37] LEPIDOPTEROUS LARVAE — FR ACKER 37 



The first instar of larval Frenatae indicates the hoinotypy of Pi 

 and vii, for no other interpretation is possible in the absence of mu 

 (Figs. 17, 18, 19, 29.) 



Pi and vii, in addition to the fact that both are primary, are both 

 ^'double" in all stages on the prothoracic and abdominal segments and 

 sometimes on the mesothorax and metathorax, while mu is always single. 



The multiple nature of Pi and vii is indicated by the newly hatched 

 larva of Panorpa, of the Mecoptera, described by E. P. Felt, 1895. Its 

 prothorax bears the same setal plan as that of lepidopterous larvae and 

 while the setae of the other segments of Panorpa are reduced in number 

 the fundamental arrangement is the same. On every segment there is 

 a pinaculum bearing four to six setae in a longitudinal row situated at 

 the base of the leg or the proleg as the case may be. The fact that this 

 is the first stage, all the setae being lost later, and that the Mecoptera 

 are usually considered more generalized than the Lepidoptera, would 

 seem to indicate that Pi originated as a multisetiferous tubercle and 

 that the setae had been reduced to the definite number two on modern 

 caterpillars. 



Scolus-bearing larvae also indicate the homotypy of Pi on the tho- 

 rax to the group called vii on the abdomen. In Saturniidae, e. g, 

 Samia cecropia (Fig. 107), each thoracic segment bears a scolus at the 

 base of the leg, and in line with these scoli are found similar ones on 

 abdominal segments 1 and 2. The latter are not present, however, on 

 segments 3 to 6. As usual, vii of the proleg-bearing segments is modified 

 into a multisetiferous plate and not into a thorn-like process. If the 

 scoli on segments 1 and 2 represented the seta "vi" or mu, they would 

 also be found on segments 3 to 6 near the base of the prolegs, as in 

 arctians and other verrucose larvae, for mu never takes part in the 

 formation of a multisetiferous leg-plate. These scoli must then be homo- 

 types of vii. But they are also indisputably homotypic with Pi on the 

 thorax. Therefore Pi and vii are homotypic and vii should be called Pi. 



Dyar (1901) interpreted this condition correctly and Quail (1904) 

 agreed with him. The views expressed in this paper are in accordance 

 with those of Quail on all the setae which he studied. A careful inter- 

 pretation of the evidence must convince one that Pi is homologous with 

 "vii" of the abdomen andlias no relation to mu. 



31 u. Mu, as has been said, arises at the first molt of Frenatae 

 between eta and the Pi group. Apparently it is homotypic with pi of 

 the metathorax, but its absence from Hepialus, Panorpa, and the first 

 stage of all caterpillars, shows that this is out of the question. What, 

 then, accounts for its presence ? 



The fact that the prolegs, and consequently the Pi group, on the 



