40 UNITED STATES NATIONAL MUSEUM BULLETIN 276 



All of these groups have been interpreted as primary setae. Primary 

 setal groups can be distinguished from secondary setae in later instars by 

 their relatively longer length, more heavily sclerotized bases, and by 

 their occurrence on pinaculumlike areas. Figure 7 illustrates the primary 

 setal groups found on abdominal segments 3-6. 



In addition to these primary setae, there are some smaller setae on 

 first instar larvae which are here interpreted as secondary. Since many 

 of them are difficult to see and they appear to have no systematic value, 

 they have not been included in figure 7. 



The terminology proposed by Hinton (1946) has been used. Terms 

 have been applied to each setal group on the basis of what seems to be 

 the most logical application of Hinton's system to Malacosoma. No 

 study of other lasiocampid larvae and related groups has been made, so 

 homologies are not positively established. 



The usefulness of chaetotaxy in separating the species of Malacosoma 

 is roughly proportional to the distinctiveness of the species in other 

 respects. In other words, the most distinct North American species, M. 

 disstria, is also the species most easily identified by setal differences. No 

 reliable setal differences have been found for identifying the other species, 

 but some setal characters are helpful when used in conjunction with 

 other characters. It is unfortunate that reliable setal characters have 

 not been found for the other species, since the color pattern is poorly 

 developed in the early instars, and setal characters would be very useful. 

 Numerous other larval structures have been examined in searching for 

 larval characters to separate the species, but none have been found 

 which are as good as certain features of the color pattern. 



The setal groups present in the first instar larva do not correspond 

 exactly to those present in the later instars. The most notable change is 

 the reduction of D2 as a distinct group on abdominal segments 1-8 in 

 the last instar of some populations. The setal groups on abdominal seg- 

 ments 3-6 are the most useful ones. The number of setae per setal group 

 is not absolutely constant, but the mean number of setae per group is as 

 follows: Dl-(4 or 5) ; D2-(3) ; SD-(2) ; Ll-(2 or 3) ; L2-(3) ; SVl-(4) ; 

 and SV2-(2). Other groups named by Hinton either are absent or in- 

 distinguishable from the secondary setae. The most useful groups system- 

 atically are abdominal groups Dl and LI. The most stable group 

 seems to be abdominal group SD which is always composed of two 

 setae — one long seta (SDl) and one short seta (SD2). 



The naming of setal groups by some system is, of course, a necessity 

 if one is to talk about them with any degree of convenience, but a more 

 important reason for naming them in this study is to use the terms in 

 referring to definite areas of the caterpillar when speaking of the color 

 pattern. For example, one could say the subdorsal area is blue except 



