OF WASHINGTON, VOLUME XIII, 1911. 183 



ciable change. The larvae which have resisted the reaction 

 of the host show no special modification, either in form or in 

 size or in the position which they occupy in the body of the 

 host. The number of parasites is quite large. In 107 worms 

 dissected I found 74 parasitized by 87 larvse. 



Toward the end of April the larva is found in the general 

 cavity or in the seminal vesicles or in the pharuyx, making a 

 rupture by the mouth opening. From this time on, fewer 

 and fewer larvae are found internally and more and more in 

 the mouth. The last are noticeably larger. Their head end 

 is turned to the posterior end of the host, while their anal 

 segments with their spines and two spiracles stick out. Under 

 these conditions the worm can hardly move itself. If it is ac- 

 cidentally thrown out on the surface of the earth, it remains 

 there, curled on itself; it cannot nourish itself; its intestine is 

 empty, and the whole body becomes clearer and more trans- 

 parent. 



The contact of the larva of the fly with the walls of the 

 mouth and the pharnyx of the host brings about an inflam- 

 matory reaction of the tissues of the worm, which results in 

 the destruction of several of the anterior segments. During 

 this time the larva of the fly increases, distending the body 

 wall of the host, which gives the whole thing a characteristic 

 aspect. When the larva has reached 10 or 12 millimeters in 

 length, its transverse diameter is greater than that of the host. 

 It rests then attached by the mouthparts and two or three an- 

 terior segments, which are plunged in the remains of the 

 worm. During the period of this rapid growth the larva is 

 plainly more active. Finally, it abandons the remains of the 

 worm, burrows deep into the earth, and at the end of three 

 or four days of free life transforms into a pupa. The imago 

 issues at the end of thirty-five to forty-two days. 



The mouthpieces of the internal larva and when it is in the 

 pharnyx plainly differ. In the latter case the median un- 

 paired piece is lacking, but the whole of the mouth armature 

 has become more complex and resembles a typical structure of 

 the fly larva. 



The different series of spines, well developed on the skin of 

 the internal larva, become less and less apparent, and finish 

 by disappearing at the moment of pupation. The lines be- 

 tween the segments become less marked; finally the spiracles 

 are very visible, and the respiratory apparatus begins to 

 function. 



It is undoubtedly the same larva which is at first an internal 

 parasite and becomes afterwards external. I have taken, in 



