OF WASHINGTON, VOLUME XVIII, 1916 157 



The frons, with its strong infolding*, forms a bridge supporting 

 the head against the thrust of the mandibles and the pull of the 

 strong adductor mandibular muscles attached to the infolded 

 portions of the epicranium forming the adfrontal suture (the 

 straight line of the Y). In the forms which have only to tear 

 delicate leaf-tissue to get the sap nourishment, and also in those 

 nibbling the thin surface cuticle of the leaf, there is much less 

 muscular tension and consequently less need of a strong buttress 

 between mandible thrust and its muscle pull. Hence the frontal 

 surface area is widened and the depth of the infolded adfrons 

 proportionally reduced. When considered in comparison with 

 the line of the epicranial suture and the character of the mandible, 

 the proportions of the frons are good taxonomic characters. We 

 find the shortest frons, the heaviest infolding of the adfrons, the 

 longest epicranial suture associated with strongest mandible, 

 possessed- by the larvae which feed on the edges of the tougher and 

 more fibrous leaves. In the biologically diversified Micro group 

 these structures indicate generic and larger divisions. 



THE SETAE. 



The setae of the epicranium are considered by Dampf of even 

 more significance than the body setae and to him belongs the 

 credit of giving them proper place in larval descriptions. Dyar 

 in 1896 (4) designated a set of Roman numerals to distinguish 

 the eleven primaries visible from the dorsal side of the head, 

 numbering them from the hind margins forward. 



Forbes and Fracker have used these numerals in their references 

 to head setae. Dampf (2), however, has shown that the setae 

 form natural groups within certain areas and has named these 

 groups after the areas upon which they are found. He counts 

 as primaries besides the eleven given by Dyar and Forbes, one 

 seta in the ocellar region, two on the hind part of the gena, and 

 several, generally unhaired tubercles, or "punctures." His sys- 

 tem with slight alterations and the addition of a set of symbols 

 to designate the individual seta, is the one adopted here. The 

 following table shows the homology of the system proposed with 

 Dyar's numerals. 



In my opinion the 3rd seta described by Dampf among his 

 Dorsolaterals (Dyar's II) does not properly belong there, but 

 should be associated with the small seta near the epicranial suture 

 (Dampf 's "vertical" seta), the unhaired tubercle or tubercles be- 

 tween them and the several secondary setae or punctures near 

 the dorsal hind margins. They form a natural and easily dis- 

 tinguished group from the others and, considered apart, give a 

 clearer understanding of the differing setae arrangements. I have 



