NO. 6 INSECT ABDOMEN — SNODGRASS 8l 



The distinct segmentation of the abdominal appendages of the 

 Sialis larva almost unavoidably gives the impression that these organs 

 are the true telopodites of the abdominal limbs. The impression, 

 moreover, comes close to a conviction when it is discovered that each 

 appendage is provided zvith internal muscles in addition to those in- 

 serted on its base. The presence of these muscles was first mentioned 

 by Hevmons (1896a) ; and it can be demonstrated by dissection and 

 staining of specimens preserved in alcohol that bundles of muscle 

 fibers are present in at least each of the first three segments, inserted 

 on the bases of the second, third, and fourth segments, but it is diffi- 

 cult to make an exact study of them without properly prepared ma- 

 terial. The slender distal part of the shaft beyond the third segment 

 is penetrated by a branched trachea, and this part of the appendage 

 might serve as a tracheal gill ; but the strong musculature of the proxi- 

 mal part of the organ, and the long hairs that fringe the segments, 

 suggest that the abdominal appendages of the Sialis larva have an 

 important locomotory function. 



In the sialid genera Chanliodcs and Corydalus the larvae are like- 

 wise provided with long, lateral abdominal appendages, a pair bemg 

 present on each of the first eight segments, and a terminal pair on the 

 last segment (fig. 36 B). In these genera, however, the appendages 

 are simple, tapering, hollow processes of the integument, unsegmented, 

 and containing no muscles. Each is supported on a lateral lobe of the 

 body wall {LB). 



The basal lobes of the lateral appendages of the Corydalus larva 

 are large and prominent as seen in a transverse section of an abdominal 

 segment (fig. 36 C, LB, LB). Each projects laterally beyond the at- 

 tachments of a set of strong tergo-sternal lateral body muscles (/) ; and 

 on the inner margin of the ventral wall of the lobe arise three muscles 

 (D, smcls), one anteriorly and two posteriorly, which are inserted on 

 the base of the distal appendicular process. Here again, therefore, we 

 find repeated the same structures that occur in the ephemerid larval 

 gills and in the thysanuran abdominal appendages. In the Corydalus 

 larva the appendage-bearing lobes of the abdomen fall in line with 

 the subcoxal lobes of the thoracic segments (fig. 36 A, Sex), rather 

 than with the long coxae {Cx) ; but the muscles of the abdominal 

 appendages (D, smcls), taking their origins in the supporting lobes, 

 can be compared only with the basal muscles of the leg telopodite 

 (A, 0, Q) inserted on the trochanter. The abdominal lobes, there- 

 fore, would appear to contain both the subcoxal and the coxal parts 

 of the limb bases. 



