THE SYRPHIDAE OF OHIO 29 



The aphidophagous species also dispense with the larval structures, 

 both the anterior and posterior tracheal connections apparently becoming 

 constricted off. But in this type there is no evidence whatever of the de- 

 velopment of special spiracles for the pupal stage. 



In the aphidophagous species the puparium becomes more or less 

 barrel-shaped or roughly conical, bulbous in front and tapering behind to 

 the posterior respiratory organ which remains as in the larva or may 

 protrude farther. 



The rat-tailed puparia are generally more elongate and less inflated 

 dorsally than the aphidophagous ones, somewhat cylindrical in shape. 

 The "rat-tail" is usually fixed in a curved position over the back, while 

 in front are the two pairs of short respiratory cornua, the more anterior 

 pair being the shorter. 



In Microdoii there is apparently less change from larv-a to puparium 

 than in the other forms. A slight amount of dorsal inflation, a general 

 hardening of the integument, and the appearence of a pair of respiratory 

 cornua at the anterior end constitute the external change from larval to 

 pupal stage. 



The appearance of puparia in this family is thus rather characteristic. 

 The "barrel-shape" of many of them; the hard, somewhat vitreous 

 surface of the pupal envelope; the short apposed tubes, or the long rat- 

 tailed structures, of the posterior respiratory organs, as well as the anterior 

 respiratory cornua of certain forms, are di.stinctive characters. 



CHARACTERS AVAILABLE FOR CLASSIFICATION 



As a basis for specific separation we have practically all of the 

 characteristics of the posterior respiratory appendage of the lar\-a remain- 

 ing the same. This structure may also assume new characteristics w^orthy 

 of note, such as greater prominence, color of certain parts, etc. Since 

 the puparium is a rigid structure, its shape and dimensions may be of con- 

 siderable value; as, for example, the proportion of height to width or 

 length, or the curve of the inflation anteriorly and posteriorly as seen 

 from the side. The mouth-parts, and in the aphidophagous species, the 

 anterior spiracles are usually unavailable. In the case of the rat-tailed 

 species, however, the anterior larval and pupal respiratory cornua both 

 present characteristics of great value. Their shape and .size as well as 

 the microscopic ornamentation of them can doubtless always be used to 

 separate the species. In the case of Microdon spp. we have the anterior 

 pupal respiratory cornua wnthout a doubt varying in size, shape, and 

 microscopic ornamentation. Most of the chaiacteristics of the larval stage 

 can also be used. 



