24 



THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



anterior notopleurals point a little less inward than this. The posterior 

 scutellars point forward and inward, crossing near their bases, and 

 have their apices between the levels of the anterior and posterior 

 pairs of dorsocentrals. 



The length of the "horns" (anterior spiracles) in proportion to the 

 length of the rest of the puparium differs according to the species. 

 I have estimated this relation roughly in the following forms (the 



r !• i length spiracle n 



fraction represents length '^-".nr;.,,.. ) 



puparium 



Chymomyza procnemis 1/12 



Mycodrosophila thoracis 1/4 



Scaptomyza adusta 1/8 



Drosophila busckii 1/5 



D. funebris 1/5 



Drosophila immigrans 1/2 



D. melanogaster 1/6 



D. quinaria 1/8 



D. robusta 2/5 



D. simulans 1/6 



In the case of C. 'procnemis there is practically no stalk to the 

 spiracle; in the others this estunate counts the definite stalk that is 

 present as a part of the horn. 



The puparia of the four genera named above are all of the same 

 general type. I have also seen puparia of Leucophenga varia, and 

 these are like the usual type. 



The adult emerges from the puparium through an opening that it 

 forces at the anterior end. A lid is lifted up, the anterior spiracles 

 being on the anterior corners of it. The splits along the sides of the 

 puparium extend as far back as the end of the flattened area at the 

 anterior end of the puparimii. Emergence is accomplished by the 

 aid of the ptilinum. This is a sac-like structure on the head, just above 

 the bases of the antennae, that is alternately expanded to a size almost 

 equal to that of the rest of the head, and then contracted until it is 

 scarcely visible. The insect appears to use this structure as a sort of 

 pump, by means of which it drags itself slowly out of the puparium. 

 After emergence the ptilinum is contracted, and in the fully hardened 

 imago it can not be seen. 



Descriptions or figures of puparia have been published by the fol- 

 lowing authors: Comstock (1893, Chymomyza amcena and Drosophila 

 melanogaster), Howard (1900, D. melanogaster), Unwin (1907, D. fune- 

 bris), Martelli (1910, D. melanogaster) , Banks (1912, D. melon og aster) ^ 

 and Malloch (1915, Mycodrosophila dimidiata and Scaptomyza adusta), 



STRUCTURE OF THE IMAGO. 



The following description is limited almost entirely to an account of 

 the chitinous structures. The internal anatomy and the soft parts ti^ 

 general are too imperfectly^ known to warrant any account of them here. ' 



HEAD. 



Several regions of the head are recognized and named, though it •■ 

 is often difficult to distinguish the lines separating them. There is-a 



