38 OHIO BIOLOGICAL SURVEY 



cases of niimicr}' by W^/iiccIla s]ip. of the host bees in whose nests their 

 larvae hve. The resemblance thus established by natural selection would 

 continue, bitt with the change of larval habit might come to stand as a 

 badge of friendship for the bees. Natural selection might operate to 

 perfect a resemblance so established, in the manner accredited to ordi- 

 nary protective mimicry. (See also pp. 40, 47.) 



AN .EVOLUTIONARY TABLE OF LARVAL HABITS 



The metamorphoses of the Syrphidac are still very imperfectly known. 

 This important field has merely been touched upon by isolated workers 

 in Europe and in North America. What little has been done is sufficient 

 to prove that (unlike the adults) an exceedingly interesting and remark- 

 able diversity of habit exists, in the larval stage, among the different 

 genera and species. 



Williston (1886; gave eight distinct classes of larval habits and cited 

 a few genera as known to him to each class. What follows is an attempt 

 to list, in tabular and evolutionary form, our present knowledge of larval 

 habits; it is based on Williston's classification and for many of the 

 additional notes I am indebted to Verrall. 



From a consideration of the various larval habits in this famil}^ and 

 what we know of the habits of man}', much more primitive, less special- 

 ized larvae in other groups, it seems likely to me that all the habits here 

 represented have been derived from an ancestral condition in which larvae 

 with mandibulate mouth-parts lived externally on the surface of vegeta- 

 tion of one kind or another, eating parts of the leaves or other plant tissue. 

 (See Fig. C. p. 41 ). This habit may have been largely given up either 

 before or after the group differentiated into what we would recognize as 

 Syrphidae. 



At the present time the habit is represented, so far as has been 

 described, only by Mcsogramma polita which feeds externally on the 

 succulent cells of corn {Zea niavs) especially in the region of the leaf- 

 sheathes or on the pollen. (See p. 46.) 



I. Larvae living externally on plants and feeding either on the ordinary surface 

 cells or on the specialized pollen grains; Mesogramuia polita. 



This I believe to be the most primitive larval habit so far discovered. Verrall 

 apparently considers this habit a specialized one, derived from the aphidophagons 

 habit, to which the larvae are supposed to have been driven by the exhaustion of the 

 supply af aphids. To me, this does not seem likely but rather, conversely, that the 

 aphidophagons habit has been derived from the more primitive, phytophagous one, 

 which is still followed by many of our most primitive larvae in other groups. From 

 feeding externally on plant tissue it is a perfectly natural step to what I would make 

 the second group. 



