38 ILLINOIS BIOLOGICAL MONOGRAPHS [38 



abdomen are farther mesad than the thoracic legs has been noted. This 

 change, which is barely indicated in the Jugatae, leaves a considerable 

 space between eta and pi on the abdominal segments. Assuming that 

 the setae are sensory in function, it is clear that the presence of a sense 

 organ in this important area would be of selective value. A discussion of 

 the method of origin of new or secondary setae in response to a need 

 on the part of the organism would be out of place here, but that they 

 do arise and are of importance to the life of the caterpillar is shown 

 by the conspicuous differences between the mature larvae of different 

 families. In this case mu appeared in the center of an area left vacant 

 by the mesal migration of the prolegs and the Pi group, and has been 

 retained because it is, like the other sense organs of the larva, an 

 adaptation to the conditions of existence. 



Sigma. Near the medioventral line of almost every lepidopterous 

 larva, sigma may be found in nearly the same position on every seg- 

 ment. So far as known it is never associated with other setae except 

 in some groups in which it is drawn into the formation of a multi- 

 setiferous leg-plate. 



Tau. The name Tau has been given to an indefinite group of setae 

 between pi and sigma. Various numbers are present, ranging from 

 one to three or four, or the entire group may be absent. So far as 

 known, only one is ever present in the first instar. This is the cephalic 

 or mesal seta of the group of three at the base of the proleg on Frenatae 

 and newly hatched Jugatae. Dyar calls all three the "tubercle vii'* 

 but a very little study of the first two abdominal segments shows that 

 the two caudolateral members of the group have a distinctly different 

 origin from the other. When four setae are found upon this aspect 

 of the proleg, as in Hepialus, the additional one is considered as 

 another member of the group Tau. In the description of this genus 

 on a previous page the individual setae of the group were named, but 

 that is scarcely necessary in Frenatae. 



VI. General Conclusions. 



The setal arrangement of every segment of the body in larvae of the 

 Lepidoptera has been derived from the same ancestral type. 



This type includes twelve primary setae : alpha, beta, delta, gamma, 

 epsilon, rho, kappa, eta, nu, pi, tau, and sigma. 



The primary setae are present in the first instar. They became 

 established before the suborders of Lepidoptera separated from each other 

 and possibly before the separation of the order from other Holometabola. 

 It is not necessary to assume, however, that there ever existed a single 



