually do in the morning hours. Upon the ground 

 they creep about until they meet some obstruction, when they burrow from one to two 

 inches below the surface, and remains as in Gastrophilus species. 



As one would suppose, these insects are injurious when they exist in considerable 

 numbers, and as many as one hundred have been observed on a single ox. Another 

 and not inconsiderable damage that they cause is in injuring the skins for leather. 

 The larvae are commonly called " grubs " by cattle men, and their presence may readily 

 be detected not only by the touch, but also by the sight, so large are the swellings 

 they cause. 



A third mode of parasitism, in the frontal and nasal sinuses, is quite as interesting 

 as the two ways just described. For this the common sheep bot-fly, CEstrus ovis, may 



be taken as an example. The fly 

 is a little less than half an inch in 

 length, nearly bare, yellowish gray, 

 with yellowish white and marbled 

 abdomen. The flies frequent places 

 where sheep are herded, resting on 

 stone walls or on trees, till the fe- 

 male is ready to deposit her young, 

 for in this and some other species 

 with similar habits the eggs are 

 hatched within the oviduct of the 

 parent and extruded alive. With 

 the appearance often of a single 

 fly in a flock of sheep the most 

 intense excitement may prevail. 

 The animals will hold their noses between their legs in close proximity to the 

 dusty waysides, or they will crowd together in hot, barren, open places with their 

 noses in the dust, in their endeavors to evade their tormentors. The flies, with quick 

 motions, eject the young larvae within the nasal orifice, whence the small, active 

 maggots crawl backward into the passages of the nostrils or throat and usually into 

 the frontal sinuses, where they remain feeding upon the mucous membrane and serum 

 till they have attained their full growth, requiring a period of about nine months. 

 When ready to change into the larvigerous pupae they dislodge themselves, and either 

 crawl out or are ejected by the animal in coughing. Falling upon the ground they 

 quickly find a sheltered spot in the soft earth or under leaves, and there remain for 

 about six weeks till they are ready to emerge as flies. 



When numerous larvae develop within the nasal and frontal cavities disease and 

 often death ensue. At the beginning they produce a thin discharge from the nostril, 

 which afterwards becomes thick and purulent ; at the same time there is a constant 

 sneezing. Later the animal constantly shakes it:: head, rubs its nose upon the ground, 

 fences, or its own legs. Still later it goes about with sunken head, with a staggering, 

 dizzy gait, with reddened and watery eyes and heavy breathing, which may culminate 

 in death. 



FIG. 539. (Estrus ovis, sheep bot-fly, larva (enlarged), and pupa. 



