20 OHIO BIOLOGICAL SURVEY 



In the type of larva adapted for aquatic or semiaquatic conditions, 

 and represented by Eristalis, and others, the body is subcylindrical in 

 shape, the integument very thin and pliable with a number of transverse 

 folds falling into groups of about five each, between which the integu- 

 ment is smooth. Near the middle of each group is a transverse row of 

 double, flexible hairs which are probably homologous with the single, 

 usually rigid, segmental hairs or spines of the aphidophagous forms. 



I take it therefore that these groups of folds or wrinkles indicate 

 body segments. There are about seven such groups clearly indicated, 

 and immediately in front of them open the anterior respiratory cornua of 

 the larva. If we consider, as in the case of the aphidophagous larvae, 

 that these anterior spiracles are borne on the prothoracic .segments, and 

 assume as in that case, two segments for the head, then this segment 

 becomes number 3 and the last one of the seven would be number 

 10. To homologize the number of the segments with that which we 

 attributed to the aphidophagous larvae (namely twelve) we may suppose 

 that two segments instead of one have become modified at the posterior end 

 into the long respiratory appendage and the part of the body which 

 bears it and on which the alimentary canal opens. 



These larvae have about six or seven pairs of well-developed prolegs 

 which are ventral projections of the body .surface over which the integ- 

 umental vestiture has become .specialized into heavy, rigid, curved and 

 retror.se hairs of varying sizes. 



There is absence in this type of larva, of hard external mouth-parts, 

 the mouth opening being a soft, fleshy buccal cavitj^ 



The body wall around the mouth is thrown out into several, rather 

 specialized, soft retractile flaps or lips which have a habit of alternately 

 opening out and introverting in such a way as to create a current of 

 water toward the movith. 



Within these flaps is found a hood-like, .striated, chitinous termination 

 of the esophageal frame-work, which reaches the surface or can be seen 

 without dissection. (Figs. 138, b; 139) Internal to this are located 

 certain hard, black, chitinized .structures which it seems to me may be 

 homologous on the whole with certain of the external mouth-parts of the 

 aphidophagous larvae (Figs. 140, 141), migrated in position and some- 

 what degenerate. 



The antennae of this type are similar to those of the aphidophagous 

 larvae, but the two .small .segments at the tip are rather elongate, cylind- 

 rical, rather than spherical, and slightly different from each other. 



The mo.st .striking modification of this type of larva, however, is in 

 the elevation of the spiracles above the general body surface. The 



