2 PROCEEDINGS OF THE NATIONAL. MUSEUM. vol.64 



in 1904 pointed out that while Fallen used the name Heteroneura in 

 1823 to designate this group, he had previously, in 1810, used the 

 same name for a genus of the Platypezidae. Accordingly Coquillett 

 proposed to replace Heteroneura Fallen, 1823, by the genus name 

 Clusiodes and to call the family the Clusiodidae. In this course he 

 has been followed by Johnson and by Malloch in their revisions of the 

 United States forms of the family. Kertesz, Frey, and Collin, in 

 publications subsequent to Coquillett's change, still retain Hetero- 

 neura as referring to the present group, v/hile Hendel, Frey, 1921, 

 and Aldrich (in litt.) prefer to base the family name on the oldest 

 validly included genus, Clusia. As the last course appears to us the 

 most logical, the family designation Clusiidae is used in the present 

 paper. 



The family Clusiidae includes small flies, varying in length from 

 2 to about 6 millimeters. Most of the species measure about 3 or 4 

 millimeters. They vary in color from pale yellow to black. The 

 thorax especially is often variegated with black and yellow pattern, 

 as sometimes also are the abdomen and legs. The v/ings range in 

 color from a yellowish hyaline to a smoky brown, and from a uniform 

 color to a variegation with as many as three or four large spots. 

 Characteristic of many species distributed among several genera is 

 an apical clouding, especially pronounced along the end of the second 

 vein. Normally the wings are fully rounded and somewhat more 

 than twice as long as wide. The distribution of the crossveins varies 

 greatly. In some of the genera the crossveins are approximated more 

 than in other Acalypterates, hence this character has been commonly 

 used as a family distinction. However, in other genera the crossveins 

 are no closer together than usual. The antennae are always yellowish, 

 the third joint more or less orbicular, with nearly apical arista. The 

 second joint usually bears a conspicuous dorsal bristle. The arista 

 ranges from pubescent to densely plumose. Most of the species nave 

 the arista openly short-plumose, but here and there a species is 

 found where the plumosity is heavily bushy. 



Great variation is common among the specimens "from Central 

 America. These variations, especially in, color pattern, are very 

 suggestive of the mutants in DrosopMla which have arisen and have 

 been perpetuated in the genetics laboratories. From their similarity 

 it seems quite possible that they have arisen in the same way — by 

 mutation. In warm damp climates the mutant flies have abundant 

 opportunity to survive, and races that might die out in a more 

 nhospitable environment continue their existence in the Tropics. 

 This renders the problem of deciding on species limits extremely 

 difficult if not quite impossible. Just where to draw the line between 

 species, just how much variation to ascribe to mutation or to Men- 



