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 
; length spiracle , . 
fraction represents length puparium Je 
Chymomyza procnemis............ 1/12 Drosophila immigrans.).4.\069) ee 1/2 
Mycodrosophila thoracis............ 1/4 D. melanogaster 2+...) 34/:).459 4.0 1/6 
Scaptomyza, adusta.. . 2. . sie cose ecas 1/8 DD) SQUIMATIO Wis sate ie hu cere eae 1/8 
Drosophite buseka, 12 Se te eB Ds'rabusta ts oe) S252 ee. eee ee 2/5 
DM PaBe Drs eG a Ee eso Mian Ne 1/5 De siiaulanie ys i005. VNU et See ee 1/6 
In the case of C. procnemis there is practically no stalk to the 
spiracle; in the others this estimate 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 puparium. Emergence is accomplished by the 
aid of the pitlinum. This is asac-like structure on the head, just above 
the bases of the antenne, that is alternately expanded to a size almost 
equal to that of the rest of the head, and then contracted until it is 
searcely 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 amena and Drosophila 
melanogaster), Howard (1900, D. melanogaster), Unwin (1907, D. fune- 
bris), Martelli (1910, D. melanogaster), Banks (1912, D. melanogaster), 
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 in 
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 
