March, 1910.] OSBURN : STUDIES ON SyRPHID^, 63 



Giistav Chagnon, of Montreal, Canada, who captured it while collect- 

 ing other Syrphid^e on Montreal Island, Sept. i, 1907. Mr. Chagnon 

 writes me that he noticed nothing unusual in the actions of the 

 specimen and that it was taken resting on a leaf in the manner char- 

 acteristic of many syrphids. I have delayed publishing an account of 

 it while working out the synonymy of Syrphiis arcuatiis Fallen and 

 a related new species, 6". perplexus Osburn, described in the present 

 number of this journal. f This specimen belongs to S. perplexus, and 

 is a normal male in all respects except those to be described. 



The main features of abnormality are: (i) The total absence of 

 the compound eye of the left side, (2) the presence of a well-devel- 

 oped supernumerary antenna on the left side, (3) a well-developed 

 supernumerary vertical triangle on the left side, (4) the distortion 

 of the head, especially on the left side, due to the suppression of 

 the eye. 



The right side of the head is quite normal in the possession of the 

 proper structures, but it is thrown a little out of balance as a result 

 of the absence of the eye of the opposite side. The eyes of the nor- 

 mal male of this, as of other species of Syrphus, are extremely large, 

 covering nearly all of the sides of the head. They meet at the top 

 of the head (the condition known as holoptic) for a large part of 

 their width. The vertical triangle is inserted, wedge-like, between the 

 eyes posteriorly (PI. Ill, Figs, i and 2). The right eye is normal even 

 to the possession of an area enlarged facets, but the absence of the 

 left eye and the consequent lack of development on that side has 

 caused the eye present to appear to extend beyond the middle of 

 the head. This is evidently due to the warping of the morphological 

 median plane of the head (Fig. 3). The face below is nearly normal 

 except that it is slightly depressed, and the antenna of the right side 

 is about in the usual position. The frons is thrown considerably out 

 of the vertical, and the left normal antenna is somewhat lower down 

 than the right one but is normal in structure (Figs. 3 and 4). The 

 color markings of the face, the facial stripe and the supra-antennal 

 spots, are normal except for the twisting (indicated by the dotted 

 line. Figs. l and 3) and that the left supra-antennal spot is reduced 

 in size by the encroachment of the additional antenna (Fig. 3). 



The supernumerary antenna is situated slightly behind and above 



t Studies on Syrphids, I, p. 55. 



