2 8 NORTH AMERICAN DIPTKRA. 



ill those, there is only a partial atrophy of the basal joint. 

 Some Cecidomyidae and Leptidae ( ' Rhachicerus) have as 

 man) 7 as twenty-eight distinct joints, and it has been said 

 there are as many as thirty-nine in some Cecidomyidae 

 (Cerodozia? ), but I can find no certain evidence support- 

 ing such a statement. Through all the Cyclorrhapha 

 the number three, or very rarely less, is constant. In 

 the nematocerous Orthorrhapha the number is usually 

 from eight to sixteen, though in rare cases there may be 

 as few as six. In such antennae the first two joints are 

 called the scape, and they are always more or less differ- 

 entiated from the remaining segments which compose the 

 flagellum. When the antennae are long, or more or less 

 filiform, the joints of the flagellum often bear whorls or 

 verticils of hairs, especially in the males, and these hairs 

 are sometimes of very peculiar structure, sometimes loop- 

 ed ; in such antennae, also, the joints may have a Icng 

 and abundant plumosity. 



The scape in all flies usually bears bristles or bristle- 

 like hairs; the joints of the flagellum seldom. Upon the 

 structure of the antennae alone, however, it is difficult, if 

 not impossible, to distinguish any of the larger groups of 

 diptera, since those with long and multiarticulate antennae 

 merge very gradually into forms with shorter antennae 

 and fewer joints. Or, the separate joints of the flagel- 

 lum may be fused so closely as to be distinguishable 

 with difficulty, so that the third joint, or the basal part 

 of the flagellum, may appear to be of a simple, uncliffer- 

 entiated structure, though upon closer examination found 

 to be composed of distinct segments or annuli ; in such 

 cases the term complex is applied to the apparent joint. 

 This peculiar structure will be readily understood by the 

 examination of the antennae of a common horsefly (fig. 

 5, 4), where the enlarged third joint is observed to be 

 composed of a large basal piece and a terminal portion 





