SPECIES HYBRID. 117 



There is no reason to suppose that the subfamily is not present in 

 the other Pacific islands, but it has not been recorded from them.* 

 With respect even to the Hawaiian Islands, Perkins (Fauna Haw., 1, 

 clxxxix) says "Not less than 250 species [of Drosophilinse] must exist 

 in the islands, and very probably double that number may occur." 

 The number and variety of Drosophilinse form the most striking peculi- 

 arity of the dipterous fauna of these islands. Nearly one-third of the 

 species of Diptera considered by Perkins to be endemic in the islands 

 belong to this subfamily. There are a number of large and strikingly 

 colored species of true Drosophila, as I have convinced myself by an 

 examination of material in the U. S. National Museum. Besides the 

 two endemic genera mentioned above, there is a genus described by 

 Grimshaw under the name Hypenomyia that is perhaps distinct, but 

 is here tentatively placed as a synonym of Drosophila, in the absence 

 of a satisfactory description. 



XIII. SPECIES HYBRID. 



Drosophila melanogaster and D. simulans are the only two species 

 of Drosophilinse that have been hybridized (see p. 14). The two 

 species are extremely similar in appearance, and there is every reason 

 for believing that they are closely related. This fact makes the 

 hybrids less interesting for study than would be the offspring of two 

 quite diverse species. In addition, the hybrids so far obtained have 

 all been completely sterile, so that their genetic behavior can no,t be 

 studied. Nevertheless, there are several facts of considerable interest 

 in connection with the cross. 



In general, the hybrids are intermediate in appearance between the 

 two parent species. This is true of the relative size of the eyes and 

 width of the cheeks and of the male genitalia. The other differences 

 between the species, except the length and shape of the egg-filaments, 

 are too indefinite for exact study; and the hybrid females do not lay 

 eggs, so that this character can not be observed. The hybrids differ 

 from both parent species in several respects, as follows : 



1. They are completely sterile, and have poorly developed gonads. 



2. The abdominal plates often are irregular and have narrower and more irregular dark 



bands than are usual. 



3. Some of the thoracic bristles are usually missing. 



4. The wings are apt not to unfold properly, or to be somewhat less convex on the anterior 



margin than is usual. 



5. The cross-veins are often broken or missing. 



The most peculiar thing about these hybrids, however, is the distri- 

 bution of the sexes. The crosses usually result as follows : 



D. melanogaster 9 X D. simulans d' = hybrid 9 9 only. 



D. simulans 9 X D. melanogaster cf = hybrid cf d' ] occasionally a few hybrid 9 9 . 



If the D. melanogaster females bear a Y chromosome (XXY), and 

 therefore give non-disjunctional exceptions, it is found that, they 



* I had overlooked the fact that Jepson (1917, Ann. Rept. Div. Ent., Dept. Agric. Fiji, 1916, 

 16) records Drosophila melanogaster from Fiji. 



