24 OHIO BIOLOGICAL SURVEY 



that any Syrphidous larva is a very specialized creature and the larvae of 

 the various habits undoubtedly derived their structure from an ancestral 

 form much more generalized than the phytophagous larva of Mesogranima 



poll t a . 



The larvae of the aphidophagous habit are structurally but compar- 

 atively little removed from the phytophagous ones although there are 

 important differences in the minute anatom}' of the two. 



The larvae of Microdon while extremely specialized are in a general 

 way nearer to the phytophagous and aphidophagous forms than are the 

 rat-tailed larvae. The body of Microdon is broader than that of 

 Mesogranima and the aphidophagous species, lacks the segmental spines 

 and possesses a very highl}^ developed marginal fringe. The respiratory 

 appendage is very similar in general appearance, but the stigmatic 

 arrangement is quite different. 



The rat-tailed larvae are extremely specialized in adaptation to 

 aerial respiration while living in an aqueous medium. There apparently 

 can be little doubt that the posterior respiratory appendage in all the.se 

 forms of larvae had a common origin. The primordial representative of 

 this structure was probably the simple termination of the two tracheal 

 trunks. These probably became separately elevated above the surface, 

 making two posterior tubes; then shifted in position until they became 

 apposed, then united ba.sally and now in the entire family they are united 

 mesad thruout their full length. This structure, at first of moderate 

 length, soft and easily modifiable, has now been modified into the short, 

 strongly chitinized structure of all the terrestrial forms, and on the other 

 hand into the long, flexible and telescopic organ of the aquatic larvae. 

 Each structural group has developed its own arrangement of special 

 spiracles. 



The chitinized structures internal to the buccal cavitj' in Eri stalls 

 aeneus (PI. VII, Figs. 140- 141), if they represent mouth-parts at all, are 

 quite degenerate. And finally in the rat-tailed species we have ventrally 

 projecting folds of the body wall, such as those in Syrphus anicrlcauus, 

 specialized into very efficient pro-legs. 



Microdon and Erlstalls differ from the phytophagous and aphi- 

 dophagous species in having an anterior pair of pupal respirator}- cornua 

 developed. 



Moulting of the larvae is incon.spicuous, probabl}' consisting of a 

 somewhat continuous process of shedding flake-like pieces. The con- 

 spicuous change from young larvae to older in S. americaniis is evidence 

 that a moulting does occur. Owing to the fact that in some species several 



