26 



Brauer {Monographie der CEstriden, p. 15) records the size of the male 

 and female at from 10 to 12'"'", or about two fifths of an inch ; the width 

 between the eyes of the male at 1"™ or oiie-twenty-fifth of an inch, 

 and of the female at 2.5"'" or one- tenth of an inch. The length of the 

 wing is Q™"", nearly two-fifths of an inch. 



Distribution. — The species occurs all over the world wherever there 

 are sheep. It is now too late to learn if it was indigenous in this country, 

 but we may believe that it was introduced with the earliest flocks im- 

 ported, whether in Mexico or on the Atlantic coast. Brauer, in 1863, 

 stated that it had then lately been introduced into Chili, South America. 



Life history and description of larva. — The interest of the flock-master 

 in this species begins when the fly buzzes around the noses of the sheep 

 and deposits its young just within the opening of the nostril. Many of 

 the older writers on this subject supposed that the fly deposited eggs, 

 but Brauer (o. (?., p. 154), in agreement with Joly and Dufour, pointed 

 out that the genus was one which deposited their young alive. Accord- 

 ing to Eiley (o. c, p. 164) Samuel P. Boardman, of Lincoln, 111., men- 

 tioned the two following independent discoveries : John Brown, " Old 

 Ossawattomie," stated in the Ohio Farmer about 1851 that he saw the 

 fly drop the perfectly formed and living grub in the nostrils of sheep. 

 About 1861 Dan Kelly, of Du Page County, 111., made the same discov- 

 ery, and records the fact in the Prairie Farmer, October 14, 1865. 

 Boardman, in 1867, received a letter from Mark Cockrill, of Tennessee, 

 who wrote of having made the discovery years previous. Eiley claims 

 (o. c, p. 165) that he obtained living maggots from a fly in 1866. 



The young larva, having been deposited within the rim of the sheep's 

 nose, soon attaches itself by means of the hooks (Plate I, Fig. 6), and 

 begins to make its way upward into the nostrils. The smallest specimens 

 collected by the writer are shown in Fig. 7e, natural size, and are not 

 much larger than when first deposited ; for the difference between their 

 size and that of eggs deposited by flies even smaller than CEstrus avis 

 is inconsiderable. These small larvae are in their first stage of growth. 

 They are little, white, elongated bodies, less than 2""" long, i. e., about 

 one-twelfth of an inch (Figs. 1 and 2). But little of their structure 

 can be seen except with a lens. They already show the division of the 

 body into eleven segments, two well-defined hooks (Fig. 3a, a), and two 

 minute terminal breathing pores (Fig. ia, a). The ventral surfaces 

 (Fig. 2) show the little spines which later on are to become strong 

 thorns; some of the spines on the sides are relatively bristle-like and 

 longer in proportion than they are later on. In their second stage of 

 growth (Fig. 7c, e) all the characters are well defined. The skin is 

 white and so translucent that the digestive organs, the respiratory ap- 

 paratus, and the fine filiform nerves and their ganglia can be readily 

 made out ; the spines on the abdomen, the hooks, and the stigmata are 

 all more pronounced. In the third stage — that of the mature larva 



