I. INTRODUCTION. 



During the early months of 1902 a collecting party from the University of 

 Nebraska spent several weeks in Costa Rica. One member of the party, Mr. M. A. 

 Carriker, Jr., did not return with the rest, but remained some months longer to make 

 collections in the wild. On his return to Nebraska he gave me three specimens of an 

 interesting parasitic larva with the following history: The specimens were obtained 

 in May, 1902, near Pozo Azul in the province of Perrio, which is located on the extreme 

 western coast of Costa Rica. The smallest one he removed from a toucan (Rham- 

 phastos tocard) which he had shot and the other two were taken from the axillary 

 region of a man in the party. While both of the latter were probably of the same 

 age, the larger of them was at least five weeks old and the host had endeavored to 

 kill them about two weeks previous to their removal, by the use of a toothpick satu- 

 rated with nicotine from a pipe-stem, a method recommended and generally employed 

 by the natives. According to a native guide the parasite is designated in that region 

 as the torcel, or screw-worm, a name already well known in the literature on the subject. 



Mr. Carriker reported that in his collecting he had met with the same species 

 rarely in a large red monkey (Cebus sp.), about one individual in thirty being infected, 

 and that the larva also occurs as a parasite, though still more rarely, in an ant-thrush 

 (Formicarius sp.). He also stated that it was common in pigs and dogs in this province, 

 and in fact all through the low country of the west coast, but it did not occur higher 

 up, being unknown in the mountains. 



The three larvae were in a perfect state of preservation, and careful examination 

 has revealed some details of structure which add to our present knowledge of this 

 species and serve to throw light on the confused or apparently contradictory reports 

 regarding the life history. One of these specimens was also just on the point of molt- 

 ing, so that I enjoyed the unusual opportunity of studying old and new skins with 

 their full armature in related positions. In addition to these three specimens I have 

 had for study two of those described by a previous student of the species in the United 

 States. The larva? represent different stages in the development of an cestrid, or bot- 

 fly, which has long been the subject of note owing to its habit of parasitizing the 

 human subject at times. 



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